We often get questions about curing in vacuum. Sure, it sounds like a great idea on paper but is it really worth it? To make a long story short, No. If you are interested in the long story, keep reading.
Curing in vacuum is a rather unusual process, and something that we do not normally recommend.
Three main reasons to avoid vacuum curing:
- There is a risk that some of the more volatile parts of the formulation might evaporate before they react & crosslink. As a result, the formulation of the adhesive may change in unexpected & unpredictable ways, and will affect the final properties.
- Transferring heat in a vacuum is more difficult than in air (or nitrogen). This poses a risk that the parts will not heat at a uniform rate, or achieve a uniform temperature. These variations will affect the final properties of the adhesive.
- From a practical point of view, the inside surfaces of the oven (and vacuum system) would soon become contaminated with outgassed products from the adhesives (mainly resins) and would need frequent cleaning.
Please bear in mind that adhesives can be fully cured in air (or nitrogen) and then exposed to high levels of vacuum with no negative effects.
As for current experiences, we are not aware of any customer that uses vacuum curing for die attach and similar processes. It has been discussed in the past, but no major customers have implemented it yet and probably for good reason.
Reasons to cure in a Vacuum
What is the problem you are trying to solve and what would lead you to cure in a vacuum anyway? The most common response would be Voids. If you are trying to solve your voiding issues there are better and easier steps that you can take to go about it.
As a starting step, we recommend to investigate the root cause. Often optimisation of dispense patterns or optimisation of curing profiles can help.
In other situations, such as capillary underfills or die attach films, curing under high pressure can help to collapse voids prior to cure. Pressure ovens such as the APT ones that we use in our labs can work great for such operations.
Choosing the correct die attach or underfill material
Another very important step is to choose the correct die attach or underfill for your package. There are epoxy products such as the multipurpose LOCTITE ABLESTIK 8361J, the low stress LOCTITE ABLESTIK 8900NCM, the hybrid LOCTITE ABLESTIK 2000 and the high thermal, Grade 0 capable LOCTITE ABLESTIK QMI529HT-LV that are famous for their void free properties. And we haven’t even touched other chemistries such as cyanate esters like LOCTITE ABLESTIK JM7000 made for hermetic packages.
Are you trying to pick the optimal material for your void free bondline and you don’t know where to start? Our product categories of Die attach pastes, Die attach Films and Underfills should be your starting point. Or you can skip all that and Contact us to get a personalised suggestion for your application needs.
Voiding is a blog post of its own. One can write papers upon papers on how you need to store, handle, dispense and cure the material to solve this issue so we’ll dedicate one of our upcoming blogs on Voiding and how to eliminate it.
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