Andover, United Kingdom – First there were light-cured adhesives, then light-activated adhesives, and now we have dual-cured adhesives. Eamonn Redmond, Director of Inseto, provides an overview of all three.
Image courtesy of DELO.
First there were light-cured adhesives, then light-activated adhesives, and now we have dual-cure adhesives. Eamonn Redmond, Director of Inseto, provides an overview of all three.
Though what we’re about to discuss has been the subject of articles before, it’s always worth having a refresher. Also, while many of the overarching principles of bonding have not changed over the years, adhesive chemistries have – which means things like open times (see later) have increased.
‘UV adhesives’ is the generic term used when referring to an adhesive that requires high-intensity light for curing. A significant proportion of these adhesives actually uses light in the visible spectrum for curing. Indeed, it is typically the visible component of the photoinitiator that is used to achieve the cure. This is useful when bonding polycarbonate, for example, as most grades of this material block UV light.
Regardless of where on the spectrum the photoinitiator lies, the simple fact is that one of the parts being bonded must be transparent to allow for 100% cure of the adhesive (see figure 1). No one likes having uncured adhesive in their products on a long-term basis, especially as a number of these adhesives are based on acrylic acid, which is rendered harmless by curing but is hazardous in its liquid state.
Figure 1 – Light curing requires one of the substrates to be transparent. (Image courtesy of DELO.)
When bonding components together, it is essential that the whole volume of the adhesive is fully cured, as uncured adhesive in the finished assembly may cause corrosion or, in the case of optical products, interfere with the light path. However, achieving full cure can be a problem due to shadow areas that light cannot reach.
In this respect, dual cure adhesives can help and there are many on the market that offer significant advantages over more traditional adhesives without sacrificing reliability, bond strength or ease-of-use. Uses include industrial displays, automotive camera modules, electric motors and even simple applications such as thread-locking. Let’s take a look at the two most popular dual cure methods.
Dual Cure: Light & Humidity
These adhesives are used when temperature-sensitive materials are being joined, but they are limited by the fact that the majority of the adhesive in the bond area must be cured by light and by the fact that the humidity-responsive portion of the adhesive cures at a slow rate (typically about 2mm every 24 hours), which is similar to the rate at which silicone adhesives cure.
They are single-part adhesives and are free of isocyanates (so present no health and safety issues) and are free of silicones too (so no impediment to subsequent adhesive bonding), unlike some acrylates. They are highly flexible, optically clear and offer excellent climatic resistance, whilst also providing excellent bond strength on surfaces such as glass, PMMA, metal pins and most plastics.
Dual Cure: Light & Heat
These are based on two diverse chemistries, epoxies and acrylates.
Epoxies tend to be hard once cured, offering increased resistance to chemical and temperature stresses due to the tight cross-linking of the polymer that occurs during cure.
Acrylates are usually softer adhesives, enabling quicker curing and greater flexibility of the cured adhesive.
Using a combination of heat and light to cure these adhesives offers a very fast fixation by snap-curing the photoinitiator in the adhesive. Subsequent heat curing ensures that there is no uncured adhesive in any shadow zones that might exist in the assembly. This fast fixation also allows increased accuracy, which is especially useful for companies that have invested heavily in high-accuracy placement machines only to see movement of the parts being bonded during the heat-cure stage.
The heat-cured stage generally involves heating the parts up to around 100°C after the light cure process. However, for temperature-sensitive materials such as some plastics, modified epoxy adhesives are available that will cure at 60°C, combining defined processes and short cycle times, despite the low curing temperature of the adhesive.
These are especially useful in applications such as automotive camera modules, or where the end product is subjected to chemical influences that would otherwise harm an acrylate adhesive. These dual-cured epoxies also exhibit very low outgassing and low yellowing, making them ideal for applications with demanding optical requirements, such as LED assembly.
Dual-cured acrylates offer similar advantages to dual-cured epoxies, namely very fast fixing and full cure after subsequent heat curing, but they also exhibit excellent impact resistance and tension-equalising properties due to their flexible nature. These are suitable for applications such as the assembly of rotary encoders, where optional fluorescing and colouring can be added to aid visual inspection.
Light and heat cured adhesives also offer increased flexibility in the manufacturing process. While heat-curing is mandatory for a small number of these adhesives, the majority offer independent curing mechanisms, allowing curing by light, by heat or a combination of the two.
They do not suffer from the same limitations as light and humidity cured adhesives, however, a downside is the minimum cure temperature is a strict 80°C, which means heating the oven to at least 83°C to avoid any potential cold spots. This is strongly recommended because if the adhesive does not reach 80°C its heat-cured portion will never cure, regardless of how long the parts to be bonded remain in the oven. The time to cure at elevated temperatures can also be a factor. For example, curing at an oven temperature of 83°C might take up to one hour, but at 150°C the cure time might be as short as 10 minutes.
Unfortunately, many modern low-cost plastics are heat sensitive and even 80°C might be a problem. Two-part cold-cured adhesives, such as epoxies and polyurethanes, can overcome this issue but the penalty is long curing times.
Light Activation
For applications where the cycle time needs to be short – i.e., to avoid lengthy humidity or heat cure times – this is where light-activated adhesives come into their own.
The process is simple. Dispense the adhesive onto substrate A, illuminate it with high intensity light for a short period of time, and place substrate B onto the adhesive. Both A and B can be non-transparent.
Illuminating the adhesive provides enough energy to trigger the curing process. However, if too much energy is provided, there will not be enough time to place the second substrate before a skin forms on the surface of the adhesive. Once this happens, it is impossible to bond the substrates.
