In part 3 of this series of posts I discussed how phenolic cured laminates are mechanically weaker than their dicey cured laminate counterparts. I pointed out some of the material properties listed on the material data sheets that explain and support this point. Whereas the phenolic systems are better at thermal management, the dicey systems are better under mechanical stress. There is no right or wrong here. The systems just perform differently under different circumstances. Understanding the differences and how they relate to the applied assembly process are important to ensure success.
On this post I would like to discuss the effect moisture has on the printed circuit board. What most people don’t realize is that printed circuit boards are hygroscopic. Boards shall absorb available moisture from the surrounding environment to the point of equilibrium. The various FR-4 laminate data sheets list a Moisture Absorption value. The values are calculated in accordance with the IPC-TM-650 22.214.171.124A specification.
The 126.96.36.199A specification basically tests a solid rectangular piece of FR-4 free of copper, no holes and the edges sanded smooth. First the sample is preconditioned, aka dry baked. Immediately after dry baking, the sample is weighed. This is the dry weight. The sample is then submerged for 24 hours in Distilled Water. The sample is removed, towel dried and then weighed. This is the wet weight. The wet and dry weights are then plugged into a formula listed in the 188.8.131.52A specification to produce the Moisture Absorption value.