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October 2006

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Subject:
From:
Miguel Vallejo <[log in to unmask]>
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Date:
Tue, 31 Oct 2006 09:16:26 -0800
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Very good assessment, Richard! I agree.
Another cause for tombstoning not related to paste is copper features(vias,
traces) under components, specially with small sizes.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Tuesday, October 31, 2006 8:49 AM
To: [log in to unmask]
Subject: Re: [TN] Bizzare Phenomena...

Steve,
Judging by the small size of most of the particles, and if I look very
closely I can see that it appears many of the particles have
agglomerated into larger particles, I would assume that these are all
very small solder balls that are being spit off of the board. Because
they are in the reflow chamber, they sit down in the oven in a molten
state continuously and if other solder spatters happen to land on them
they are absorbed into the larger solder balls. This is why it appears
that there are large solder spatters, when in fact they are not, just
accumulations of small spatters.

Why is it happening? BTSOOM.
My first guess is that at the higher temperatures there is some
spattering occuring as the paste goes into reflow. This can be
attributed to outgassing of paste solvents and/or moisture in the paste,
but there are many other possible causes, including outgassing from the
pwb, printed solder paste not entirely on the pads, and many other
potential causes. Something is obviously causing the molten solder to
spatter, and these are only a few things that come to mind. Your profile
does climb from 190 C to 250 C in about 3 or 4 minutes, and this might
be enough to cause this type of spattering if the flux volatiles are not
burning off fast enough. At first I wondered if excess solder paste was
smearing the bottomside of the pwb during the printing process and
falling off at reflow, but what would cause it to fall off? So, it is
probably paste spattering.
Whether the cause is boiling off of paste solvents or extraneous
moisture from some other source, I am 99% sure that it is related to the
tombstoning issues you mention. Fixing this problem will probably fix
the other.

I guess I would try going with a slightly longer profile with a more
linear ramp-up and see if it improves. You may also want to evaluate
different solder pastes.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Tuesday, October 31, 2006 8:57 AM
To: [log in to unmask]
Subject: [TN] Bizzare Phenomena...

Mornin' All!

This morning I have seen something that I have never, ever have seen
before. It's been a little slow the past week, so we decided to do our
maintenance on our reflow ovens. After we opened the oven up, we found
many, many solderballs laying on the bottom of the oven right around
zone 7 - 8, right at the beginning of the spike zone. Check-out:

http://stevezeva.homestead.com/files/Oven_Solderballs.jpg

http://stevezeva.homestead.com/files/Oven_Solderballs_2.jpg

http://stevezeva.homestead.com/files/Oven_Solderballs_3.jpg

http://stevezeva.homestead.com/files/Oven_Solderballs_4.jpg

We've been having tombstoning problems when we've run the two lead-free
jobs that we have here. I've been working with the paste vendor sending
my profiles and plots, but I haven't been able to solve the problem. The
solderballs in the oven are lead-free solderballs.

We are using a Heller 1809, and my profile set-points are:

120    140    160    180    190    200    230    240    250
120    140    160    180    190    200    230    240    250

Belt speed 26 in per min

The paste is a SAC 305 with a water washable no-clean flux. I think my
profile is pretty reasonable, and the last board we ran is pretty
standard, .062" thick, with no bizzare copper planes. The board finish
with both jobs has been ENIG. The last job I ran we were tombstoning
0805's, the one before that we were tombstoning 0402's. With both jobs
we've had major tombstoning problems. I've slowed the beltspeed down,
sped it up, ramped up slower, ramped up faster, changed this, changed
that, but still haven't resolved my tombstoning.

Of course, I'm the only one that's having this problem according to my
paste vendor...

I wouldn't have imagined that I would see the solderballs in the oven
like we have...

Have any of you ever seen something like this before?

-Steve Gregory-

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