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?
Board baking will remove moisture and it does help to prevent delamination if done correctly, so in a sense baking does reduce the potential of delamination but it does not restore anything.
Board baking will destroy solderability. By baking the boards, the growth of the intermetallic compound is accelerated and depending upon the final thickness of the plating on the boards, one could end up with exposed intermetallic layer which is no longer solderable. This is where the coating durability categories are defined and utilized.
How often can the boards be baked? I would hesitate to answer this as the boards should be kept in a dry environment or dry box if they are to be stored for any length of time. If baking is deemed necessary after the assembly process then the baking process can be implemented at that point in time. I would not recommend continuously baking the boards.
The ability of the laminate material to absorb moisture is very high; hence the boards should be soldered immediately after the baking cycle. The laminate will absorb all the moisture removed in a baking cycle within the following 8 to 12 hours. This is from work we did at DEC when I was there.
If more information is needed, please don't hesitate to contact me.
- The ability of the board to be soldered without delaminating or
- The solderability of the board.
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At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.
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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