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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