The time it takes for the skin to form on the adhesive is called the open time. It is measured from when the illumination ceases to when the skin forms. Increasing the energy provided to the adhesive, whether by increasing the intensity of the light or by illuminating for longer, reduces the open time.
Unfortunately, the relationship between the activation energy and open time is not linear. Factors such as substrate material, colour, smoothness and reflectivity all have an impact on the open time. The technical datasheet of the adhesive will indicate a range of open times. For example, the technical data sheet for DELO KATIOBOND 4594 states that an open time of 15 to 20 seconds results from an illumination time of 3 seconds when using a DELOLUX LED lamp with a light intensity of 200 mW/cm2, measured at the adhesive.
Changing the lamp, the illumination time or materials will affect the open time, which should be measured. Also, it’s worth noting that if one the materials being bonded is metal, then it will be necessary to heat it up slightly, to say 35°C. This is because the heat generated within the adhesive during the activation process will be conducted away and slow down the reaction significantly. It may even prevent the reaction completely.
Once the substrates have been joined, full cure will take place over time, typically 24 hours. However, we did say above that light-activated adhesives are ideal for shortening the cure time, and they are because the cure process can be accelerated by:
Depending on the geometry of the parts being bonded, additional light curing is possible. For example, if there is a fillet of say 0.5mm of adhesive around the joint, a second light cure process can be carried out immediately after bonding (see figure 2). This increases the bond strength, allowing the assembly to be moved on to the next process.
Alternatively, the assembled parts can be heated (also shown in figure 2). This can seem contradictory as light-activated adhesives are used for the very purpose of eliminating heat from the process. But even the addition of low levels of heat can have a significant effect on the cure speed. As a general rule, for every 10°C increase in cure temperature, the cure time is halved. So, increasing the temperature of the bonded parts to even 45°C (i.e., safe enough even for the modern temperature sensitive materials I mentioned earlier), can reduce the cure time to 6 hours, while ensuring that there is sufficient handling strength in the adhesive to safely carry out the next (assembly) process step.
Figure 2 – Above, an illustration of a camara module being bonded to a substrate. Following activation of the adhesive and placement of the module, further curing can be achieved using light shone through the sides of the fillet or through the application of heat. (Image courtesy of DELO.)
A recent development on the light-activated adhesives front is ‘activation on the flow’, announced by DELO in 2022. The technology combines adhesive dispensing and pre-activation in a single process step and is considered particularly suitable for bonding and encapsulating temperature-sensitive electronic components.
Another benefit of adding irradiating to the dispensing step is that the exposed adhesive areas can be additionally irradiated (as discussed above) and fixed after joining. This provides immediate initial strength, preventing the adhesive from flowing out and the components from slipping, which allows them to be further processed immediately. Whether with or without additional light fixation, the adhesive cures reliably to final strength without any additional process step, even in undercuts and shadowed areas. For further information on this see FAST magazine news from 25th July 2022.
Summary
So, what has this refresher taught us? Traditional light (only) cured adhesives need one of the substrates to be transparent. Light and humidity dual-cure adhesives overcome the problem but take time to cure. Light and temperature dual-cure adhesives take less time to cure but cannot be used with many heat-sensitive materials.
Light activation, the new kid on the block, initiates the curing process (and marks the start of the ‘open time’). Full cure takes time, but the process can be accelerated using light (subject to it being able to penetrate) or heat (subject to the substrates not being heat-sensitive, though lower temperatures can be used compared to light and temperature dual-cure adhesives).
‘UV adhesives’ is the generic term for adhesives that require high-intensity light for curing. For many though, it is light in the visible spectrum that does the curing. (Image courtesy of DELO.)
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This article is featured in Issue 3, 2023 of Fast Magazine and is re-produced here with the kind permission of the publisher.
Inseto is exclusive distributor for DELO‘s range of adhesives throughout the UK and Ireland.
Andover, United Kingdom – Inseto, a leading technical distributor of equipment and materials, has supplied the Institute for Compound Semiconductors with a variety of semiconductor manufacturing equipment for its new 16,150ft2 cleanroom situated adjacent to its Translational Research Hub (TRH).
SUSS MicroTec MA8 Gen4 mask aligner, part of the equipment supplied to the Institute for Compound Semiconductor by Inseto.
The TRH, which opened for business in May 2023, is a 129,000ft2 facility with a mix of flexible laboratory and office space where industry and experts can come together to solve complex global challenges. The ICS’s new 200mm fabrication line is being used by researchers in the ICS and commercial partners to trial, develop and scale up new compound semiconductor devices.
The equipment supplied by Inseto includes an MCS8 manual resist coating system, an MA8 Gen4 mask aligner, SD12 and AD12 wet processing systems, and an HMxSquare 9 photo mask cleaner (all from SUSS MicroTec), a PlasmaEtch PE-75 plasma treatment system and an ADT 7900 wafer dicing saw. In addition, Inseto’s engineering team managed equipment installation and operator training.
Dr Angela Sobiesierski, ICS Operations Director said: “Inseto was able to offer us a range of state-of-the-art equipment that has enabled us to scale up our fabrication line from 150mm to 200mm – something that was at the heart of our move to the new cleanroom.”
The ICS was a founding member and key partner in the development of CSconnected, the first compound semiconductor cluster in Europe. The institute provides small to medium scale fabrication
capacity to complement activity at other cluster partners, with the expertise and capability to translate academic excellence through to practical, manufacturable devices and integrated subsystems.
