Most people have now heard of a previously little-known polymer called Bisphenol-A, often referred to by its acronym, BPA.
BPA has been widely used for years to soften plastics, including those that line canned foods, despite being a known endocrine disruptor and potential carcinogen. About three years ago, new studies came out showing that levels previously considered safe actually were not. I won’t go into the whole story, but if you’d like to know the gory details, the Wikipedia article has a fairly detailed overview of the health issues surrounding Bisphenol-A.
So why bring this up? It turns out that BPA was involved in some drama in electronics.
As most of you know, integrated circuits have to be encapsulated in some sort of material. This material protects the IC from the elements as well as providing a means to attach the IC to other electronics. There are several types of materials used in the encapsulation process (usually referred to as packaging), but the most frequently used are plastics. This is because they are easy to process and are relatively cheap. There are other ways to package ICs, but these other methods are not used as often because of expense and other reliability factors.
In the ’60s, a very common packaging material was created from BPA. It was referred to as BADGE, or bisphenol-A diglycidal ether.
It turns out that BADGE has a fairly low glass transition temperature. The glass transition temperature is where there is so much kinetic energy in the individual molecules that they start to wiggle around, independent of the other molecules they’re connected to. At this point, the coefficient of thermal expansion changes. This means that when the package is heated up, it will expand a lot more than when an similar temperature increase occurs below the glass transition temperature.
Because BADGE has a low glass transition temperature, it would expand and break the bondwires connecting the die to the associated pads. When the package was warm, it would expand and create open circuits so that the part wouldn’t work. When it was cool, the package would shrink and the bondwires would make contact with the pads, allowing it to function normally again.
Can you imagine what a pain troubleshooting that must’ve been?
BADGE is obviously not used much for molding compound anymore, having been replaced by epoxy novolac compounds.
So next time you’re worried about nasty chemicals in your soda bottle, you can be relieved that at least one of them also isn’t in your electronics.