Inseto

Month: July 2019

Adhesives – All Costs Considered

10th July 2019

Adhesive Considerations: The Total Processing Costs: 10 Things to Factor in besides the actual adhesive costs (IKB-042).

When selecting an adhesive for use in a manufacturing scenario, i.e. on a production line, the starting point is to consider the properties of the materials you’re planning to bond, the desired bond strength and the end application’s environment, such as the presence of heat and/or chemicals. 

Having identified the ideal adhesive type, it is all too tempting to then start shopping around for the most attractive per-volume cost; or for your company’s purchasing department to set about the task. 

However, it is essential to consider the bigger picture. There may be costs associated with transportation, storage and curing of the adhesive. Also, as ‘time is money’ it too should be factored in. In short, the adhesive with the lowest cost per volume may not be the lowest cost to use. So, what should be considered when determining the total processing cost? 

Much is driven by how the adhesive cures, which will be in one of three ways:

  • With the application of high intensity light (either broad spectrum or UV);
  • With the application of heat; or
  • At room temperature (RT).

Regarding the last of these, and for the purpose of this article, RT-cured shall also be taken to mean ‘air-cured’ and ‘moisture-cured’. 

In addition, we do not explore dual-cure adhesives in any great depth, but the pros and cons can still be derived in what follows.

DELO Adhesives Product Range

#1 – Adhesive Cost

As a rule, light-cured adhesives tend to be quite expensive, due to the high cost of the photo-initiator that is responsible for speed of cure. Comparatively, heat-cured adhesives might be less or more expensive, depending on the chemistry, and RT-cured adhesives tend to be the lowest cost. Also, whatever the curing method, be mindful of minimum order quantities (MOQs). As a minimum, you should perform volume-for-volume comparisons and, for short shelf-life adhesives, expired stock should not be used and should be disposed of; which for certain adhesives can prove costly.

#2 – Transportation

Neither light- nor RT-cured adhesives should incur particularly expensive transportation costs, however different countries have varying regulations on what can and can’t be put through their postal services. Heat-cured adhesives quite often require refrigerated or frozen transportation.

#3 – Storage Temperatures

As per the transportation issue, heat-cured adhesives usually require cold or frozen storage; so there will be running costs associated with a refrigerator or freezer (see #10), plus the capital outlay if you don’t already have one. This can be particularly expensive if the adhesive needs to be stored at -40°C, in which case an industrial freezer is required. Light- and RT-cured adhesives normally require only standard cabinet storage.

#4 – Adhesive Preparation Time

RT-cured adhesives, such as silicones, have minimal preparation time, but for two-part systems, the component parts (i.e. the resin and the hardener) will need to be mixed. One-part epoxies are simply dispensed, but may need to be conditioned from frozen, whether from -18 or -40°C (the two usual storage temperatures). This could take several hours, and the thawing process cannot be expedited due to the possibility of moisture getting into the adhesive. Careful, well-timed planning is required to ensure that there is no disruption to the production process and that only the necessary volume (or slightly more to err on the side of caution) is defrosted. Light-cured adhesives normally require no preparation time.

Adhesive Preparation Time

#5 – Dispensing Equipment

This is normally dependent on the manufacturing requirements, including the level of control needed with regards to ambient temperature and desired drop bead size. Simple time-pressure systems are most often the easiest and simplest way to dispense an adhesive, especially one-part adhesives, but low viscosity adhesives are susceptible to small changes in ambient temperature, and a relatively expensive volumetric system may be required. Again, this is very application-specific. For higher volume users, two-part adhesives can require expensive dispensing equipment to ensure that the correct ratios of resin and hardener are dispensed.

Adhesive Dispensing Equipment

#6 – Curing Equipment

Understandably, RT-cured adhesives (which include anaerobic’s) require no specialist equipment. Heat-cured adhesives require ovens, with their associated running costs and capital outlay (if you don’t already have one). Where light-cured adhesives are concerned there are trade-offs to be made. For instance, halogen lamps emit light across the full optical spectrum and can be used for curing a wide variety of adhesives. The lamps tend not to cost too much but the bulbs (with their lifetime of about 1,000 hours) do. Conversely, LED lamps have a higher capital outlay but far lower running costs because the bulbs draw less power and have longer lives (>10,000 “on” hours). Be mindful of the fact that the LEDs emit light of a specific wavelength (typically 365, 400 or 460nm) so the bulbs may need to be matched to the adhesive being used.

Adhesive Curing Equipment

#7 – Curing Time

Adhesives cured by high intensity light will do so in seconds, and in a production scenario this easily offsets the higher cost of the adhesive. Heat-cured adhesives can require that the bonded parts spend up to two hours in the oven. NB: heat-cured adhesives will have a minimum cure temperature. It is advised not to set the oven to this temperature though. It is most likely that the oven won’t be of a uniform temperature throughout. Accordingly, tests should be performed to determine the optimal oven temperature that is above the minimum cure temperature of the adhesive. RT-cured adhesives require the longest curing time; from 24 to 72 hours.

