Polymer Decompression
Time-dependent reduction of internal mechanical stress in cured polymer layers occurs when the substrate is held at or above its glass transition temperature. High-density multi-layer boards experience viscoelastic resin relaxation during the lamination and curing cycles. This material behavior allows the epoxy or resin to reorganize its molecular structure, reducing the risk of warpage and delamination.
Lamination Sinking
During the hot press phase, the resin flows into the voids around the copper traces and then begins to relieve the built-up mechanical stress. If the laminating press does not allow sufficient time for this relaxation, residual stress remains trapped in the cured panel. This trapped stress can cause the board to warp or twist during subsequent thermal processes like soldering or drilling.
After lamination, the temperature is cooled slowly to prevent the rapid buildup of new mechanical stress. This slow cooling step is necessary to ensure the dimensional stability of the finished multi-layer substrate.
Stress Resolution
Measuring the changes in substrate flat-ness before and after thermal exposure indicates the effectiveness of the stress relief phase. Boards that undergo proper relaxation during curing show minimal change in dimension and retain trace alignment. This stability is essential for the long-term reliability of fine-pitch electronic assemblies.
