ANALYSIS OF FAILURE MECHANISMS OF LEAD FREE ALLOYS UNDER CONTINUOUS MONITORING

摘要

The reliability of two lead-free solder alloys, with different surface finishes (immersion silver (IAg), electroless nickel immersion gold (ENIG) and organic solderability preservative (OSP) are assessed under accelerated temperature cycling (ATC) conditions. The tests are performed in accordance with IPC-9701A industrial standards in an effort to increase the product life of electronic components during operating conditions. The investigated solder alloys are Tin-Silver-Copper (SAC) and SAC plus a dopant. The test vehicles consist of a daisy chain of ceramic surface mount chip resistors type 2512 and ball grid arrays (BGAs) which were monitored individually for electrical failure (>1000Ω) using high sampling rates event detectors. The components were subjected to a symmetric thermal profile 0°C to 100°C of 100 minute duration with 30 minute dwell times in a controlled environmental temperature chamber. Metallographic cross sectional analysis using scanning electron microscopy (SEM) was used in order to establish failure mechanism and to evaluate the solder joint cracking and creep-fatigue damage after thermal cycling. Intermittent failures were observed during the continuous monitoring prior to the occurrence of permanent open circuits. The doped SAC alloy demonstrated superior long-term reliability under ATC stimuli. The superior performance of the doped alloy was attributed to enhanced mechanical properties of the solder joint at the printed circuit board pad interface.