#8 – Overall Process Cycle Time (includes dispensing time)

Whilst Tip #7 discussed the curing time (important in its own right), there are other factors to consider. In general, the higher the number of parts to be assembled, the shorter the cycle time needs to be. So, all things considered, light-cured adhesives will always provide the shortest cycle time, given their speed of cure. At the other end of the scale, RT-cured adhesives will always have the longest cycle time, simply because they take longer to cure.

Sometimes the cure time can be reduced by adding heat, and a general rule-of-thumb is that for every 10°C increase in curing temperature, the curing time halves. The converse is also true, so RT-cured adhesives can be even slower than normal in ambient cooler temperatures, e.g. in a non-temperature-controlled environment in winter. One-part adhesives fall somewhere in between these two extremes, depending on the cure temperature that is specified on the technical data sheet.

#9 – Work in Progress (WIP) Space and Jigs

Tips #7 and #8 discussed curing times and cycle times. These will have a direct bearing on not only the space required but, for some builds, you may not be able to commence a build stage if jigs are unavailable. Again, light-cured adhesives cure so quickly that work can be moved on at a pace and jigs are not tied up for too long. Projects for which heat-cured adhesives are used can take longer (an hour or two) and projects for which RT-cured adhesives are employed will typically require at least 24 hours. Note: dual-cure adhesives (specifically light and heat) are proving to be a popular means of freeing up jigs. The adhesive can be ‘flash cured’ (normally with UV light) so that the assembly can be handled and taken to the oven.

#10 – Utility Costs

Here, we’re mainly talking about your electricity bill. As RT-cured adhesives don’t require any special storage or curing equipment the electricity requirements are minimal. Light-cured adhesives incur a small to medium charge depending on whether LED or halogen bulbs are used. Heat-cured adhesives have the largest electricity overhead because of refrigerator/freezers for storage and ovens for curing.

Summary

As outlined above, there is a myriad of factors to be considered – collectively rather than in isolation – when choosing the best adhesive for an application. Following these guidelines will ensure that the most cost-effective solution is delivered for each and every application. Also, as per our opening gambit, the starting point must still be a consideration of the properties of the materials you’re planning to bond, the desired bond strength and the end application’s environment. These, combined with the above 10 tips, will ensure you end up with a fit-for-purpose adhesive (chemistry) plus a cost-effective curing method to meet your manufacturing and business goals.

DELO Adhesives Testing

Author

Date

Version

Author

Eamonn Redmond

Date

23 October 2018

Version

IKB042 Rev. 1

Download

Choosing an Adhesive – How to Choose the Right Adhesive

10th July 2019

Choosing an Adhesive: There are many factors that go into choosing the right adhesive for a particular application. This document offers guidelines on how to proceed with this sometimes tricky process (IKB-035).

Choosing an Adhesive – How to choose the right adhesive?

Quite often, the last thing to be considered in a new project is the adhesive. After all, how difficult can it be to glue two things together? If it was really that simple, this document would not exist, and unemployment figures would increase (but probably not dramatically – there aren’t too many people who make a living from selling “glue” (legally, that is)).

The factors that influence the choice of an adhesive can be broken up into three categories:

Pre-Cure

Cure

Post-Cure

Some factors could fit into more than one category, equally there will be some debate as to which category a property belongs!

It is usually a matter of eliminating possibilities to see what is left – for example, to bond two very large items, a heat-cured adhesive will not usually be chosen. Think of the oven size required to bond the various components that make up the wing of an airplane.

Selection of DELO Adhesive Containers
Choosing an adhesive: Selection of DELO Adhesive Containers

Pre-Cure

In no particular order, here are some factors that should be considered at the pre-cure stage when choosing an adhesive:

Surfaces are bonded, not materials – an adhesive bonds to the surface! So is the surface plated? Does it oxidise easily, like copper?

Some materials should be avoided at all costs, e.g. PTFE (aka Teflon), most Polyethylene’s (PE) – there is a reason adhesive bottles are made from PE!

If it is not possible to choose a material that is easy to bond to, then consideration must be given to pre-treatment. This can range from simple batch plasma cleaning to expensive in-line plasma ovens. Other possibilities include flaming, corona treatment, and sand-blasting.

Cleanliness – do the surfaces need to be cleaned? Always a vital consideration, and highly recommended by Inseto.

Storage of the adhesive a) at the customer’s facility, and b) in a cleanroom (not always needed) – recommended adhesive storage temperatures range from RT (∼23°) all the way down to -40°C. At the lower extreme, an industrial freezer is required, so lots of £’s need to be spent. Adhesives that need -18ºC storage can be stored in commercially available freezers, so all Purchasing Managers will be happier with this choice!

Adhesives that need to be stored below 0ºC require specialised transport, packed in large Styrofoam boxes containing ∼10Kgs of dry ice. There are only two courier companies willing to transport these, so transport costs can be very high.