Matt Brown, Director of Inseto, commented: “We are delighted to have been awarded this work and to have provided so much of the equipment being used by the Institute for Compound Semiconductors in its new cleanroom at the Translational Research Hub.”
Dr Sobiesierski added: “The post-delivery support from Inseto has also been great, working closely with our team and the OEM engineers involved to ensure that installation and commissioning proceeded smoothly.”
SUSS MicroTec MCS8 manual resist coating system (and partly in frame on the righthand side, an AD12 wet processing system), part of the equipment supplied to the Institute for Compound Semiconductor by Inseto.
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About the Institute for Compound Semiconductors
The Institute for Compound Semiconductors (ICS) is a world-class compound semiconductor research and small-scale manufacturing facility. Available to all on an open access basis, ICS provides a platform for
fabrication on wafers up to 200mm in diameter, where scientists and industry work together to prepare projects for translation into the production environment. Part of Cardiff University, the UK’s facility of choice is internationally recognised for stimulating, facilitating and enabling CS research
Figure 1: Ultrasonic wirebonding is an ambient temperature (‘friction weld’) process, so does not run the risk of damaging the cell. Bond integrity/quality is the product of the physical force and ultrasonic energy (i.e., power over time) applied. These parameters are controlled to ensure the bond takes and can even be adjusted to account for some levels of surface contamination.
Electromobility (e-mobility), the move away from using internal combustion engines in cars, bikes, buses and trucks etc., requires batteries to store and release power. The properties of a battery, or rather a battery pack, are therefore largely responsible for setting the vehicle’s performance. Storage capacity defines range and the rate at which power can be released sets acceleration.
In addition, the rate at which the battery pack can accept power determines charge times (in the case of battery EVs [BEVs] and plug-in hybrid EVs [PHEVs], for example) and how much power can be recovered through regenerative braking (in the case HEVs, PHEVs and range extended EVs [REEVs], for example).
A typical battery pack in something the size of a car comprises multiple battery modules, connected using bars, bolts and/or heavy gauge cables, arranged in parallel and series combinations to produce the desired energy and power characteristics. Each module can contain anywhere between a few and more than a thousand cells.
Consider these stats for the battery pack of the Tesla Model S Plaid. It has 99kWh total capacity, of which 95kWh is usable. It has five identical power modules. Each module contains 22 rows (i.e., series connections) of 72 cells placed in parallel (i.e., within each row). This equates to 1,584 cells per module and 7,920 for the pack. Each cell is a Panasonic lithium-ion 18650-type, so larger in diameter and length than a standard AA cell.
Not surprisingly, a vehicle’s battery pack accounts for much of its cost. For a BEV, that can be more than one third. With material costs more or less the same across the industry, all battery pack manufacturers are keen to make their products as cost-effectively as possible. However, they cannot compromise on durability or safety. And regarding this last point, ISO 26262 functional safety standard applies to the battery management systems (BMS) within or working alongside battery packs.
Connections
As mentioned in the Tesla battery example, several cells are placed in parallel. This is typically achieved by connecting the terminals of the cells to busbars. This tends to be done in one of two ways.
Laser welding. Each busbar is placed in physical contact with the respective terminals of all cells to which it is to be connected. Tooling can be an issue to account for any cell height tolerances. Also, as it is a traditional weld process, the objective is to heat metals until they fuse together. Here, there’s a risk that localised heat from the welding process penetrating the negative terminal can alter the cell chemistry and lead to catastrophic thermal runaway. NB: cell positive terminals are ‘floating’, so less vulnerable because of the air gap.
Ultrasonic wirebonding (see figure 1). The process is already dominating power electronics manufacturing as a flexible and robust method of making electrical interconnects in hybrids, switches and regulators; and it dominates the microelectronics industry. Also known as ‘friction welding’, there is minimal localised heating to the wire or battery surface and the process copes far better with tolerances in cell height (relative to the busbar). In addition, there are several industry specifications relating to wirebonding and bond quality that are being adopted within the automotive sector. For example, MIL-STD-883E, Notice 4, Method 2011.7 is a test to measure bond strengths. See figure 2.
Figure 2: Above, MIL-STD-883E, Notice 4, Method 2011.7 states the minimum forces to be applied during a bond wire pull test depending on the material and its diameter. This is a non-destructive test, and the bond wire is pulled midway along its length. Some wire bonders can perform the test, using a hook, as part of the bonding process so the quality of every bond can be checked during production.
As mentioned, durability and safety are key and, in this respect, ultrasonic wirebonding has an edge over welding. Depending on the vehicle’s intended environment of operation, the battery pack may be subjected to significant vibration and mechanical shock. Any interconnect technology used at the cell level must withstand the external forces expected, to ensure a good operational lifetime.
The bond wire tends to be high purity aluminium, with a diameter of between 0.2 and 0.5mm and has a degree of softness and flexibility (annealing). Note: multiple bonds can be made side by side to accommodate high currents.
As for safety, with a suitable diameter, a bond wire can act as a fuse and a failing/shorting cell will effectively self-isolate, thus reducing the risk of fire or explosion.
In Practice
As mentioned, keeping manufacturing costs down is an imperative. But it’s not just the likes of Tesla keeping a keen eye on production costs. For example, Steatite’s Power Business Unit has recently taken delivery of an Asterion EV wire bonder – see figure 3. Steatite specialises in the creation of custom battery packs, which often need to be of a particular size and shape.
The bonder gives Steatite the ability to establish electrical connections using wire bonding in up to half the time spot welding would take, in many cases. In addition, wire bonding makes possible the creation of battery packs with high discharge capabilities and improved performance, as a result of the very low resistance of the bond wire.
