|Ask the Experts|
October 3, 2011
Bare PCB Shelf Life
Is the full shelf life of bare PCB's restored, or reset after it has gone thru a baking cycle? How many times can bare PCB's be baked to restore shelf life?
|Expert Panel Responses|
I'm sure there will be different types of answers for this question. Is the full shelf life of bare PCB's restored, or reset after it has gone thru a baking cycle? How many times can bare PCB's be baked to restore shelf life? I'm curious as to what is meant by shelf life of the bare boards. IPC-J-STD-003 and IPC-A-600, describe the Coating Durability for three categories, 1, 2, and 3. Category 1 is for boards which will be soldered within 30 days, category 2 is for boards which will be soldered within 6 months and category 3 is for boards which will be stored over 6 months. Therefore the questions I would ask are what shelf lives do you want to restore?
Vice President, Technical Director
The 'IPC-1601 printed board handling and storage guidelines' states: "If process controls are ineffective, and printed boards have absorbed excessive moisture, baking is the most practical remedy. However, baking not only increases cost and cycle time, it can also degrade solderability of the printed board which requires extra handling and increases the likelihood of handling damage or contamination. In general, both the printed board fabricator and the user should strive to avoid baking by practicing effective handling, packaging, storage, and process controls..." The document also states "Baking is NOT recommended for OSP coatings, as it deteriorates the OSP finish. If baking is deemed necessary, the use of the lowest possible temperature and dwell time is suggested as a starting point." Baking also accelerates solid diffusion between metals, and increases intermetallic growth. This can lead to a "weak knee" or other solderability issues if the intermetallic layer reaches the surface and oxidizes. Safe and effective PCB drying can be accomplished without damaging high temperatures using ultra low humidity drying enclosures that produce an atmosphere containing less than 0.6g/m3 water vapor. This creates a 'moisture vacuum' that releases the previously absorbed moisture from the PCBs while protecting against oxidation and intermetallic growth. These enclosures are also suitable for safe storage of PCBs for unlimited times. For additional information you can refer to documents at http://www.superdry.info/.
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