Reliability Modeling to Enable Damage Assessments for Plated Through Holes

Abstract

A modeling approach to assess the reliability characteristics of the copper used in printed circuit boards (PCBs) is presented. Plated Through Holes (PTHs) for electrical connections across PCBs are formed by drilling the PCB and electroplating the hole, often using a copper (Cu) finish. Although the mechanical strain on the Cu in the center of the PTH may be insignificant to disrupt circuit functioning in a single reflow soldering cycle, more substantial damage accrues over multiple reflow cycles. Loads applied after the reflow cycles are shown to contribute to subsequent mechanical disconnects. Efforts to quantify the damage to the Cu are hindered by the difficulties of measuring direct strain or displacement inside a PTH and extrapolating the damage to the Cu in the first three reflow cycles. The assessment of damage to the Cu is enabled by finite element analysis. The authors' finite element model is defined as a single load system based on PTHs with flanges on every side of the laminate board connected by a hollow cylinder, a configuration treated as symmetric from the center of the board. The critical area of focus is the inner ring of the PTH. Stress and strain measurements in the region of interest are obtained using numerical simulations. The two-dimensional axi-symmetric model is validated against a three-dimensional full model and several test cases. Reliability assessments relying on undamaged circuits are shown to be less accurate than estimates incorporating Cu damage following three reflow cycles. An increased probability of intermittent connection for the PTH's remaining use is suggested. Accordingly, the authors propose that damage assessments of copper finishes be incorporated into models of PCB reliability.