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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