I agree with your observations. We do prebake some boards, including all
field returns requiring rework with convection or radiation transfer tools.
But, not as a standard practice.
J-STD-001 broaches the subject, saying that it can be done. The HDBK-001
expands on the information. Remember the IPC standards are consensus
standards. The committees working on them understand that standard
requirements end-up on contracts. So, work of the committees respects "The
Principles of Standardization". These are listed on the inside front cover
of the J-STD-001. The principles make it very difficult to make suggestions
or recommendations within the text of a standard.
I don't think there is a consensus for a standard requirement. They may be a
consensus on good advice, for now. But, in a book it would become obsolete
as materials improved, or changed (things will get worse with Pb-free solder
for awhile). This is a good place to get current advice.
My advice is to bake boards when you have reason to be concerned.
1. Bad past experience
2. High layer count and very expensive-rare components
3. Field return that requires hot air rework
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Stadem, Richard
Sent: Monday, October 10, 2005 10:34 AM
To: [log in to unmask]
Subject: [TN] (tn)(pwb) When do you need to bake a circuit board prior
to assembly? Are there standards?
Fellow technetters, some questions.
I know that it is sometimes necessary to pre-bake pwb's when there is a
chance that moisture could damage the board when it goes into reflow, or
when it is subject to hot-gas rework. I understand most OEM's and CEM's
do not bake as a matter of standard procedure simply because they
usually do not need to, and baking will diminish solderability. Nearly
all of the delamination issues I have ever seen were invariably caused
by some type of prepreg lamination deficiency. Most of the time nearly
any standard .062" multilayer FR-4 pwb can be reflowed without pre-bake
without danger of delamination, at standard 63/37-based reflow
temperatures. Typically the boards are still dry, and even if they are
not, the prepreg holds up during reflow and rework.
Are there standards for pwb handling that spell out what type(s) of
prepreg materials should be used for different types of pwb's , what the
proper storage methods are, when pre-baking should be performed, at what
temperature and time, etc? Is there a standard that tells us that
certain board types or configurations should be baked (or which ones do
not need to be)? Is there anything out there that provides proper
storage methods for pwb's?
If no standard exists, should not there be one?
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