I'm trying to bond semiconductor samples (Si and Ge) area ~1-2cm^2 to fiberglass printed circuit board (PCB) We use West system marine epoxy for other things. 105 resin 209 hardener (long cure time.) So that's what I used. (mixed at standard ratio)
I wanted a controlled thickness for electrical isolation. So I also tried adding some fillers to the epoxy. Glass beads (9.8 mil .. kinda thick IMO, sprinkled on surface.) Alumina oxide, 240 grit. (~1 part Al2O3 to 2 parts epoxy, by weight.) All samples (but one Ge) were from an old piece of Si wafer. Samples and pcb were cleaned in acetone, and scrubbed with cotton tipped applicator. Epoxy mixed. applied and samples pushed into place.
And allowed to cure for 24 hours.
They where then dunked into liquid nitrogen (LN2). After a few dunks, warming in room air, the samples glued with Al2O3 filler had fallen off.
After more dunking the samples held with raw epoxy and beads fell off. A few more tortures that also included warming more rapidly with a heat gun. And I'd lost everything but the Ge sample.
As a final abuse the Ge sample was taken from LN2 and placed into a cup of warm water, several times. It remained attached.
All the bonds failed at the Si interface, and the epoxy remained attached to the PCB (except for the glass beads, that failed everywhere.)
So what's wrong?
I then thought about cleaning. The old Si samples may have all sorts of hand grease.
The final difference is that the Si samples are polished on both sides, while the Ge was only onthe top... the bottom was rough.
wow, that was a long question, (sorry)
I made up a new batch of samples today to try and answer those questions. They will cure over the weekend.
I also wonder if I need a different epoxy? The West 105 stays somewhat pliable.
I don't know if that's a good or bad thing.