The following is text contained in IPD-HDBK-001. As this is a "handbook" I
would not read it as requirement. This would merely be a guideline.
If you choose to use the information below give thanks to SMTnet, and do not
hold anyone responsible for any failures, especially me (grin).
Franklin
=================================================
During fabrication and storage, both components and PWBs will often absorb
water. If left in the device, this water will vaporize at soldering
temperatures and can lead to PWB delamination, soldering voids (especially
in PTHs), and device cracking.
For PWBs, the bakeout removes water accumulated during the fabrication
process and absorbed during storage. Recommended baking times and
temperatures 2 are given in Table 7-1. Longer bakeout times and higher
temperatures are not recommended, as they can degrade PWB and component
solderability.
Water re-absorption begins immediately upon removal of the PWB from the oven
and is linearly related to RH. For a storage environment of 20°C and 30% RH,
a maximum interval of two to three days is recommended with the interval
decreasing with increasing humidity .
Plastic encapsulated devices, especially ICs, also have a tendency to absorb
water from the air, which is violently released during soldering. Typically,
1000 ppm of absorbed moisture is considered a maximum content beyond which
device failure due to body cracking may result. Bakeouts similar to those
used for PWBs have been successful in eliminating these defects. After
baking, the parts again begin to absorb water. Recommended maximum storage
times after bakeout as a function of RH at 25°C 4 are given in Table 7-2,
based on the time to achieve 800 ppm of water (see 5.0.9).
Table 7-1 Baking Times and Temperatures
Baking Temperature Baking Time
120°C 3.5 to 7 hours
100°C 8 to 16 hours
80°C 18 to 48 hours
Table 7-2 Maximum Storage Times After Bakeout
Relative Humidity Maximum Recommended Storage Time
36% 20 days
40% 11 days
50% 7 days
==========================================
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard
Sent: Monday, October 10, 2005 9:34 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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