[TriEmbed] reflow hazard: parts "float"

[Brian]

Pete,

It looks like you have roughly-square brass contacts placed on round 
pads (judging by the visible edge of the pads).

Make your brass contacts the same shape as your pads, or vice-versa. 
Make the pads slightly larger than the contacts. Then surface tension 
will be on your side and you shouldn't see any migration during reflow.

-B

On 01/10/2017 01:33 PM, Pete Soper via TriEmbed wrote:
> 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
>
>
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