Conditioning time – any adhesive stored in a fridge or freezer will need to be thawed to room temperature before it can be used. The technical data sheet will advise this conditioning time, but do not, under any circumstances, force the thawing, for example, by placing the adhesive container on a radiator! This will induce moisture into the adhesive, and cause a lot of problems both during and after cure.

Processing time, or pot life – this can vary widely from adhesive to adhesive. Generally Inseto offers adhesives with working a life of 48 hours to 12 months!

Mixing – if a two-part adhesive is chosen, the resin will need to be mixed homogeneously with the hardener. For small to medium volume applications, this can be achieved by purchasing the adhesive in a “twin-pack” – it looks a little bit like the barrels of a shotgun! For high volume applications, the adhesive will be supplied in large (10L / 20L) containers, and expensive, specialised mixing equipment is required to dispense from these.

Viscosity – too low, and the adhesive can’t be controlled; too high, and it won’t move after leaving the container. Horses for courses. Also, care needs to be taken when choosing an adhesive based solely on its viscosity. Look at the small print on the data sheet to see how the viscosity has been measured. Unless the test method is identical to another adhesive that might be under consideration, any comparison is invalid.

Flowing behaviour – this is not defined in any data sheet! It’s very difficult to quantity, but it can have a big effect on processing. Basically, what happens to the applied adhesive as the ambient temperature increases?

Application of the adhesive – manual or automatic? This is usually determined by the quantity to be assembled. For small volumes, a manual process is usually sufficient. For larger volumes, repeatability is key, so an automatic process is usually required.

The next factor is the method of application – dispense, jet, stamp, screen / stencil print, spray, brush? Each has its own advantages and disadvantages, and sometimes the nature of the adhesive will rule some in / out. For example, moisture sensitive adhesives should only be dispensed.

Finally in this section, it is important to consider how the waste, or unused adhesive, can be disposed. The easiest way is to somehow cure what is left in the container and then dispose of it in normal household waste – a cured adhesive is a plastic! But there will always be some unused adhesive that must be disposed, and engaging the services of a specialist disposal company can be expensive.

Cure

An adhesive is cured by one or more of the following methods: heat, light, moisture (sometimes mistakenly called air curing – it’s actually the moisture in the air that causes the adhesive to cure), mixing a resin and hardener, or, in the case of anaerobic adhesives, the absence of oxygen and the presence of metal.

Sometimes the cycle time dictates which adhesive is chosen. For example, the automotive industry and the mobile phone industry both require a large number of parts to be made in the shortest possible timeframe. So if one part needs to be assembled every 5 seconds, an adhesive that needs 30 minutes to cure will not be used – think of the size of the oven that would require!

Sometimes the materials being bonded dictate the adhesive. It is not possible (under normal circumstances) to bond two opaque parts together using a light-cured adhesive – how does the light get to the adhesive? When bonding to Polycarbonate, it is usually not possible to use a UV adhesive, as most grades of Polycarbonate block UV light (the trick in this instance is to use an adhesive that also contains a VISIBLE light photoinitiator). In addition, some plastics can’t withstand a high level of heat, so the adhesive needs to be cured at 60°C – 80°C maximum.

High intensity light is generally the fastest way to cure an adhesive. Cure times range from 1 second to 60 seconds, depending on the adhesive. The downside is material cost – due to the high cost of photoinitiators, this type of adhesive tends to be expensive. Having said that, the process cost, which is considerably more important than material cost as it is a true reflection of the cost of manufacture, is ALWAYS lower for light-cured adhesives, if the volumes are high enough.

The slowest curing mechanism is moisture. The cure rate for these adhesives is 2mm every 24 hours, so, depending on the geometry of the parts being bonded, the actual cure time can be days! Some, but not all, moisture cured adhesive can have their cure rate accelerated by adding heat, or increasing the moisture content in the ambient atmosphere

Between these two extremes lie heat-cured, cold-cured, and anaerobic-cured adhesives. Usually, anaerobic curing is the fastest of these, anywhere between 15 seconds and 5 minutes. Heat-cured adhesives will usually cure anywhere between 60°C and 180°C, while cold-cured adhesives cure at room temperature after mixing. The cure schedule for most cold-cured adhesives can be accelerated by using heat – as a general rule-of-thumb for epoxies, every 10°C rise in temperature reduces the cure time by half. The reverse is also true, so be careful when curing cold-cured adhesives in the wintertime when some ambient temperatures can drop below 20°C.

When using a heat-cured adhesive, remember to include the heating-up time for the assembly. If a technical data sheet states that an adhesive cures at 120°C for 30 minutes, curing only starts when the adhesive reaches 120°C. This is especially critical when bonding metal, as it can act as a heat sink and remove heat from the adhesive.

One final point concerning the curing of heat-cured adhesives: Be very careful of curing at the bottom edge of the process window. For example, if a data sheet gives a minimum cure temperature of 80°C, it is usually for a good reason – below this, the adhesive just will not cure, no matter how long it remains in the oven. If the oven is set to 80°C, it is almost 100% guaranteed that not all areas within the oven are actually at this temperature. So if the parts to be bonded are placed in one of these areas, the adhesive will never cure. It is better to set the oven to cure at 82°C -84°C to ensure that the adhesive cures.

