• Every laminate supplier we use recommends that the bare boards be baked prior to assembly.  This is regardless of material type, but is more important with polyimide as it more readily absorbs moisture than Fr-4 materials.  The time and temps that the laminate suppliers recommend vary, but generally they are in the range of 250 degrees fahrenheit for 4 - 6 hours.  It is important to make sure that the boards get above 212 F and stay there for the 4 -6 hours.  racking boards (as opposed to stacking) will make the baking more efficient.  Use the boards within a few hours of baking or they will re-absorb moisture.

• The reason for the bake is that PWB materials do absorb moisture.  The laminate spec. sheets spell our how much for each laminate.  If you do not believe this try this simple test.  Take a board from stores and weigh it using the most accurate scale you have in house.  Bake the board as recommended above.  Re-weigh the sample.  The weight loss is indicative of how much water is in the board.  For grins and giggles, try both Fr-4 and polyimide materials.

Other items that affect moisture (besides how the board was built):

• Packaging.  Most of our customers are very specific about what type of packaging they will accept.
• Storage conditions.  What our customers do here varies widely.  Some store their inventory in nitrogen dry boxes prior to use.  On the other hand, we were once called to a customer's plant because of our "delamination problem" only to find out they were storing the boards on a pallet outdoors, in Florida, in the summer!  Talk about moisture absorption.
• Oh yeah, baking.

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