印制板通用规范 / Generic Specification for Printed Boards

This part of IEC 62326 specifies test methods for evaluating electrochemical migration (ECM) resistance of printed board materials and finished boards. ECM includes conductive anodic filament (CAF) growth and surface dendritic migration under bias and humidity conditions.

Conductive Anodic Filament (CAF): Conductive copper salt filament growing along glass fiber reinforcement under electrical bias and moisture. Surface Dendrite: Metallic dendritic growth on PCB surface between biased conductors. Time-to-Failure (TTF): Duration until insulation resistance drops below threshold.

Coupon with parallel conductor pattern: 0.25 mm spacing, 25 mm length minimum. For CAF testing: PTH pairs at specified spacing (0.5 mm to 2.0 mm). At least 10 test vehicles per condition. Include both inner layer and surface patterns. Reference coupon for baseline IR measurement.

Clean specimens with IPA rinse and dry at 105°C for 1 hour. Apply flux if simulating assembly process. Solder reflow simulation if applicable. Store in desiccator before testing. Baseline insulation resistance measured at 100V DC, must exceed 1 GΩ.

Apply DC bias voltage (typically 10V to 100V) across surface conductors. Test environment: 85°C/85% RH in temperature-humidity chamber. Continuous monitoring of leakage current at intervals ≤ 1 hour. Failure criterion: IR drop below 1 MΩ or 3 orders of magnitude decrease.

Apply DC bias (typically 50V to 100V) between adjacent PTHs or inner layer conductors. Test conditions: 85°C/85% RH for 1000 hours minimum. Monitor insulation resistance continuously. CAF failure identified by sudden IR drop followed by partial recovery pattern. Cross-section analysis required to confirm CAF.

Record time-to-failure for each test vehicle. Calculate median TTF using Weibull or lognormal distribution. Report TTF at 50th percentile (median) and 1st percentile (early failure). Plot IR vs time curves for all specimens. Identify failure mechanism through post-test analysis.

Surface ECM: no failures within 1000 hours at rated conditions. CAF resistance: median TTF > 1000 hours at 85°C/85%RH/50V. No early failures (< 100 hours) permitted. Post-test cross-section must show no evidence of filament growth for passing specimens.

Cross-section failed specimens through migration path. Polish to reveal filament morphology. SEM/EDS analysis to identify chemical composition of migration product. Document filament growth direction relative to glass fibers. Photograph at multiple magnifications. Determine root cause: material defect, process contamination, or design issue.

Report shall include: material identification (resin system, glass type, copper foil), specimen geometry and spacing, test conditions (T/RH/bias), preconditioning details, individual TTF values, statistical analysis results, failure analysis findings, comparison to previous lots or baseline data.