optical reliability of film adhesives

Optical reliability of film adhesives

Adhe­sives in film for­mat are an increas­ing top­ic of inter­est. They have sim­i­lar for­mu­la­tions to tra­di­tion­al die attach pastes and assem­bly pastes but they come in sheets with cus­tom dimen­sions that can be applied direct­ly on the die or sub­strate. These flat and even con­duc­tive and non con­duc­tive die attach films ensure per­fect uni­for­mi­ty and a very con­trolled bond line thick­ness. Addi­tion­al­ly, there’s min­i­mal over­flow dur­ing assem­bly and cus­tom die-cut designs can also be produced.

Indus­try trends for elec­tron­ic devices can be sum­ma­rized as:

  • Minia­tur­iza­tion
  • High pow­er dissipation
  • High­er reliability
  • Thin­ner devices

All these trends have cer­tain draw­backs and chal­lenges for die attach and assem­bly pastes.

Minia­turi­aza­tion means that it is eas­i­er to con­t­a­m­i­nate the adja­cent wire­bond pads by flow and bleed from the adhesive

High­er reli­a­bil­i­ty faces dif­fi­cul­ties because of the dif­fi­cult set-up and process main­te­nance that can lead to incom­plete cov­er­age, uneven fil­let height, suck back, and oth­er issues. In addi­tion to that, the warpage of the die makes dies hard­er to stack, while con­t­a­m­i­na­tion of the die sur­face from the adhe­sive is a possibility.

Thin­ner devices are prone to incon­sis­tent fil­lets that can reduce reli­a­bil­i­ty. The posi­tion accu­ra­cy is very impor­tant to avoid tilt while the con­t­a­m­i­na­tion of the wire bond­ing pads by flow and bleed from the adhe­sive still lurks.

All of the above prob­lems can be resolved by die attach film adhe­sives due to their con­trolled flow.

To add to that, adhe­sives in film for­mat can also be Opti­cal­ly clear. Films such as ATB100U and ATB100US Series show high time zero trans­mis­sion. But what about reli­a­bil­i­ty? How do they fare against com­mon or accel­er­at­ed oper­at­ing con­di­tions? That’s what we tried to measure.

Light trans­mis­sion data will be pre­sent­ed, both from “time zero” and after reli­a­bil­i­ty test­ing. In par­tic­u­lar, the con­duct­ed tests on the cured coupons are:

  • 125°C 1000h
  • 85°C, 85% RH 1000h
  • MSL3: 30°C/60%/192hrs fol­lowed by 3 time reflow (260°C, N2condition)
  • Ther­mal Cycling ‑40°C –125°C, 1000times, 30min cycle

Sam­ples were lam­i­nat­ed on glass slides and mea­sured before and after reli­a­bil­i­ty using a bare glass slide as ref­er­ence. We will study the effect of dif­fer­ent reli­a­bil­i­ty tests on the trans­mis­sion of var­i­ous films

We see that the US series looks over­all bet­ter with much less yel­low­ing after Heat stor­age and High tem­per­a­ture humid­i­ty test­ing. So why would you ever choose the U? Well, reli­a­bil­i­ty is not the only thing that matters.

LOCTITE ABLESTIK ATB U films are cur­able epoxy based films with slight­ly infe­ri­or opti­cal prop­er­ties that are also based on a ther­moset chem­istry. This makes them soft­er and eas­i­er to attach void free.

The rea­son the ther­moset films turn yel­low with heat is that the cross-link­ing caused by the cur­ing agent can break down. This effect is still present with the “skip cure” films – but it is very very much reduced.

LOCTITE ABLESTIK ATB US films are skip cur­able films which even though are opti­cal­ly supe­ri­or have down­sides due to their ther­mo­plas­tic chem­istry. They make up for it due to their sta­bil­i­ty in harsh environments.

The pros and cons of each chem­istry and cur­ing process can be found in the fol­low­ing table.

Cure (eg. ATB-120U)• Good dic­ing
• Good gap-fill­ing
• No ther­mal-bud­get concern
• Long assem­bly process time
• Non-wet­ting on large die
• Suck-back & over-flow
• Rel­a­tive high warpage after cure
Skip cure (eg ATB 120US)• Rel­a­tive high UPH (as pre-bake & cure process can be skipped.)
• Broad D/A process win­dow
• Easy die stacking
• Wire-bond­ing prob­lem (e.g. NSOP, die-shift, etc)
• High risk of EMC pen­e­tra­tion
• Ther­mal-bud­get per­for­mance con­cern for mul­ti-die stacking

To sum­ma­rize, we have explained why die attach films such as LOCTITE ABLESTIK ATB 120U or LOCTITE ABLESTIK F125E can be a great replace­ment for tra­di­tion­al die attach pastes and assem­bly adhe­sives, espe­cial­ly when it comes to stacking.

These films come with great opti­cal prop­er­ties that remain con­stant after reli­a­bil­i­ty test­ing and can vary depend­ing on the chem­istry and cure type. Addi­tion­al­ly, they come in dif­fer­ent thick­ness­es start­ing from 10um and 20 um and lead­ing up to 25um or even 30um.

Do you have a die stack­ing or assem­bly appli­ca­tion and you are strug­gling to choose the best prod­uct? Please con­tact us and we’ll be hap­py to do an appli­ca­tion pro­file analy­sis with you and sug­gest the best pos­si­ble candidate.

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About George Kountardas

George is a Jack of all trades with an unappeasable inquiring mind. Obsessed with new products and technologies, he is always pushing forward for better, faster and more efficient applications. Always learning something new.

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