Post Cure

There is potentially a very long list of “things” that the cured adhesive must provide. It is worth noting that despite the technical values that appear on a data sheet, these usually apply only to the adhesive, and the adhesive will behave very differently when it is bonding two parts together. The complete assembly must be tested by the customer to ensure reliability. After all, one customer’s aluminium might be a very different (grade of) aluminium than another customer is using.

Here are some of these “things”, again in no particular order!

Bond strength – this is usually one of the first factors considered when choosing an adhesive! How strong must the join be? Most customers will reply “as strong as possible”. There are two elements to “bond strength”: cohesive bond strength, and adhesive bond strength.

Adhesive bond strength is defined as the bond strength of the adhesive to the materials being bonded. In every case, this has to be tested in the specific application under discussion. No two materials give the same bond strength, so an adhesive that bonds well to stainless steel might be terrible with aluminium. Even within a generic material such as Polycarbonate, care needs to be taken. There are in excess of 100 grades of Polycarbonate available, and each will probably bond differently! Generally, the technical data sheet will point in the right direction, so it’s a very good starting point, but only a starting point.

Cohesive bond strength is more straightforward, it is the inherent strength of the adhesive, and is independent of the materials that it is bonding. It will be specified on the data sheet, and is measured simply by curing a test coupon of the adhesive and then pulling it apart until it breaks! The force required to break it is the strength of the adhesive, called the tensile shear strength (sometime just the tensile strength).

Flexibility – there are different ways to indicate the flexibility of an adhesive: elongation, Young’s Modulus, sometimes shore hardness. Elongation is the easiest to both measure and visualise – it is calculated in the same test that determines the tensile strength of the adhesive. The percentage that the test coupon has strength when it breaks is the elongation, also specified in the data sheet – simple!

Temperature resistance – this will usually be specified on a data sheet, and depending on the application, will be more or less important to each customer. For those working in the military, aerospace and automotive fields, the temperature range of interest will be from -40°C or -55°C to +125ºC or +150ºC. For commercial applications, it will be much narrower, usually from -20°C to +80°C.

For tests such as temperature cycling and thermal shock, no data sheet will state “our adhesives survive” these tests. The temperature rating specified on the data sheet is a very good guideline (for example, an adhesive that is only rated to 80°C should not be considered for an application that must work up to 120°C.), but it not an absolute guarantee. Again, the complete assembly must be tested to ensure these requirements are met.

Chemical resistance – there are so many different chemicals that could possibly come in contact with the bonded assembly that it is absolutely impossible to test for all of them. Most application areas will have their own set of chemicals that the adhesive must survive. For example, in the automotive industry, this set usually consists of petrol, diesel, AdBlue, engine oil, brake fluid acetone, and maybe DI water. However, the mobile phone industry will have a completely different set of fluids, coffee, alcohol and water being the most frequent!

As a general guideline when choosing an adhesive for an application that needs a high chemical resistance, keep away from flexible adhesives. These tend to have a less rigid molecular chain once cured, so it is easier for chemicals to penetrate the adhesive and attack the bond. Hard, rigid adhesives generally offer the best chemical resistance.

Applications that will see changes in temperature over their lifetime need to consider how the adhesive expands and contracts with these temperature changes. Two very important characteristics of the adhesive need to be consider here – the glass transition temperature (Tg) of the adhesive, and the coefficient of thermal expansion (CTE) of the adhesive. These are directly related to each other.

The Tg is defined as the temperature at which the mechanical and physical properties of the adhesive start to change. Below the Tg, the adhesive network is in a solidified, relatively immobile condition. Above the Tg, the polymer chains of the adhesive may move slightly relative to each other, independent of the degree of cross-linking (i.e. curing). This may cause the adhesive to “soften” slightly above the Tg, but the degree of softening varies according to the adhesive.

In an ideal world, the chosen adhesive should have a Tg in excess of the operating temperature of the end application. As the Tg is exceeded, the adhesive softens, even if it is only very slightly. As the operating temperature comes back down below the Tg, the adhesive will harden slightly. Doing this thousands of times during the lifetime of the application will induce stress into the adhesive, causing the join to fail more quickly than it should.

CTE is defined as how the size of an object changes with changes in temperature, and is usually expressed in parts per million (ppm). Technical data sheets should contain two CTE values – ∝1 for the CTE below the Tg, and ∝2 for the CTE above the Tg. They will nearly always be significantly different values!

When bonding similar materials together, e.g. glass to glass, CTE is not an issue as both materials will expand and contract at the same rate. However, when bonding dissimilar materials such as glass and aluminium, the trick is to choose an adhesive with a CTE that is somewhere in between the CTE’s of the glass and the aluminium – around 4ppm for glass and 23ppm for aluminium. While it may not sound much, this can be a huge difference for large pieces.

Summary

As evidenced above, there are A LOT of things to take into account when choosing an adhesive! Like most things in life, there will be a compromise between some of these. It’s usually a good idea to create a list of “must have” properties, and “nice to have” properties – it can add clarity to the choosing process. Good luck!

