Barrel Cracking

Thermal Stress

Volumetric expansion of resin systems during thermal cycling drives high out-of-plane mechanical strain into the thin copper lining. Standard FR-4 laminates exhibit a coefficient of thermal expansion near fifty parts per million per degree Celsius below glass transition, which increases dramatically above that transition temperature. When repeated solder reflow operations subject the assembly to elevated temperatures, the differential expansion between copper plating and base laminate creates fatigue fractures.

Insufficient plating thickness or poor copper elongation properties accelerate crack propagation across the entire circumference. Fabricators manage this mechanical risk by controlling acid copper bath chemistry and plating current density to ensure uniform deposit thickness along the hole core.

Acceptance Standard

Microscopic microsection evaluation under IPC-TM-650 Method 2.1.1 verifies structural integrity after thermal stress testing. Acceptance criteria for Class 3 high-reliability electronics prohibit any circumferential fracture exceeding internal wall thickness limits. The presence of a single full crack across the copper barrel results in lot rejection during quality auditing.