We are required to run the Hygroscopic Dust Test (part of NEBS testing).
The test is performed to simulate 30 years of environmental exposure.
The hygroscopic dust is applied as a liquid spray and is then dried for
24 hours. After the liquid is dried the board is powered up at humidity
levels between 70% and 80% and is required to be 100% operational.
Sensitive circuitry could fail due to low impedances paths created at
high humidity. In fact I have seen an intermittent failure on some of
the more sensitive clock circuitry during these tests. 
 
I need to specify a sealant to apply around the BGA devices with
sensitive circuitry. This bead of sealant around the BGA should prevent
the liquid spray from entering the pin field under the BGA. I know that
conformal coating the entire board is out of the question here so please
only suggest solutions that seal around a specific BGA device. Does
anyone have a recommendation on the type of sealant that should be used
or you have used in the past? Obviously, I will be calling my CM to
discuss this issue but I wanted some feedback from the forum also. 
 
Thanks
Tony  Cosentino
Tekelec
[log in to unmask]
919-460-3656

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