Steatite’s engineers have been successfully spot- and arc-welding battery pack components for several years. Spot-welding is, in particular, suitable for the vast majority of the company’s products, in which some parts are up to 3mm thick. Wirebonding is therefore a complement to welding in the manufacture of most battery packs.
On a general note, for any high value manufacturing process, the ability to rework process steps to improve assembly yield is important, especially in the initial prototyping and pilot production stages. In this respect, wirebonding has the edge. Failing or weak bonds can be easily reworked. Moreover, wire bonders like the Asterion EV can automatically perform wire pull tests to verify the bond has taken.
Reworking a failed or imperfect weld is more problematic as there will be more surface material to remove and the cell will be exposed to another temperature process as it is rewelded. Also, depending on its design and once in place, a busbar might not allow access to individual joints for rework purposes.
In summary, welding and wirebonding both have key roles to play in the construction of EV battery packs. As for which process to use, this depends on the pack architecture, ease of access to the parts to be connected, whether or not fuses are required, the ability to accommodate reworks, volumes being manufactured, production costs (including time) and the end application.
Figure 3: Above, the Asterion EV wedge wire bonder in situ within Steatite’s Power Business Unit.
Pressure sintering using an AMX Automatrix X-Sinter P54.
The EV sector is constantly striving for higher power densities (in terms of W/m3) and in particular with regards to the electronics for controlling power, both within vehicles and within charging stations. Reducing the volume of a component reduces the quantity of materials to build it, thus reducing production costs.
Power modules, as used in inverters for example, are required to switch high voltages at high frequencies into loads that draw hundreds of Amperes. These requirements have made silicon carbide (SiC) the semiconductor material-of-choice, and EV power modules typically contain several SiC-based MOSFETs or IGBTs.
In addition to the requirements to switch more power, there is also a need for deep power cycling and providing high reliability operation in relatively harsh environments. All of these factors place challenges on device packaging – certainly if industry is to get the most from all that SiC has to offer.
Although SiC devices boast low power losses these are relative to very high (achievable) power densities. Essentially, SiC-based high power switching structures run hot, and the heat must be dissipated. Moreover, SiC-based die can run far hotter than the melting point of even specialist solders.
Sintering – Materials & Processes
As readers will be aware, pressure sintering is an alternative to soldering but let’s take a detailed look at the materials and how they affect/govern the sintering process.
A sinter paste comprises monometallic particles (typically less than 1µm in size), an organic compound (with an evaporation temperature of about 150˚C) and possibly an oxygen reduction agent. Sinter paste OEMs have their own recipes but common to all is that the monometallic particles account for about 90% of the volume.
The material of choice is currently silver. However, all paste OEMs are conducting R&D into copper. The cost of copper is about 5% that of silver, but its use presents many challenges because of how readily the metal oxidises, preventing the formation of good mechanical bonds – unless the entire process is performed in an inert atmosphere.
The most popular method of applying the sinter paste to the substrate (which is typically direct bonded copper, DBC) is using a stencil, where the holes are usually 5 to 10% larger than to die to be placed. The applied paste will typically be between 100 to 120µm thick, though some device manufacturers are experimenting with 80µm.
The next stage is pre-drying. This tends to be in an oven with a nitrogen atmosphere. The substrate is heated to between 120 and 130˚C for 30 minutes. This removes any moisture present and results in the thickness of the paste reducing by about 20% while still remaining tacky, as the organic compound is still present.
Figure 1 – Above, a cross-section of the sintered die attach BLT.
After drying, the substrate is moved to a pick and place machine. Here, many OEMs are using hot-head tools (at about 100˚C) as it improves the bond quality. Note: the underside of the SiC die is already metalized with silver.
Next, it’s to the sinter press. If the substrate is large, it is advisable for the press to have a pre-heating stage. This reduces the risk of thermal stress (warping). Also, if the substrate is a large thermal mass, it will take longer to reach the sintering temperature, which is about 250˚C.
The sintering process is relatively quick, at about 3 minutes. The top of the die is protected using a thin film of Teflon and the sinter tool head presses down, applying a pressure of between 15 and 30MPa. The thickness of the paste reduces by a further 50% as the silver particles bond to produce a bond line thickness (BLT) of between 30 and 40µm – see figure 1.
It is advisable to allow the substrate and die to cool while still in the sinter press to avoid oxidisation, before going into test; followed by wire bonding (which might also include tests) and moulding (chip encapsulation). Figure 2 illustrates some of the equipment used in the packaging of sintered die.
Figure 2 – Equipment used in the fabrication of a power electronics component/module requiring die to be sintered to the substrate. Not shown, but a recent industry development is the use of a foil with a similar composition to a paste. Printing and pre-drying can be skipped, but the foil is expensive and requires special handling. The technique is more for R&D work than volume production.
Results
The main method of determining how well the die has attached to the substrate is to perform a mechanical shear test. A shear strength of more than 30MPa should be achievable at room temperature and more than 20MPa during a hot test (typically 100 to 120˚C).
One way of increasing shear strength is to silver plate the DBC substrate in the areas where the sinter paste will be applied. This means the interface will be between the silver on the underside of the die, the silver sinter paste, and the silver plate on the substrate. In destructive tests performed by AMX Automatrix, shear strengths of up to 70MPa have been recorded. Without silver plating, dies were shearing at about 55MPa.
Figure 3 – Above a SAM image of a number of sintered dies. The voids look like bubbles and the dark patches are a sign of delamination.
