Evaluation and Modeling of Electrochemical Migration on Printed Circuit Boards

Abstract

Electrochemical migration (ECM) is the growth of conductive metal filaments on a printed circuit board (PCB) through an electrolyte solution under a DC voltage bias. ECM can cause a reduction in surface insulation resistance (SIR) between adjacent conductors and lead to intermittent or catastrophic circuit failures.

To evaluate the current leakage and electrochemical migration behavior on printed circuit boards, IPC B-24 comb structures were exposed to 65°C and 88% relative humidity conditions under direct-current (DC) bias for over 1000 hours to determine the effects of some key factors to ECM. The key factors include: solder alloy (eutectic tin-lead and lead-free), board finish (organic solderability preservative versus lead-free hot air solder leveling), spacing (25 mil versus 12.5 mil), and voltage (40 V versus 5 V bias), solder mask (using and not using), flux solids content. In-situ measurements of SIR, energy-dispersive spectroscopy after testing and optical inspection before and after test were used. The relationships between the electrical behavior and electrochemical behavior of solder alloys were established. The long term electrochemical behaviors of tin-lead and lead-free solders were obtained. The morphology and distribution of migrated species, including Sn, Pb, Cu and Ag were investigated. Compared with solder alloy, board finishes played a secondary role in affecting SIR due to their complexation with or dissolution into the solder. The competing effect between electric field and spacing was also investigated. Solder mask was found to stabilize SIR and reduce the chances for ECM to occur due to its "walling effect". Compared to low solids flux, medium solids flux increased the characteristic lives of PCBs due to their encapsulation effects.

The prolonged SIR decline of Sn-3.0Ag-0.5Cu soldered boards was simulated by three-dimensional progressive and instantaneous nucleation models, whose predictions were compared with experimental data. The kinetics of the electrochemical migration process between copper traces in deionized water was investigated using electrochemical impedance spectroscopy and cyclic voltammetry. The rate limiting step was identified before and during dendritic growth. The time to generate an embryonic dendrite was measured experimentally on copper traces and modeled using Nernst Planck equations, which matched experimental results well.