For more detailed information on any of these topics, e-mail enquiries@inseto.co.uk

For further information on our full adhesives, please click HERE or to visit the DELO website, click HERE

Author

Date

Version

Author

Eamonn Redmond

Date

15 June 2017

Version

IKB035 Rev. 1

Download

Encapsulation Adhesives

10th July 2019

Knowing which adhesives to use to protect sensitive IC’s and wirebonds can be a little bit of a challenge! This article provides an overview of the options available to customers, and advises on how to cure the adhesives, what the properties of the cured adhesives are, and the advantages and disadvantages associated with the different options (IKB-013).

In the microelectronics industry, encapsulation adhesives are used solely to protect bare silicon and the associated wire bonds. There are usually two adhesive sets to choose from – heat-cured or light-cured! However, recent advances from Delo have combined the two curing mechanisms so that in certain instances, it is more beneficial to use these newer adhesives.

Dam & Fill, or Globtop!

The “globtop” process consists of dispensing an encapsulation adhesive onto the top surface of the silicon and allowing it to form into a dome that covers both the IC and the wirebonds. The globtop is then cured, whether by heat or by light.

However, as ICs get bigger, the area to be protected also increases, and then the dome becomes too high to control. In this case, a “Dam & Fill” process is then used: a high viscosity encapsulant adhesive is dispensed around the area to be protected, forming a wall or “Dam”. A low viscosity, chemically-compatible encapsulant adhesive is then dispensed into the central area within the Dam, until the whole volume within the Dam is covered – this is the “Fill” process.

As a general rule-of-thumb, Globtop is used for areas up to 2mm x 2mm, and then Dam & Fill takes over.

Globtop
Dam & Fill

Heat-Cured Encapsulation Adhesives

These are based on two diverse chemistries, epoxy and acrylate. 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.

Inseto’s heat-cured encapsulation adhesives are:

Globtop: DELO MONOPOX GE765

Dam: DELO MONOPOX GE785

Fill: DELO MONOPOX GE725 & GE727

The properties of these encapsulant adhesives are summarised here:

DELOMONOPOX

Note: GE730 is no longer available.

1. Properties of the uncured adhesives

These adhesives come in a range of viscosities depending on the use for which they are designed: low-medium viscosity for Globtop, low viscosity for Fill, and highly thixotropic materials for the Dam. They are normally supplied in 30ml syringes, and dispensed using simple time / pressure systems on X-Y tables – minimising capital expenditure. Because they need to be stored at -18°C (and therefore shipped in dry ice), 30ml syringes need to be conditioned to room temperature for up to 60 minutes. Never force the thawing process – this induces moisture into the syringe, potentially leading to voids when cured.

Some encapsulation adhesives need the substrate to be heated up to ∼80°C in order to improve the flowing behaviour of the adhesives. This is not necessary with any of the Inseto materials. In addition, some Dam materials have a relatively low minimum dispense height before they start to slump, especially at the start of the curing process when heat is first added. With the GE785 from Inseto, this is not an issue – tests in the lab have shown that this adhesive is stable up to a height of at least 7mm, and heights of 3mm are standard in production environments.

2. Curing method

These adhesives cure over a wide temperature range: 100°C to 180°C. Picking which temperature to use will be decided by the materials being protected, and the throughput / cycle time required. The minimum cure temperature is 100°C, but the adhesives then need 4.5 hours to cure (not including the time it takes for the adhesive to reach the required temperature). At the opposite end of the scale, they can be cured at 180°C for approximately 10 minutes (again, not including heating-up time). Most applications will fall somewhere in between the two – a typical cure schedule is 150°C for 20 minutes. Because the speed of cure can have an effect on the properties of the cured adhesive (too fast and stresses can be induced into the adhesive and materials being protected), for some very thin applications using 100°C can actually be very beneficial, despite the long cure time.

3. Properties of the cured adhesives

Heat-cured encapsulation adhesives offer the highest reliability of all materials. The properties of Inseto’s family of heat-cured anhydride epoxies for Globtop and Dam & Fill are summarised here:

  • Standard operating temperature range: -65°C to +180°C
  • Glass transition temperature (Tg): >170°C
  • Coefficient of Thermal Expansion (CTE): ≤25ppm
  • Decomposition temperature: >300°C
  • Ionically pure: <10ppm Na, Cl, K
  • Excellent chemical resistance
  • JEDEC MSL Level 1
  • RoHS / REACH Compliant

The Tg is very important for High Rel applications in the military and automotive markets, as cycling multiple times across the Tg during the operation of the end-product can have serious repercussions on the lifetime of the part. Every time the Tg is crossed the adhesives softens or hardens slightly, which imparts stress into the adhesive.

