Inspection Method
Quality assurance coupons positioned on circuit board panel borders supply destructive test samples evaluated after lamination, plating and solder assembly steps. A metallographic microsection reveals internal structural features, plating thicknesses and barrel crack defects through cross-sectional microscopic examination. Metallurgical mounting resin encapsulates the coupon, which is ground, polished and chemically etched to expose copper grain boundaries and plating interfaces.
Standardized under IPC-TM-650 test method 2.1.1, this analytical technique confirms compliance with IPC-A-600 and IPC-6012 acceptance criteria. The method is limited to discrete sampling points on panel margins and cannot evaluate non-destructively the entirety of an active circuit board assembly.
Preparation Process
Sample preparation requires precise cutting, potting in liquid epoxy resin, mechanical grinding with successively finer silicon carbide papers, and diamond polishing. Improper polishing creates metal smearing or edge rounding that masks fine structural defects such as microvoids or inner layer separation. Chemical etching with ammonium hydroxide solutions highlights electrodeposited copper grain structures, revealing interfaces between foil cladding and plated hole walls.
Technicians perform optical microscopy at magnifications ranging from one hundred to one thousand times to measure plated copper thickness in barrel walls and capture pad junctions. Automated image analysis software quantifies plating voids, resin recession, thermal stress cracks and microvia target plating integrity.
Acceptance Standard
Plated through hole integrity dictates electrical reliability across thermal cycling tests and harsh field operations. Cross-sectional microsectioning detects subsurface flaws such as knee cracks, barrel separation and resin recession before boards reach final assembly.
