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In a message dated 10/5/99 10:57:50 AM US Eastern Standard Time,
[log in to unmask] writes:
> I'm in search of a little insight, if you will. I'm witnessing random
solder
> skips on the bottomside of my panels. Everything is clean, but I've
> switched
> mask types from the glossy to the matte finish. Why would that cause
random
> solder skips on the small chips and resistors on the bottomside 805's and
> 603's?
> I'm trying to put some technical reasoning behind just witnessing the
> problem.
>
> Jason Smith
Jason,
I have seen this when the matte finish mask keeps the gap between part
and PCB a little higher than before, allowing a small trap for liquid flux
under the part. If the preheat cycle is exactly the same, there can be a few
random areas where the amount of flux trapped is enough to still have a very
small amount of volatile left at wave contact. The result is a small "air"
bubble as that liquid turns to gas, and that bubble encases the chip at one
or both ends, thus preventing solder contact. I had to build some simulated
assemblies substituting glass for the PCB in order to see this happen. I had
the biggest problem with SOT's, but sometimes 0805's as well.
Jon Moore
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