Shear strength is also a measure of the presence of voids beneath the die, noting here that to see voids requires a scanning acoustic microscope (SAM). Voids can lead to delamination as a result of thermal cycling. Figure 3 shows a SAM image of voids and delamination.
Voids are expressed as a percentage of die area. However, the figures regarding what are acceptable percentages have been inherited from industry standards for attaching die (usually Si) using solder. For SiC, far more power is being handled (to the extent there’s likely to be a bang in the event of delamination) and there’s a far greater need for efficient heat dissipation. Many believe there should be much tighter requirements where acceptable void percentages are concerned, and although sinter processes (for electronic component die attach) have not been around long enough for standards to be set, early results are looking good.
Thermal cycling is an essential test, and AMX is aware of some OEMs subjecting sintered components to extremely aggressive tests. For example, some cycle between -55 and 250˚C, noting that -40˚C is the automotive industry’s current low temperature limit. And the jury is out on what should be an upper temperature test limit for SiC, though AMX is aware of one company that is testing to 300˚C and their power modules are performing well, i.e., no reduction in shear strength and therefore no indication of delaminating. Also, although 1,000 cycles are standard, many are subjecting their designs to 10,000 cycles.
Summary
Sintering technology has come a long way in a relatively short period of time, and the use of silver sinter paste has produced some great results and continues to do so. Copper sintering is on the horizon though, driven by the far lower cost of the metal.
Inseto, a leading technical distributor of equipment and materials, has supplied and installed a Kulicke & Soffa (K&S) Asterion EV wedge bonder at Leicester-based BPC Electronics to support its expansion into sectors in which battery packs are used.
BPC, a contract electronics manufacturer, is renowned for its electronic design and PCB assembly services and has the usual manufacturing equipment one would expect of a CEM; including pick and place machines, solder reflow lines and a vapour phase oven.
The company’s investment in the Asterion EV gives BPC additional assembly capabilities and, in particular, the ability to manufacture battery packs for electric vehicles (EVs), other forms of e-mobility and the static storage of power (on the domestic and small industrial scale) from renewables such as wind and solar.
The EV is part of a family of Asterion wire bonders (the largest of which has a bondable area of 760 x 1440mm), the members of which are all ideal for establishing the numerous electrical connections found in battery packs. The EV has a bondable area of 300 x 860mm, pattern recognition capabilities and is driven by a direct-drive motion system that requires minimum maintenance, provides for extremely tight process control and delivers high repeatability.
Mike Pitt, Sales Director of BPC Electronics, commented: “We’re already serving many sectors that use battery packs. We’re doing so by manufacturing circuit boards and cable harnesses, plus we do box builds. Offering battery pack design and manufacturing services is a logical expansion for us.”
BPC built a dedicated and spacious room for its Asterion EV, as the company envisages having a steady throughput of battery packs of varying shapes and sizes, and has already started building packs with 6, 12 and 24kWh capacities for a number of static energy storage applications. The company also plans to lease out time on its new wire bonder. “We have ambitious expansion plans,” concluded Pitt. “These include purchasing additional wire bonders in the future, as all the indicators are that our domestic markets are keen to have battery packs and associated products, such as battery management systems, that are designed, built and can be supported by companies here in the UK. Despite government incentives, investing in e-mobility and energy storage still requires a leap of faith. We give customers peace of mind because we’re a UK-based OEM that can develop, manufacture and supply most of what’s needed.”
Inseto has supplied and installed a Kulicke & Soffa (K&S) Asterion EV wedge bonder at Leicester-based BPC Electronics to support the CEM’s expansion into sectors in which battery packs are used.
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About BPC Electronics
Established in 1993 and with more than 25 years of experience, BPC Electronics is one of the UK’s leading companies for printed circuit board, LED assembly and electronic contract manufacturing.
Based in Leicester, the company offers a variety of printed circuit board (PCB) manufacturing and design services, ranging from production and testing to contract electronics manufacture (CEM).
BPC supplies a vast range of PCBs covering simple single-sided boards through to flexi-rigid multilayer boards and has extensive experience of handling many different types of materials from ceramic to PTFE and everything in between.
Andover, United Kingdom – Inseto, a leading technical distributor of equipment and materials, has supplied and installed a Kulicke & Soffa (K&S) Asterion EV wedge bonder at Steatite’s Power Business Unit. Steatite specialises in the creation of custom battery packs, which often need to be of a particular size and shape, and the company has been designing and manufacturing lithium battery packs, COTS battery modules, and portable power and energy storage systems for more than 30 years.
The Asterion EV gives Steatite the ability to establish electrical connections using wire bonding in up to half the time spot welding would take, in many cases. In addition, wire bonding makes possible the creation of battery packs with high discharge capabilities and improved performance, as a result of the very low resistance of the bond wire.
Above, the Asterion EV wedge battery bonder in situ within Steatite’s Power Business Unit.
“Wirebonding complements our other manufacturing capabilities,” comments Dave Carlton, Head of Technical Sales at Steatite’s Power Business Unit, “and the Asterion EV was selected for its large bondable area [300 x 860mm], high resolution [0.1μ] and its ability to perform non-destructive bond wire pull tests whilst bonding. Also, the support from Inseto – from their recommendation of the Asterion EV through to operator training – has been excellent.”
Carlton goes on to say that power failure is not an option in many of the critical applications for which Steatite designs and manufactures battery packs. Steatite battery packs can, for example, be found in products and equipment used in medical, industrial, oceanographic, energy and transport applications.
