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In readiness for the EU’s likely ban of chip encapsulation adhesives containing substances of very high concern, DELO’s chemists have produced alternatives that are not only SVHC-free but are also better suited to assuring high reliability. Eamonn Redmond outlines the required properties of chip encapsulation adhesives for harsh environment applications and discusses a just-launched SVHC-free product.

Encapsulation is preformed to protect bare semiconductor dies that have been attached to a substrate. It also protects any wire bonds, made to establish electrical connections between the top surface of each die and the substrate.

There are two methods for encapsulation, ‘GlobTop’ and ‘Dam & Fill’ – See figure 1.

For high reliability applications, heat-cured adhesives are used as opposed to light-cured. This is because the chemistry of a heat-cured encapsulant tends to have a low coefficient of thermal expansion (CTE) as well as a high glass transition temperature (Tg) – both of which assure a long operating life in the field.

By far the biggest advantage of heat-cured is a wide operating temperature range. Military applications typically require an operating temperature range of -55 to 150°C. Many other harsh environment applications, such as medical and industrial, have adopted this range too.

In addition to having to cope with the temperature extremes of the environment, there is also the issue of generated heat. For example, all heavy-duty solid-state power switching modules used in aerospace, automotive and power grid applications contain silicon carbide (SiC) or gallium nitride (GaN) die. Even with efficient thermal management in place, die temperatures can easily exceed those expected in military applications.

Most high reliability applications also call for the highest level of moisture resistance (JEDEC’s MESL 1). This is because if moisture gets into the encapsulant, its expansion (‘popcorning’) during high temperature cycling can damage the chip. Damage can also be caused, over time, by outgassing and many applications demand that chip encapsulants have measured ionic impurity levels of less than 10ppm.

It is also worth noting that adhesives used for high reliability chip encapsulation are also used for protecting sensitive sensors in harsh environment / industrial applications because, in addition to the above benefits, they are also resistant to oils and most chemicals.

SVHCs

Much is required of a chip or sensor encapsulation adhesive to ensure high reliability, and the properties that make it fit for purpose are down to its chemistry and, specifically, the inclusion of compounds now formally classified as ‘substances of very concern’, or SVHCs, because they are hazardous to health and/or the environment.

The European Chemical Association (ECHA) is constantly reviewing and working towards banning the use of SVHCs in not just adhesives but also paints, detergents, cleaning products etc.

For chip encapsulation adhesives, the main compound in question is an anhydride epoxy called hexahydromethylphthalic anhydride.

All SVHCs are regulated under the Registration, Evaluation, Authorisation and Restriction of CHemicals (REACH), an EU regulation that came into force on 1st June 2007 and replaced multiple European directives and regulations with a single, unified system. One of REACH’s goals is to make “…the people who place chemicals on the market (manufacturers and importers) responsible for understanding and managing the risks associated with their use.”

If, or more likely when, the ban goes ahead, it will force manufacturers based in Europe to cease making products containing even small traces of SVHCs. Or, to put it another way, this is not like the lead-free solder directive introduced a few decades ago. That ban was placed on the users, not the manufacturers.

Aware of the likely ban of SVHCs, some manufacturers have taken the opportunity to develop products that will not only be fully compliant, but which also have properties that make them even more suitable for harsh environment applications. And the results so far are very impressive.

For example, DELO MONOPOX GE765 is a popular GlobTop encapsulant. It contains hexahydromethylphthalic anhydride which, as mentioned above, is an SVHC. The SVHC-free alternative for this encapsulant is the recently announced DELO DUALBOND GE7065. Table 1 compares the cured properties of the two adhesives.

Of those properties, the lower CTE and higher Tg and operating temperature make the SVHC-free one well suited to applications where heat resistance is a must.

Benefits are also gained during the encapsulation process. See table 2. The greatly reduced particle size of GE7065 means it flows well between fine-pitch bond wires. Also, while the cure temperature is 10°C higher this is offset by the reduced cure time. Moreover, GE7065 is ‘light fixable’ and exposure to UV light will freeze the adhesive in place, allowing the board/substrate to be moved without risk of the GlobTop dome slumping.

Be Prepared

The ban is almost certainly coming, and a decision was meant to have been made by ECHA in June 2020 but was delayed by COVID-19.

That said, there is no need to panic because ECHA will provide notice. Also, as demonstrated above, SVHC-free products are available now; the result of manufacturers and distributors working hard to ensure users requiring chip encapsulation adhesives for use in high-reliability electronics are supported ahead of the ban.

Lastly, while we only compared an SVHC-containing and an SVHC-free GlobTop adhesive above, equally impressive improvements have been made on the Dam & Fill front too. Again, greater in-field and manufacturing benefits have accompanied the change to SVHC-free encapsulants. For further information, we invite readers to check out our ‘Likely Changes to Chip Encapsulation Adhesives’ Knowledge Base guide (online at www.inseto.co.uk). It contains useful links, to ECHA and REACH for example, and we will be maintaining/updating the guide as more becomes known about the ban.

 

This article was written in close collaboration with Inseto (UK) Limited and appeared in the summer issue Fastening & Assembly Solutions and Technology (FAST) magazine.

It appeared in print, is online here and is reproduced on our site with the editor’s kind permission.