Re: gEDA-user: "soldermask via caps"

[Stefan Salewski <mail@xxxxxxxxxxxx>]

On Fri, 2010-05-21 at 22:09 +0200, Dietmar Schmunkamp wrote:

> 
> I tried this approach on my last board (the exposed pad of the IC I
> choose is the only GND connection) and I failed miserably. My intention
> was to reflow the paste from the bottom side via a via in the pad from
> the bottom side of the board. It turned out that I used too much paste
> and got shorts between the pins and the exposed pad. I finally asked for
> professional help and will get a 2nd board back some time next week.
> The people I tlaked to ensured me that without reflow your yield will be
> close to 0 to solder an exposed pad from the reverse side.
> 

Indeed that was one of my fears -- generating shorts by using to much
solder. My first idea was to use solder wire (no paste) and add the melt
solder from the backside. It is very difficult to estimate the amount of
solder this way, so shorts may occur. It may work, I do not know how
good the solder can skip or fill the area without copper surrounding the
large center pad. But I should have no problem if I put solder paste in
the right quantity on the pad below the device. But I have still two
problems: Fixating the device in the exact position, and transferring
heat. Soldering a few outer pad of the device to fix it is not really
good, because so I can not check if the center thermal pad is really
soldered. I think I will try to fixate the device with mechanical
pressure instead. Transferring heat should work I guess, I will put a
0.2 mm copper wire into the via to increase thermal conductivity.

Some other people have used very large vias, 1mm or so, so that they can
put a fine soldering iron into the via to reach the device. This way
visible inspection of the result is possible.

Of course I can use a hot plate to heat my whole PCB board, if the other
idea fails. 

I will only try to make the first prototype this way...

Best regards,

Stefan Salewski




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