“With the Asterion EV we have the peace of mind that every cell leaving our factory having been wire bonded is safe, reliable and robust,” concludes Carlton. “This is enabling us to maintain our reputation for battery pack performance quality, a reputation we’ve secured over decades in the power business.”
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About Steatite
Steatite Ltd is a market leader in the design, production, test and supply of rugged industrial computers, custom battery packs, MANET radio systems, advanced wideband antennas, and imaging technologies, all ideally suited to harsh operating environments.
Steatite has a rich heritage and strong reputation for creating technology that meets the operational demands of its customers. Operating from four dedicated UK facilities, the company has full in-house design, engineering, manufacturing and testing capabilities, ensuring that the Steatite name is synonymous with quality and reliability.
Part of the Solid State Plc group of companies (AIM:SOLI), Steatite’s web address is www.steatite.co.uk
Andover, United Kingdom – Lincoln-based RF and microwave design consultancy Cogent RFMW enhances its semiconductor verification and characterisation services through the use of a SemiProbe probe station, supplied by Inseto.
SemiProbe Probe Station Testing High-Frequency Devices at Cogent RFMW
Inseto, a leading technical distributor of equipment and materials, has supplied a SemiProbe probe station to RF and microwave design consultancy Cogent RFMW, enabling the company to enhance and extend its semiconductor verification and characterisation services.
Used in conjunction with benchtop T&M instrumentation, the probe station is being used for DC tests – including pulsed and continuous IV curves, Gummel plots, breakdown voltages and current gain – and RF tests that include small and large signal S-parameters, noise figure and load-pull linearity. All tests can be performed on wafers or die.
“The semiconductor industry is changing in the UK, and skills shortages are making it difficult for many manufacturers to do everything in house, even if they have the equipment,” comments Malik Ehsan Ejaz, Founder and CTO of Cogent RFMW. “At Cogent we have much sought-after skills, and the SemiProbe station supplied by Inseto plus equipment from Interligent and Focus Microwave enable us to support UK-based IC OEMs.
Ejaz, formerly Principal Engineer at CSA Catapult, goes on to say that skills shortages are the biggest hurdle manufacturers face.
Matt Brown, Managing Director of Inseto, adds. “The supply chain has had to re-invent itself in recent years to bring real value to customers. For instance, in 2021 we established a Process Development Laboratory, which is now in almost constant use, plus we’ve introduced equipment operation training courses. What Cogent RFMW is doing is another great example of how the industry is changing. A consultancy that is fully hands-on with advanced equipment and has the expertise to support semiconductor OEMs.”
Based in Lincoln, Cogent RFMW is well positioned to serve semiconductor manufacturers in the Midlands and the North.
Inseto is exclusive distributor for SemiProbe in UK, Ireland and Northern Europe.
For more information on SemiProbe Probe Stations – Wafer Probing Equipment, please click HERE.
E-Mobility Technology International Publication – Read all about the process development of “Wire Bonded” battery connections in the Summer 2022 edition.
For any type of vehicle, or electrotechnical machine for that matter, in which the drivetrain is largely electric, the battery is its defining component. It determines performance. Also, the reliability of the battery pack is a major contributor to the overall reliability of the EV or machine. There is also the cost aspect. In the case of an EV, the battery pack accounts for almost one-third of the vehicle’s cost. And over a third of the cost of the battery pack is in its manufacture.
Understandably, battery pack manufacturers are seeking to get the most from well-established and proven manufacturing technologies to make increasingly dense and complex battery packs, while assuring high reliability and keeping costs as low possible.
As readers will be aware, a battery pack comprises several battery modules that are connected (using bars, bolts or heavy gauge cables) in parallel and series combinations to produce the desired energy and power characteristics. Each module contains several cells – of pouch, prismatic metal can or (most commonly) cylindrical type – connected to a busbar.
The two most popular methods for connecting cells to busbars are:
Laser welding. Each busbar (typically a complex shape) is placed in physical contact with the respective terminals of all cells to which it is to be connected. The contacting surfaces are then fused with a weld.
Ultrasonic wirebonding. This is an ambient temperature ‘friction weld’ process. The controlling variables that determine the process are the ultrasonic energy, the bonding force applied by the wedge and the bond cycle time.
Both methods have clear benefits. And both have some limitations. For instance, laser welding is non-contact, and the beam can be projected into spaces that cannot necessarily be accessed by a wedge bonder. On the downside, tooling can be an issue to account for any cell height tolerances. Also, as it is a traditional weld process, there’s a risk of localised heat from the welding process penetrating the negative terminal and damaging the cell. Note, a cylindrical cell’s positive terminal is ‘floating’, so less vulnerable to heat transfer because of an air gap.
As for ultrasonic bonding (see figure 1), it is an ambient temperature process (that can be used to place wire or ribbon) so there is minimal localised heating and no risk of damaging cells. Also, the process copes far better with tolerances in cell height (relative to the busbar) and, when wire is used, by matching its cross-sectional area and length to a maximum desired current each cell is effectively given its own in-line fuse.
This is a great safety feature, as a shorting cell will self-isolate and protect the module. However, due to the application of ultrasonic energy in the wirebonding process, all components (e.g. cells, busbar etc.) must be clamped sufficiently to ensure no harmonic resonance can occur to compromise bond integrity.
Finite element analysis reveals where best to attach wire bonded connections to battery cells
While there are many ‘laser weld versus wirebonding’ debates taking place in the industry, both techniques have their respective places; and industry bodies, equipment OEMs and distributors alike are working closely to help battery pack manufacturers identify, develop and optimise the best processes for their applications.
