I have seen this problem as well, from a well known and respected supplier.
It happened during qualification testing. We did not observe the same thing
with a different paste formula from the same supplier. We had used it for
several years without issue. So, we here hopeful when we started shopping WS
flux. I returned he paste and they said they could not duplicate my problem
and the paste met their specifications. So, we chose another suppliers WS
paste. 


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Goulet
Sent: Friday, November 28, 2008 10:37 AM
To: [log in to unmask]
Subject: [TN] Solder Paste inconsistency

I would have to believe that others must have seen solder paste
inconsistencies or my vendor has issues.
After having a problem with one paste and checking all the paste handling,
storing, neading, process temp and humidity etc we changed to another paste
but from the same company as we returned the other paste and gave them
another chance with this new and better product.
The first batch came in jars and everything was going perfectly. The second
batch came in 600 gram tubes and I swear it was the old material with the
new label. I don't know if they were testing us or that the company just
can't control their process. The paste from the tubes exhibit the same high
viscosity, where it doesn't separate of the squeegee blade. I tested the
refrigerator using a temoerature mole over a 2 day period and found it was
well within spec. A second note is that we are seeing voids under the 64
pads BGAs, the smaller body with 256 balls but not the larger BGA also with
256 balls. The profile hasn't changed, three solder pastes are all water
soluble tin/lead, two from one company and I'm going to compare another
vendor's paste to this one next week. The type of void looks like a donut,
most are 20% or less but I don't want any or less than 10%. This type of
void indicates flux was successfully dried out at the edges or migrated
toward the center and was trapped. The vendor says water solube should be
ramp to peak unlike a No-Clean which should have a ramp soak profile. From
what I see there must be a gray area here like quickly rising to 120c then
have a steeper ramp to at 1.5C/sec to 200C then a gradual ramp of 1C/sec up
to the entrance of the reflow zone. This should slowly remove the volitiles
and eliminate  the voids. I understand that uncured UV mask can also cause
this but I would think the visual evidence would be quite different where
you'd have 1-3 small voids around the edge. The boards and BGAs were baked
so it is not moisture.

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