This brings up an interesting question. How does this compare to ESD with
regards to problems on the boards.? I don't doubt that moisture in the
boards may cause problems but to what level? Is this something we should put
lots of money and time into? We can bake our PTH boards before wave solder
and the finished boards before submerging in dielectric oil, but is this all
relative to the level of humidity in the building (fab area) and how long
the bare boards have been on hand?
Thanks in advance for your wealth of information.
Phil Nutting
Manufacturing Engineer
Kaiser Systems, Inc.
High Voltage Power Supplies That Work(tm)
126 Sohier Road
Beverly, MA 01915
ph: 978-922-9300
fx: 978-922-8374
[log in to unmask]
-----Original Message-----
From: Jack Crawford [mailto:[log in to unmask]]
Sent: Thursday, January 25, 2001 6:03 PM
To: [log in to unmask]
Subject: Re: [TN] PCB Baking
There is support for this in IPD-HDBK-001 w/Amendment 1 Handbook and Guide
to Supplement J-STD-001 (Includes J-STD-001B to C Comparison). I've copied
text below, and there are also pointers to other good reference documents.
(Ok, I know--two HDBK responces in one hour, but it's a nice book with a
tremendous amount of information to support soldering requirements; 165
pages and only $35 for IPC members.) Jack
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
==========================================
APEX - the industry's premier trade show in Electronics
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--------
Jack Crawford, IPC Director of Assembly Standards and Technology
2215 Sanders Road, Northbrook IL 60062-6135
[log in to unmask] <mailto:[log in to unmask]>
847-790-5393
fax 847-509-9798
>>> [log in to unmask] 01/25/01 04:40PM >>>
Hi TechNetters,
Is there any standard (IPC or otherwise) that details the requirements and
the process of baking bare PC boards prior to assembly? It is standard
practice to bake boards that have been stored in high humidity conditions
for long periods. These boards will have absorbed moisture and will need
drying before wave soldering to prevent outgassing. This should not be an
issue with surface mount boards that will be reflowed. Given that the baking
process will accelerate pad oxidization, is there any good reason to bake
SMT boards?
Harinder Jassal
Manager, Manufacturing Engineering
Aimtronics Corporation (Delta)
Tel: 604-946-9666
Direct: 604-940-5712
Fax: 604-946-7611
[log in to unmask] <mailto:[log in to unmask]>
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