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Subject:
From:
David Hillman <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, David Hillman <[log in to unmask]>
Date:
Thu, 13 Aug 2015 15:04:27 -0500
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Hi Steve - yes, you can peel copper away from solder as solder is not a
structural material. IMHO your solder coverage is outstanding. My guess is
that the product team is having circuit functional issues and thinking they
have a grounding/thermal issue which is not the case given the amount of
solder coverage you achieved.

Dave Hillman
Rockwell Collins
[log in to unmask]

On Thu, Aug 13, 2015 at 2:56 PM, Steve Gregory <[log in to unmask]> wrote:

> 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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