Equally important is the CTE, as there are usually multiple CTE’s in a typical assembly (FR4 or ceramic substrate, die attach adhesive, IC, wirebonds, and encapsulation adhesive). Therefore, having a CTE that is as close as possible to the CTE’s of the constituent parts also helps minimise stress, due to the fact that the finished device contracts and expands with (sometimes severe) changes in temperature. The graphic below shows the range of CTE’s possible in a typical application, and illustrates the issues of wire breakages:

Properties of the cured adhesives

Because the majority of these adhesives are used to protect bare semiconductors, it is important that any outgassing from the adhesive is kept to a bare minimum. All the encapsulation products in the Inseto range of materials meet this requirement, with measured ionic impurity levels of <10ppm.

As well as protecting the IC and wirebonds from physical damage, encapsulant adhesives also must be able to withstand a wide range of chemicals, including water (one of the most difficult substances to protect against!). Inseto’s encapsulants all meet the highest level of moisture resistance, JEDEC’s MESL 1 standard. A phenomenon known as popcorning can happen and cause catastrophic device failure. If moisture gets into the encapsulant, it can expand during hot temperature cycling and cause the adhesive to bubble. If this gets too extreme, the bubbles “pop”, just like popcorn!

Equally important is the adhesives resistance to a range of chemicals, e.g. those encountered in the automotive industry – oil, petrol, AdBlue, diesel, etc. The illustration below shows just how chemically resistant these adhesives are:

Properties of the cured adhesives

Light-Cured Encapsulation Adhesives

The overall purpose of light-cured encapsulants is the exact same as heat-cured encapsulants: to protect the IC and the wirebonds. The major difference is that the light-cured adhesives do not meet the highest reliability properties of the heat-cured adhesives, so they are used in less demanding applications such as Smart Card manufacture.

The overall purpose of light-cured encapsulants is the exact same as heat-cured encapsulants: to protect the IC and the wirebonds. The major difference is that the light-cured adhesives do not meet the highest reliability properties of the heat-cured adhesives, so they are used in less demanding applications such as Smart Card manufacture.

Inseto’s heat-cured encapsulation adhesives are:

Globtop: DELO KATIOBOND 4670

Dam: DELO KATIOBOND DF698

Fill: DELO KATIOBOND 4670 (4670 is suitable for both Dam & Fill and Globtop)

1. Properties of the uncured adhesives

The Dam is highly thixotropic and therefore must be dispensed from a large volume (800Gm) cartridge. Therefore a pressure tank plus a simple pinch valve must be used with this adhesive, using an X-Y table to manoeuvre the substrate. The Fill is available in 1,000ml bottles, so a pressure tank is required for this also. However, unlike the heat-cured encapsulants, the light-cured ones need only to be stored in cool conditions, at <10°C, so significant savings can be made on transport (dry ice) and storage containers.

2. Curing method

These adhesives are cured by UVA light between 320nm to 400nm. High intensity LED lamps must be used, as sunlight will only harden the adhesives and will not provide any bond strength. Cure times are <20 seconds at an intensity of 160mW/cm2. While there is extra investment required for the LED lamps (<£10K), the significant advantage of light-curing over heat-curing is process time. For medium to high volume applications, the added cost of dispensing equipment and LED lamps is soon recouped, and process time and cost is significantly cheaper.

3. Properties of the cured adhesives

While light-cured encapsulation adhesives do not offer the same “highest reliability” that is achieved with the heat-cured adhesives, they are suitable for all other applications.

Light-Cured Encapsulation Adhesives

Light-Fixed, Heat-Cured Encapsulation Adhesives

In keeping with other very recent developments in the field of dual-curing adhesives, a light-fixing component has been added to the heat-cured matrix of the High Rel encapsulants to enable fixing in place of the encapsulation adhesive, followed by a subsequent (mandatory) heat cure of the adhesive. This prevents any movement of the encapsulant, whether it is Globtop or Dam & Fill, between the initial dispensing process and the hardening of the adhesive.

The curing process is a combination of the two already outlined: UV flash on the surface for 5 seconds at 200mW/cm2, and then oven cure at 150°C for 30 minutes. Comparisons against the existing GE725 Fill show no degradation in chemical resistance:

DUALBOND / DELOMONOPOX

Conclusions

For optimum protection for bare semiconductors over extended periods of time in the most demanding applications, the best option is to use heat-cured encapsulation adhesives. They offer significant advantages over light-cured products, and are easier to process (if not as quick!). For less demanding applications, and where volumes are so high that very short cycle times are required, then light-curing the adhesives offers a significantly better choice.

New developments, both recent and in the near future, that combine the best of both worlds, should also be considered.

For further information on our full range of encapsulation, glob top adhesives, please click HERE

Author

Date

Version

Author

Eamonn Redmond

Date

30 May 2017

Version

IKB013 Rev. 2

Download

Dual Cured Adhesives

10th July 2019

Dual cured adhesives: As joining and sealing applications become more and more complex, the demand for adhesives with multiple curing mechanisms is significantly increasing. This article provides an overview of the different dual curing adhesives manufactured by Delo, their hybrid chemistries, curing methods and typical applications (IKB-012).

Delo has developed an extensive portfolio of dual cured adhesives based on dissimilar chemistries, which 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.