For instance, where wirebonding is concerned, wedge bonder OEM Kulicke & Soffa (K&S) offers ultrasonic modelling (see figure 2) to identify the exact bonding responses expected from the unsupported end cap, eliminating the need for customers to invest in complex tooling and fixtures.
Industrialisation
The challenges associated with battery module welding (including wirebonding) were explored in a presentation delivered by the UK Battery Industrialisation Centre (UKBIC) at the Battery Test Expo at Silverstone, United Kingdom, in May 2022. Based in Coventry, UKBIC is a 20,000m2 manufacturing research facility. It is used for the trialling of new materials, cell formats, module and pack structures, and manufacturing processes.
UKBIC’s presentation highlighted that, to achieve high consistency and yield in welding battery modules, cells and modules must be designed to be more work-friendly for the welding process (irrespective of which type) in terms of their geometries and choice of materials.
Also highlighted were challenges associated with investment costs, access to equipment, knowledge and skills. Regarding these, OEMs and the supply chain are stepping up to the plate. As mentioned, K&S offers ultrasonic modelling to help customers develop their processes. Also, Inseto (which is supporting UKBIC through partnership with the supply of a K&S Asterion EV hybrid wedge bonder) has invested in its own wedge bonder for use in a process development laboratory. Both the investment and provision of a laboratory for customers to use are not standard practice for a distributor. Nor is bonder use training. Something else Inseto offers.
Several battery pack manufacturers have already used the laboratory to develop and optimise their processes, establishing the ideal mix of wire gauge and bonder control parameters. They have also been exploring how to handle surface contamination, as the cleanliness and plating quality of commercial cylindrical cells often used for battery packs can be problematic, irrespective of weld method. As the battery cells are live during assembly (typically 30% charged), the cleaning process must not short the cells or impact the internal battery chemistry.
Similarly, UKBIC is focused on support – and more than just technical. For instance, the centre aims to help manufacturers rise to other challenges, such as helping manufacturers create safety policies – as many policy makers may not be fully aware of the safety regulations associated with battery manufacturing. Procedures will need to be in place for transportation, storage and handling.
In summary, these are exciting times for the e-mobility sector and whilst there are many challenges ahead it’s encouraging to see how industry bodies, OEMs and distributors are rising to these.
Andover, United Kingdom – Inseto, a leading technical distributor of equipment and materials, has supplied and installed a Kulicke & Soffa (K&S) Asterion EV hybrid wedge bonder at the UK Battery Industrialisation Centre (UKBIC).
The UK Battery Industrialisation Centre (UKBIC) offers and develops wirebonding using a K&S Asterion EV wedge bonder, supplied and installed by Inseto.
The Asterion EV gives the national battery manufacturing development facility, UKBIC, the ability to provide a wider range of welding technologies to its customers. The new bonder – specially created to support battery module manufacturing – complements the facility’s existing laser welding capability, meaning the facility can now offer different welding technologies dependent on the application.
Andrew Britton, UKBIC’s Business Development Manager, said: “We’re delighted to be collaborating with Inseto on the installation of this new bonder at UKBIC, meaning that we can offer more welding choice to our customers. The bonder also features a non-destructive inline pull test capability to check weld quality. Also, with wirebonding, cells can be reworked and recycled more easily at end of life.”
Matt Brown, Managing Director of Inseto, added: “We’re delighted to be collaborating with UKBIC so that they can offer wirebonding as a means of interconnecting the many cells in a battery pack. Laser welding and ultrasonic wirebonding processes both have roles to play in battery pack manufacturing, but it’s the latter’s ability to place suitably sized wires that can act as individual fuses for each and every cell that’s got people interested. Also, there’s no need to pre-form complex busbars, which is the case for laser welding.”
The K&S Asterion EV, one of the most advanced bonders in the battery sector, is ultrasonic and uses ambient temperature ‘friction welding.’ It can place and bond aluminium wire in the 100 to 600µm diameter range and copper wire in the 100 to 500µm diameter range.
MAIN ENDS
About UKBIC
The £130 million UK Battery Industrialisation Centre (UKBIC) battery manufacturing development centre was opened by the Prime Minister in July 2021. The unique national facility provides the missing link between battery technology, which has proved promising at laboratory or prototype scale, and successful mass production. Based in Coventry, UKBIC welcomes manufacturers, entrepreneurs, researchers and educators, and can be accessed by any organisation with existing or new battery technology – if that technology brings green jobs and prosperity to the UK.
In addition to funding from the Faraday Battery Challenge through UK Research and Innovation, UKBIC is part-funded through the West Midlands Combined Authority. The facility was delivered through a consortium of Coventry City Council, Coventry and Warwickshire Local Enterprise Partnership and WMG, at the University of Warwick, following a competition in 2018 led by the Advanced Propulsion Centre with support from Innovate UK.
POWERING UP – Boundaries are being pushed in all engineering sectors, but few more so than with regards to power electronics. Matt Brown, Managing Director of Inseto and Andy Longford, Technical Consultant at PandA Europe explain.
For every problem solved in the power semiconductor industry, it seems that at least one other is introduced. Where performance is concerned, the goals are typically to increase power density and to operate at higher switching frequencies. These goals have been met by moving from silicon (Si) to wide bandgap materials, and silicon carbide (SiC) has become extremely popular.
To date, only relatively simple structures such as diodes and mosfets have been fabricated in SiC and are commercially available. These tend to be in standard packaging and that is what limits their operating temperature range. However, many applications are calling for more advanced structures and/or the ability to cope with higher temperatures.
