Cross-Sectional Preparation
Metallographic examination requires the extraction of a specific region from a printed circuit board to reveal the internal structure of copper platings and solder joints. Thermal microsectioning uses high temperature exposure to simulate the stresses of assembly before the physical cutting and mounting of the sample occurs. This controlled heat application induces micro-cracks or separation if the bond between the substrate and the copper exhibits poor adhesion or improper chemical deposition.
Material integrity fails when the expansion of the dielectric material exceeds the elastic limits of the metallic interconnection during these thermal cycles.
Evaluation Procedure
Technicians embed the cycled coupons into an epoxy resin to secure the sample for precise mechanical grinding and polishing. A rotating abrasive disc removes layers of the board until the internal planes lie exposed for optical observation under a microscope. Analysts check the resulting surface for evidence of barrel cracking, laminate voids, or insufficient plating thickness.
Chemical etchants reveal the grain structure of the copper and highlight any discontinuities caused by the earlier heating phase. Consistent lighting and magnification settings allow for accurate measurement of intermetallic compounds at the interface of the lead and the pad.
Compliance Benchmarking
Quality standards govern the acceptable limits for these visual irregularities based on the target reliability class of the product. Industrial specifications define the minimum required copper thickness to ensure electrical conductivity despite the thermal fatigue generated by the prepress cycle. Inspection protocols demand that every hole wall remain free of copper separation after the sample reaches specified temperature peaks.
Boards showing cracks larger than the allowed thresholds fail to meet the performance criteria established for high-density interconnect applications. Internal defects detected through this method offer definitive evidence of fabrication flaws that demand corrective action in the plating line.