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. For light cured adhesives (Ultra Violet & Visible Light), achieving full cure can be a problem due to shadow areas that the light can’t reach. Heat-cured adhesives do not suffer from this problem, but some components can be sensitive to temperatures even as low as 90-100°C. Two-part cold-cured adhesives such as epoxies and polyurethanes can overcome this issue, but the user then must suffer from the long curing times that these adhesives require, thereby increasing cycle times and reducing throughput.

Dual Curing Adhesives

Diverse Curing Mechanisms – What’s Available?

Dual-cured adhesives overcome these issues in different ways, depending on the chemistry of the adhesive. Currently, hybrid light + heat cured adhesives appear to be the preferred solution, but other chemistries such as light + humidity and light + anaerobic adhesives are gaining in popularity.

Diverse Curing Mechanisms for dual curing adhesives.

Light + Heat Cured Adhesives

These are based on two diverse chemistries, epoxy and acrylate. 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 the user 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 for the user – this is especially useful for companies that have invested heavily in high-accuracy placement machines, only to see that investment wasted due to movement of the parts being bonded during the heat-cure stage.

This 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 (some are NASA-qualified) 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 these adhesives also exhibit excellent impact resistance and tension-equalising properties due to their flexible nature. These are ideally suitable for applications such as the assembly of rotary encoders, where optional fluorescing and colouring can be added to aid visual inspection.

Light + 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 either by light, or by heat, or by a combination of the two.

Dual curing adhesives using light and heat curing mechanisms.

Light + Humidity Cured Adhesives

Light + humidity cured adhesives also enable a fast fixation by light, but the secondary curing mechanism in this instance is humidity, so there is no requirement for additional curing equipment. The majority of the assembly should be capable of being cured by light, to achieve optimum bond strength, but for applications such as mobile phone displays, where only the black border prevents light getting to the adhesive, then humidity curing at a rate of 2mm / day is the ideal secondary curing mechanism, enabling immediate handling after the initial light-cure process.

These are also single-part adhesives, and are free of isocyanates (no Health & Safety issues) and silicones (no impediment to subsequent adhesive bonding), unlike some acrylates. They are highly flexible, optically clear adhesives offering excellent climatic resistance, whilst also providing excellent bond strength on surfaces such as glass, PMMA, metal pins and most plastics.

Adhesive light curing equipment

Anaerobics

These are one-part adhesives that afford a relatively high temperature resistance (up to about 200°C) and tensile strength of about 15MPa. They cure in the absence of oxygen and the presence of a metal (though not all metals) and are frequently used for magnet bonding and thread-locking. Curing is at room temperature and, thanks to recent developments within the adhesives industry, any exposed adhesive (i.e. in contact with air) can be cured using UV light. Storage is at room temperature.

Dual curing adhesives using light and anaerobic curing methods to cure.

Conclusions

Dual-cured adhesives provide increased flexibility in the manufacturing process, giving end users more freedom when designing the manufacturing flow. They eliminate the unwanted possibility of uncured adhesive in the end product, and offer maximum placement accuracy when building complex assemblies on expensive equipment.

For further information on our full range of dual cured adhesives, please click HERE

Author

Date

Version

Author

Eamonn Redmond

Date

11 November 2016

Version

IKB012 Rev. 1

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Basic Adhesive Types

9th July 2019

Basic Adhesive Types: A simple guide to the different adhesive types used in engineering and electronics today (IKB-041).

Basic adhesive types: One Part Epoxies

Depending on the exact chemistry, a one-part epoxy will require a typical curing temperature of between 60 and 150°C. It may also require light curing, which is normally best done with high intensity UV lamps. One-part epoxies afford excellent chemical and moisture resistance. Other benefits include high thermal resistance (more than 200°C) and a high tensile strength (of up to 57MPa), and the higher the cure temperature the stronger the bond formed. As for drawbacks, even with the application of heat, cure times can be relatively long. Light curing is much quicker. Also, once cured, the bond is rigid. Regarding storage, the latest one-part epoxies to join the market require a temperature of less than 10°C and some can be as low as -40°C.

Placement of the adhesive is typically by automated dispenser before the board goes into the pick-and-place machine. A consideration here though is the time between the two activities. If it is likely to be a long time, you may need to assess the epoxy’s sensitivity to moisture.

Regarding storage, the latest one-part epoxies to join the market require a temperature of less than 10°C and some can be as low as -40°C. Curing temperatures vary from 60 to 150°C and curing time will typically be less than an hour.

Basic Adhesive Types being dispensed

Two Part Epoxies

These offer bond strengths of up to about 28MPa, are relatively easy to process using a mixing system, have a shelf-life of up to about a year and can be stored at room temperature. In addition, once mixed, they cure at room temperature too. The application of heat can shorten the curing time and, as with one-part epoxies, the resultant bond is relatively inflexible.

Two Part Epoxies for rapid bonding in engineering applications.

