Material Separation
Laminar cracks occurring within the internal dielectric layers of printed circuit boards represent a failure of structural integrity during thermal stress. These core fracture zones propagate along the glass fibre weave when mechanical or thermal expansion forces exceed the interlaminar bond strength of the resin matrix. Such internal splits often remain invisible to surface inspection techniques while compromising the long-term reliability of plated through holes.
High-density multilayer boards face increased risk of this phenomenon due to the complex coefficient of thermal expansion mismatch between copper features and the surrounding substrate.
Processing Mechanism
Fabrication of these boards involves a hot press cycle that forces resin into the prepreg to bond the constituent layers together. Insufficient pressure or premature cooling prevents complete resin flow and leaves microvoids trapped between the weave and the metal layers. Subsequent thermal excursions like reflow soldering cause these trapped pockets to expand and pull the resin away from the reinforcement fibres.
Operators calibrate press parameters to ensure uniform pressure distribution and adequate dwell time to prevent the formation of these weak points.
Reliability Impact
Performance requirements dictate that circuits remain stable under operational temperatures to prevent latent short circuits or open connections within the stackup. Cross-sectional micro-analysis after high-temperature thermal cycling testing acts as the primary validation tool to verify the absence of these internal fissures. Detected instances of this separation signal a breakdown in the curing process which renders the board incapable of meeting standard solderability and service life specifications.
