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November 2004

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Subject:
From:
Ken Patel <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Ken Patel <[log in to unmask]>
Date:
Thu, 4 Nov 2004 10:29:18 -0800
Content-Type:
text/plain
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text/plain (103 lines)
Steve and all,
Our CM is using no clean AIM solder paste and we think that there is no
problem from that angle. When I personally went and looked the board, the
quality person told me that she is seeing black color on the pad. I laughed
at saying that can't happen on my boards after problem was fixed in the
industry 5-7 years ago and what you are seeing must be reflection (optical
illusion) of black molded QFP or similar components on a very shinny solder
joint. And also ALL fab house knows the black pad phenomenon and they
control the phosphorous content in nickel bath to prevent this from
happening!

I looked board from different angles, took to different microscopes and also
used couple of regular magnifying glasses to clear any doubts about the
blackish looking pads. And indeed it turned out black/dark gray. We found
some locations were you can see thru naked eye. What surprised me was only
passives were affected, mostly on one side of the board. Also one pad was
affected 1/2 making solder joint half as there was de-wetting on the black
part portion.

Few equations for TechNet experts:
        (1) Why only one side was affected more than other?
        (2) Why only few locations are affected and not the entire board?
        (3) Why problem persists more near the edge of the board then the
center of the board?
        (4) Why name black pad was given - is it due to black color looking
pads?
        (4) Can this be reworked?

We have given board for cross section to confirm the problem. I will update
you with the finding.

Re,
Ken Patel


-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Wednesday, November 03, 2004 2:22 PM
To: [log in to unmask]; [log in to unmask]
Subject: [TN] Solderability issue...

Hi Ken and TechNetters!

Ken sent me some pictures of some solder problems he's seeing on some
assemblies and wanted me to post them on my page. I got the pictures up. Go
look at:

http://www.stevezeva.homestead.com/files/solderability_1.jpg

http://www.stevezeva.homestead.com/files/solderability_2.jpg

http://www.stevezeva.homestead.com/files/solderability_3.jpg

I have a few questions; are you seeing this quite a bit, or just very
random? What kind of solder paste is it?

I had some strange looking joints one time in the past when I was out in
California building memory modules. We were using water soluble paste at the
time. One of the inspectors called me said that she was seeing cold solder.
I went and looked at the assembly, and saw what really did look like cold
solder. But the strange thing was that it was occuring on a couple of leads
on a part, and all the rest of the leads were fine...including the ones
right next to the leads that looked cold.

I was thinking;"Something doesn't add up, convection heat doesn't work like
this...how can one lead have cold solder, then the one right next to it is
fine?"

I started looking a little more, and took a pair of tweezers and gently
probed the cold solder joint. To my suprize, there was a solid fillet of
solder that was wetted to the lead and pad beneath a layer of what looked
like cold solder. Things were getting really strange now.

To make a long story short, what it turned out to be was that the screen
print operator was using a wet rag to clean the underside of the stencil
periodically. It was an old MPM SP2400 that didn't have an automatic
cleaning system. The operators heart was in the right place, but the rag he
was using was overly wet. When he wiped the bottom of the stencil, some of
the water was trapped in the apertures. So on the next print, the was enough
water to alter the chemistry of the past that gave us these random cold
looking solder joints.

After I took his wet rag away, gave him some decent wipes and alcohol, the
problem went away.

The point of this story is that maybe whatever they're using to clean the
stencil with might becoming trapped in some of the apertures, and the
screwing the paste up on the next couple of prints.

Just a WAG...

-Steve Gregory-

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