The one property everyone expects from an adhesive is that they stick things together. It is true when you buy some sort of glue for your house and it is true when you are using an adhesive on your PCB.
Often we are asking the adhesives on the PCB to carry out some additional functions: conductivity (thermal or electrical), reliability (thermal cycling or drop/shock), environmental protection, etc. First and foremost, these materials need to adhere … sometimes they don’t.
There are a variety of reasons why this occurs. The most common that we see (especially when dealing with SMAs and thermal adhesives) is contamination on components. Components are typically molded – sometimes there is mold release left behind on the components. Mold release is designed to not allow adhesives to stick – therefore the adhesive can fail. This is often observed when you have components from one reel falling off in a wave soldering process, but none of the other components. The adhesive isn’t smart enough to know when or where to fail. These contaminates can be confirmed by examining the components for these compounds.
The most common method is to use Fourier-Transform Infrared (FT-IR) spectroscopy. This technique can identify chemical compounds on the surface of the components and can help to determine why an adhesive is not sticking to a surface.
So next time you have an adhesion problem, look to the surfaces as well as adhesives.