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
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Jack Crawford, IPC Director of Assembly Standards and Technology
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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
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