Interrelating Surface Insulation Resistance Test Patterns

Abstract

For reliable telecommunication systems, Bellcore recommends that Surface Insulation Resistance (SIR) be monitored at key points in printed wiring board and circuit pack manufacturing. The Bellcore SIR criteria are based on the old ‘Bell System’ test pattern having 0·025 inch conductor line widths, and 0·050 inch conductor spacings. Since divestiture, many equipment suppliers have suggested using different test patterns, or even conductors on actual product for SIR testing. Also, with the trend to high density packaging and smaller conductor spacings, the Bellcore pattern now represents old technology. This work confirms and advances prior work suggesting pattern translation based on the SIR per square concept. Essentially exact SIR per square correlation has been found over an order of magnitude of pattern conductor space widths. Critical experimental techniques to modify the FR‐4 epoxy surface appropriately and an important theoretical hypothesis involving shadowing (proven experimentally) are developed in this work.