Hello everybody.

Any  idea about random presence of microballs (diameter 0.1 to 0.5 mm) 
beneath some chips (R and C 0805, 1206,1210, 1812) ?
We use a SnPb process with a vapor phase oven.
We use same types of PCB, same process on same SMT line in a clean room 
from years and the balls are randomly present from some months now. 
They could be present in one lot of PCB and not the following; beneath a 
component on one PCB and not on the same component on other PCB; we have 
balls one day and not the day after; the balls disappear for weeks and 
re-appear after; ...
I have checked different lots of solder paste, origin of PCB's, all 
parameters of printing, P&P and the cleaning (co-solvent station). The 
thermal profiles are same.
As it's hi-rel production I cannot change easely products and process.
I suspect a process parameter but which ?
I ask myself if there is not a ghost in the line .. :-)

Best regards.

CANTAGALLO Luigi
Technology Engineer


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