Sounds like possibly the pad was not in the solder paste layer and wasn't
cut. As far as inspecting, I had to do this in a former life. We inspected
the pad with a 5DX, but not just for the presence of solder, but for solder
coverage percentage. It was in the specs to ensure 70% solder coverage. I
also learned alot about vacuolar porosity. You might want to inspect a
sample every 10 boards or so with X-ray.....if you have the ability.



-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Bob Wilson
Sent: Monday, October 04, 2004 3:01 PM
To: [log in to unmask]
Subject: [TN] Solder pad under chip


Hi!

I have a chip on a board (Linear Technology part
number LTC3414) found here:
http://www.linear.com/pdf/3414f.pdf

It has a pad (pin number 21) that is underneath and a
note that says: "To maximize the thermal performance
of the LTC3414, the exposed pad should be soldered to
a ground plane."

I built the footprint and made sure the pad was
soldermasked and included the pad shape in the
pastemask file.

The engineer was trouble-shooting and de-soldered the
part from the board.  It looks like there was no
solderpaste or solder on the pad.

Here's the (dumb) designer question:  How would the
solder get to that pad underneath? Should we add a
requirement note saying that the part should be
inspected or x-rayed to see if the pad is indeed
soldered?

Thanks for all and any suggestions.

Regards,

Bob Wilson
www.cstone-design.com
[log in to unmask]

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