Acrylates

Predominantly transparent, these are one-part adhesives and are cured using heat and/or light, have a shelf-life of between six months and a year and, once cured, exposed surfaces remain tacky. At about 30MPa, acrylates are more flexible than epoxies. Storage requirements will vary from 0°C to room temperature.

Two-part Polyurethanes

As with two-part epoxies, these are relatively easy to process and curing can be accelerated with the application of heat. Shelf-life is about six months and, once cured, the tensile strength can be up to 23MPa. Perhaps the biggest drawback to polyurethanes is their low UV resistance (susceptibility to sunlight) and relatively low heat resistance, which tends to be limited to 125°C. This limitation also restricts the max heat curing temperature, capping it at about 80°C.

Two-part Polyurethanes are tough with elastic properties and offer good strength under static and dynamic conditions.

Anaerobics

These are one-part adhesives that afford a relatively high temperature resistance (up to about 200°C) and tensile strength of about 15MPa. They cure in the absence of oxygen and the presence of a metal (though not all metals) and are frequently used for magnet bonding and thread-locking. Curing is at room temperature and, thanks to recent developments within the adhesives industry, any exposed adhesive (i.e. in contact with air) can be cured using UV light. Storage is at room temperature.

Silicones

Silicones are stored at room temperature, and cure at it too – though very slowly (typically at a rate of about 2mm per 24 hours). Once cured, they remain very flexible and can tolerate up to 600% elongation in some cases. They have a very high temperature resistance (up to about 300°C). On the downside, silicones outgas during curing and, post-cure, the resultant bond strength is not that great; at between 2 and 5MPa.

Silicones provide excellent vibration protection and resistance to high temperatures for electronic applications.

For further information on our full adhesives, please click HERE or to visit the DELO website, click HERE

Author

Date

Version

Author

Eamonn Redmond

Date

01 October 2018

Version

IKB041 Rev. 1

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Adhesives for PCB’s

7th July 2019

We take a look at some of the ways adhesives are used for PCB and electronic assembly applications (IKB-040).

PCB Adhesive: Component Fixing

For component fixing pre-reflow in PCB assembly, it is often necessary to tack small SMDs, such as resistors and capacitors into place, to prevent their movement during reflow, as even a minute drift away from the solder paste pads will compromise quality. The best type of adhesive to use here is a heat-cured, one-part epoxy. These offer high thermal resistance, as they need to withstand three reflow passes (the industry norm) at circa 220°C.

Placement of the adhesive is typically by automated dispenser before the board goes into the pick-and-place machine. A consideration here though is the time between the two activities. If it is likely to be a long time, you may need to assess the epoxy’s sensitivity to moisture.

Regarding storage, the latest one-part epoxies to join the market require a temperature of less than 10°C and some can be as low as -40°C. Curing temperatures vary from 60 to 150°C and curing time will typically be less than an hour.

PCB Adhesive: Socket Securing and Pin Coating/Sealing

This is done mainly to prevent moisture ingress and to reduce the risk of dendritic growth. There are two options available for securing/bonding pins/sockets. The first is to use a light-cured epoxy (see photo). These are popular as they cure very quickly in the presence of a high intensity light-source, making them ideal for high volume, rapid-turnaround production runs. The second option is to use a dual-curing (but again one-part) epoxy. High intensity light can be used for a rapid, initial cure, so that the board can be handled. This is then followed by heat curing – some of which can be time spent in the solder reflow oven – and will cure any epoxy ‘shadow zones’ (i.e. areas the light could not penetrate).

Adhesives for sealing and securing electronic components.

PCB Adhesive: Coil Fixing

The objective here is primarily to provide additional strength (see photo). Choke coils have a relatively higher mass than most electronic components, and bonding can protect against the effects of shock and vibration.

A two-part epoxy would be best to use, applied manually before soldering. There are no major curing issues, as the soldered joints will hold the coil in place while the adhesive cures (which is typically viscous and won’t run) at room temperature. Storage pre-use is typically at room temperature too.

Silicone encapsulation of PCB's. using high purity electronic grade silicone's.

PCB Adhesive: Ferrite Bonding

Here, the bond might be ferrite-to-PCB or ferrite-to-ferrite (e.g. the two halves of a transformer integral to the board). Because at least one surface is metal, the best adhesive to use would be a one-part anaerobic, a common use for which is thread-locking. Curing is at room temperature in the absence of oxygen, so any visible adhesive (i.e. at the edge of the bond) will remain viscous/liquid and should be removed. However, recent anaerobic adhesives to join the market can also be UV cured, meaning the excess adhesive can be treated.

PCB Adhesive: Casting / Potting

This is typically for circuitry intended for use in harsh environments; to protect the PCB from moisture and contamination (see photo). The adhesive-of-choice would be a two-part epoxy, as they are relatively low cost and a significant volume might be required for some applications. Pre-use storage and curing (which takes about 24 hours) is at room temperature.

Potting of PCB's using epoxies for harsh environments.

For further information on other example electronic applications for our adhesives, please click HERE.

Author

Date

Version

Author

Eamonn Redmond

Date

01 October 2018

Version

IKB040 Rev. 1

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