[TriEmbed] reflow hazard: parts "float"

[Pete Soper]

I thought it might be of interest to see the photo below of some brass 
sheets "walking" away from their original positions when reflow 
soldered. My instinct was to tape the sheets down with kapton, but I 
decided to take a chance. Notice how the underlying pads are exposed in 
three cases and the middle sheet rotated clockwise. The hot flux in the 
solder paste tends to lift the sheets and then as the volatiles boil 
away there's mechanical force (or maybe thrust?). On the other hand, 
there isn't the pattern of stains that I got with a test run that did 
involve tape. So with the sheets held down by tape the volatiles come 
out low and some of the flux material accumulates on the board.

I'll reposition the worst cases with a hot air tool, but this PCB will 
be at the bottom of the "spares pile" and I'm getting out the kapton 
tape now. This is one of the boards for Rod Radford's robot contest 
arenas for IEEE Southeastcon. After imagining over-assertive robots 
hammering the PCB pads with sharp points as part of probing them to 
discover what components are connected the brass "armor" become part of 
the spec.

http://triembed.org/blog/wp-content/uploads/2017/01/brass-grows-legs.jpg

-Pete