Hi Matt,
Different epoxies present different degrees of moisture absorption. You
should be able to find moisture absorption expressed as a percent by
weight in material spec sheets.
Baking is a double edged sword. Baking removes moisture, and other
volatiles, but at the same time excessive baking degrades the material.
I met a person at IPC (Chicago) last year that told me that he hooked up
a spectrometer to the exhaust of his oven. Water evolved for the first
four hours then C02 started to be seen in the exhaust. His conclusion
was that the material was degrading after four hours. He did not give
specifics. I anticipated that aggressive baking may degrade material
(and also surface finish for that matter) and have been cautioning
against aggressive baking.
As you know, in response the material damage explicit with lead-free
assembly, we have developed a method to find delamination in our test
coupons using capacitance. One of the early findings was that moisture
can also be seen using capacitance measurements of test coupons. Usually
we are able to differentiate moisture loss from material damage by the
capacitance profile that it presents.
The bottom line is moisture may contribute to delamination but material
degradation due to excessive baking can also contribute to delamination.
My sense is that moisture induced delamination presents as adhesive
delamination that is horizontal cracks that resemble blisters and are
located at a laminated interface (b-stage to c-stage or copper, or along
bundles of glass). It may be that the vapor pressure exerted by trapped
moisture induces a mechanical failure at laminated interfaces.
Material degradation induced delamination more often presents as
cohesive failure that may exhibit cracks with vertical sections, often
bifurcating and spanning across b-stage and c-stage material, even
through glass bundles, and tend to propagate with sharper angles. This
failure mode appears to be more a chemical degradation of the epoxy
system.
Sincerely,
Paul Reid
Program Coordinator
PWB Interconnect Solutions Inc.
235 Stafford Rd., West, Unit 103
Nepean, Ontario
Canada, K2H 9C1
613 596 4244 ext. 229
Skype paul_reid_pwb
[log in to unmask] <mailto:[log in to unmask]>
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Matt Byrne
Sent: Tuesday, January 13, 2009 8:36 AM
To: [log in to unmask]
Subject: [TN] Moisture Content of PWB's for Assembly?
I'm looking for a target measured level of moisture in epxoy/plass PWB's
before soldering assembly. Baking before assembly is used to remove
moisture, but what's considered "safe" to avoid excess z-axis expansion?
I found mention of 0.5% moisture in the resin in an archived TechNet
note.
Any hard data out there?
MB
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