" It was told to to us that there is less than 50% coverage on the belly pad."

1) What occurred that made the customer look?
2) How was this 50% determined/detected by the customer?

Ahne.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Thursday, August 13, 2015 12:56 PM
To: [log in to unmask]
Subject: [TN] Crude destructive test

Hi All,

So here's the story; we're building a board that has a Linear Technology
LT3652 12-pin MSOP device on it, and it has a belly or grounding pad on it.
The board is designed with five 20-mil via's beneath it (bad idea), and they are through via's not cap plated or tented. So they start having problems with the part. The footprint on the board has the belly aperture much larger that the pad on the device itself. I have a 5-mil thick stencil because there are 20-mil pitch QFP's on the board, and I print 100% of the area of the belly pad on the board (which is .160" X .082"). My reflow profile peaked out at 228 C. and I was above reflow for 63/37 for 112-seconds. While I did not have a thermocouple on that part, I did have it nearby because there are some large D-Packs in the vicinity and I wanted to be sure I was getting adequate reflow on those.

So the complaint is that we're getting insufficient solder on the belly pad. It was told to to us that there is less than 50% coverage on the belly pad. We x-rayed many boards and while we could see that sometimes the vias didn't fill, we saw evidence of solder on almost 100% of the area of the pad. We could tilt the board and under high magnification see beneath the part and tell that solder had wet to the belly pad on the part, but what we could not tell was how much it actually covered the pad. We removed parts with hot air and could not find one part that did not have solder wet to 100% of the pad.

So my boss asked me to take a board and physically pry one off to see what it looked like. So I got a heavy exacto blade and slipped it beneath the edge of the part and pried. The corner broke and then the top half part with all the guts split off, a couple of leads in the corners broke off too, but the belly pad stayed on the board. I was able to get the exacto blade under a corner of the belly pad and I could slowly peel it away from the solder joint, and this is what it looked like:

http://stevezeva.homestead.com/LT3652_Belly_Pad.jpg

So I saw a few small voids (which was not unexpected), but to my untrained eye I don't see much wrong. I do wonder a little about me being able to peel away the belly pad like that, should I have been able to do that? Or does the solder fracture along the intermetallic layer when you apply peeling forces like that?

Thanks in advance for any input...

Steve

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