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Effect of Temperature Cycling Parameters on the Durability of Pb-Free Solders
Keywords: solder joints, thermal cycling, health monitoring
Solder joints provide critical electrical and mechanical connections in the electronic assemblies. To assure reliability of electronic hardware, it is important to understand the reliability of solder joints under use conditions. The conversion to RoHS compliant materials has raised concerns about the reliability of lead-free solders for use in electronic product. One of the primary mechanisms of solder joint failure is fatigue due to fluctuations in temperature resulting from operational and environmental conditions. Fatigue of solder arises from cyclic strain within the solder joint due to different rates of temperature expansion in the solder and interconnected structures. To address concern of lead-free solder, CALCE has conducted extensive testing to examine the impact of dwell time and mean temperature on the solder joint reliability. In this investigation, leadless ceramic chip carriers (LCCC): 68 and 84 pins; were assembled with SAC305, SN100C, Sn61.5Pb36.5Ag2 (SPA), 95.4Ag3.9Cu0.7 (SAC397), Sn96.5Ag3.5 and SnPb solders. The assembled boards were then subjected to different temperature cycling profiles of 25 to 125oC, -25 to 75oC, -50 to 50oC and 0 to 100oC with varying dwell times (15, 75, 120 minutes). Resistance of the packages is monitored by data logger and thermal cycling tests are continued till all the components on the board fail. The results indicated that lowering the mean cycle temperature increases the thermal cycling durability of the solders dramatically. Results on the impact of cyclic mean temperature and dwell time revealed that dwell had a larger effect when the medium cyclic temperature is lower. Extended low temperature dwell for the temperature cycle -25 to 75oC was found to be more damaging than expected, particularly for SnPbAg. For maximum temperature of 125oC, dwell time increase from 15 to 75 minutes did not have much effect. Results indicated that SAC305 was slightly less durable than SAC379 but was more reliable than SN100C and SnPbAg solders. SnPbAg solder was less durable than SnPb.
Michael Osterman, Research Scientist
University of Maryland-CALCE
College Park, MD

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