Surface finish effect on reliability of SAC 305 soldered chip resistors

Abstract

Purpose

The purpose of this paper is to assess the long‐term reliability of lead‐free Sn96.5Ag3.0Cu0.5 (SAC305) under accelerated temperature cycling (ATC) conditions. Test vehicles consisted of commercial 2512 ceramic chip resistors soldered to printed circuit boards (PCBs) using three different Pb‐free surface finishes: organic solderability preservative (OSP), immersion silver (IAg) and electroless nickel immersion gold (ENIG).

Design/methodology/approach

Two populations of solder joints were monitored continuously during a thermal cycle of 0°C to 100°C with a ramp rate of 10°C/min and a 30 min dwell at the temperature extremes. One population was cycled to 2,500 cycles, the other population was cycled to 8,250 cycles. Failures were defined in accordance with the IPC‐9701A industry test guidelines and failure data are reported as characteristic life, η. Microstructural evolution was characterised using metallographic techniques and back‐scattered scanning electron microscopy (SEM). Fractography was performed on post‐cycled resistors to determine failure mechanisms.

Findings

The results showed that the lifetime of the chip resistors could be ranked as follows: OSP>ENIG>IAg after 8,250 thermal cycles, when>73 per cent of the population had failed. It was found that the relative performance of OSP finishes improved with longer periods of cycling. This was attributed to diffusion of copper into the bulk solder and which subsequently, over time, acted as a dispersion strengthening mechanism. The relatively poor performance of IAg finishes was attributed to void formation.

Originality/value

The paper provides new and valuable information to end‐users about the long‐term reliability of lead‐free solder on three commonly used lead‐free surface finishes. The performance of each surface finish is explained in terms of microstructural evolution and void formation.

Citation

Collins, M.N., Punch, J. and Coyle, R. (2012), "Surface finish effect on reliability of SAC 305 soldered chip resistors", Soldering & Surface Mount Technology, Vol. 24 No. 4, pp. 240-248. https://doi.org/10.1108/09540911211262520