Hi, Matt
I have done a lot of work in this area, including working on the IPC
subcommittee D-35, where we are developing a standard that will provide
guidelines for proper packaging, storage, and handling of various types
of PWBs.
I have found that for most substrates a moisture level less than 20% of
its saturated
moisture weight is usually sufficient to prevent damage during reflow,
wave solder, or hot gas rework. Of course, this is a guideline. There
are probably some IPC-4101B/xx substrate materials out there where a
moisture content
above 20% of saturated weight can lead to damage during thermal
excursions, but I cannot find any information or data suggesting that.
So there will be a caveat in that a sample should be
tested to qualify the process for a worst case scenario consisting of
"unusual substrate materials" combined with "unusual thermal
requirements".
Note that polyimide materials can retain and absorb much more moisture
at a much faster rate.
Rather than repeating a lot of information that has been discussed on
this forum, I am giving you some links to the Technet archives that deal
with this subject. There are many more, and you can see them by going to
the archives and typing in "moisture content" in the search box.
http://listserv.ipc.org/scripts/wa.exe?A2=ind0508&L=TECHNET&P=R79791
http://listserv.ipc.org/scripts/wa.exe?A2=ind0808&L=TECHNET&P=R69509
http://listserv.ipc.org/scripts/wa.exe?A2=ind0711&L=TECHNET&P=R30486
The last link below has information on how to determine the optimum bake
time of a given PWB, to ensure it is always below 20% SMC at the time it
goes into assembly. By doing this once, you know you are never
overbaking the PWB.
http://listserv.ipc.org/scripts/wa.exe?A2=ind0712&L=TECHNET&P=R32195
And here is a link to an excellent article that describes rates of
moisture adsorption and dessication in organic laminates:
http://www.calce.umd.edu/articles/abstracts/1999/ingresslam.htm
As you can see, the curves show that baking past the point at which 80%
of the saturated moisture content has dissipated is rather pointless, as
the rate of dessication is so small that further baking is probably
counterproductive in terms of the reduced internal moisture as opposed
to the amount of reduced solderability.
When baking immersion silver and immersion tin PWBs, loosely wrap the
PWB in aluminum foil. This does not reduce the rate of dessication, but
has been proven to reduce or eliminate reduction in solderability due to
oxidation.
Good luck,
Richard
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Matt Byrne
Sent: Tuesday, January 13, 2009 7:36 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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