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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