In theory, any semiconductor device structure that can be fabricated in Si can, with suitably adjusted manufacturing processes, be made in SiC and a far higher (5x to 10x) power density should be achievable. Tests are necessary to confirm that theory can be put into practice. The University of Warwick, for example, has a dedicated Power Electronics Applications and Technology in Energy Research (PEATER) group for research into SiC power electronics and the development of fabrication processes for bipolar SiC power semiconductor devices. Expertise covers fundamental materials research, device simulation and optimisation, fabrication, characterisation, packaging and reliability testing.
Semiconductor Device, image courtesy of University of Warwick
It is worth noting that thermal cycling is an important factor. For example, AEC-Q100, the qualification test for packaged integrated circuits for automotive applications, details four ambient operating temperature ranges. The widest range is -40oC to 150oC. But SiC-based devices will, subject to packaging, be able to operate at far higher temperatures. It is not hard to imagine a SiC die at the heart of an automotive component in a vehicle working in a cold environment cycling from sub-zero temperatures to more than 300oC several times a day.
As part of its work with the EPSRC (Engineering and Physical Sciences Research Council) Centre for Power Electronics, the university has already produced SiC insulated-gate bipolar transistors (IGBTs) with breakdown voltages of 10kV, noting that silicon IGBTs are typically rated up to about 5kV. In addition, full gate control has demonstrated the viability of the technology for applications including high voltage DC transmission.
Testing and characterisation are performed using a SemiProbe PS4L probe system and other equipment in the university’s dedicated wide bandgap semiconductor characterisation facility. Thanks to a degree of automation, the tests are highly repeatable and are producing a wealth of data. This means that for any given manufacturing process and desired device performance goals it is possible to determine the yield from a SiC wafer.
Make the connection
To benefit from a SiC semiconductor die’s ability to operate at a high temperature – noting that SiC power semiconductors can operate with junction temperatures in excess of 500oC – it is best to attach it to a substrate, such as copper, which will provide good heat dissipation. However, solder cannot be used because it has a melting point lower than the kind of temperatures the SiC die might reach. For example, the popular lead-free SAC alloy (96.5% Sn, 3% Ag and 0.5% Cu) melts at about 230oC. Also, at only a few tens of Watts per meter-Kelvin (W/(m.K)), solder is not that good a thermal conductor.
An alternative to soldering is sintering, a manufacturing process that is a combination of heat and pressure. Silver is currently the most popular sintering material. Its melting point is about 960oC and the thermal conductivity of a sinter paste is very good (between 130 and 250W/(m.K)).
Silicon Carbide Die attached using Silver Sintering
The sintering process is as follows. A sinter paste comprising monometallic particles, of less than 1μm in size, and a resin are printed (in a similar way to solder paste) onto a substrate in patterns corresponding to the shapes and locations of the SiC dies. The substrate is heated to evaporate the resin and the dies are placed. Pressure, which can be up to a few tens of MPa, is applied as a downwards force. It serves two purposes. First, a lower temperature can be used to bond the materials, yet the bonds achieved will be able to operate at a higher temperature in the field. Second, it reduces the risk of voids, which can lead to delaminating and cracking over time. Note: checking for the presence of voids requires the use of a scanning acoustic microscope as x-ray equipment does not work; the power required to penetrate the metallic substrate is so high it makes the die virtually invisible.
Sintering for die attach is a relatively new technology. For example, the North East’s Driving the Electric Revolution Industrialisation Centre (DER-IC) – funded by UK Research and Innovation and set up in 2020 – has just taken delivery of an AMX P100 sinter press. It employs a Micro-Punch system that allows equal force to be applied to multiple dies of varying thickness. Compared to using a single (flat) punch, this mitigates against the risk of voids forming beneath thinner dies.
DER-IC will provide open access facilities and aims to bring together the UK’s technology and manufacturing expertise in electrification research and development. It is believed DER-IC is the first facility in the UK to take delivery of a sinter press as advanced as the AMX P100.
Supply chain
In many cases, those developing next generation SiC-based power devices are in unknown territory. They have volume (space) and ideal performance characteristics in mind, but can such a device be made?
Few organisations will want to invest in cap-ex equipment, which is why the services offered by the North East’s DER-IC have such an important role to play. The Compound Semiconductor Applications Catapult, a DER-IC for the Southwest and Wales, offers services too. Its Power Electronics Laboratory is heralded as one of the country’s most advanced and comprehensive modelling, characterisation, integration and validation facilities for power electronics innovation. It also provides advanced packaging capabilities for power electronics.
Distributors are doing their part too. For instance, Inseto has a Process Development Laboratory at its headquarters in Andover. In 2021, the company invested in a Kulicke & Soffa Asterion wedge bonder to join its materials test and plasma cleaning equipment. A major OEM of power ICs is, at the time of writing, using the facility. Also, several companies have been using the facility recently to develop battery cell wirebonding processes. These companies range from existing battery pack manufacturers exploring new processes to start-ups doing prototype runs. In all cases they have access to equipment they cannot yet afford to invest in.
That, plus access to expertise.
Empowered
In summary, the production of next generation power semiconductor devices requires new types of equipment and processes. But that is to be expected considering the use of materials such as SiC. What is interesting is the amazing level of collaboration between industry and academia, and roles being played by OEMs and distributors to de-risk programmes. Engineers have access to expertise that is confirming if custom (and potentially high volume) power devices can be fabricated. And if so, how.
Inseto is proud to reproduce the above article first published in Electronics Weekly on 27th April 2022, with the kind permission of the editor.
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