The Effect of Dwell Time on Thermal Cycling Performance of High Reliability Solder Alloys with a −55/125 °C Test Condition

ABSTRACT The past decade has seen the development of commercial, third-generation, high reliability Pb-free solder alloys designed to meet the requirements of higher temperature use environments. Most of these offerings are based on the Sn-Ag-Cu (SAC) system, with major alloying additions of bismuth (Bi), antimony (Sb), or indium (In). These elements, individually or in combination, promote additional precipitation, solid solution, or dispersion strengthening that can resist microstructural degradation at elevated temperatures or during aggressive thermal cycling. Results from the literature show that an increase in thermal cycling dwell time can decrease the thermal cycling reliability of SAC solders. Because these high reliability solder alloys are designed for extended operation at higher temperatures, it is important to understand their behavior and characterize their reliability at extended thermal cycling dwell times. This paper presents the initial results from an experimental program designed to compare thermal cycling results for high reliability solder alloys using a typical dwell time of 10 minutes to an extended dwell of 60 minutes. The 10-minute data were generated in the initial phase of testing and preliminary results were published previously. The current data are based on a thermal cycling test condition of −55/125 °C (TC7 in IPC-9701B) and the test vehicles are a 192-pin chip array ball grid array (192CABGA) and an 84-pin thin core ball grid array (84CTBGA). The high reliability alloys were found to outperform the prevalent SAC305 (Sn3.0Ag0.5Cu) consistently. With the 192CABGA, SAC305 showed a moderate loss of reliability with increased dwell time, but some high reliability alloys had comparable performance with 60-minute and 10-minute dwell times, and others performed much better with the 60-minute dwell. With the 84CTBGA, the high reliability alloys have comparable performance with 60-minute and 10-minute dwell times, with only one alloy performing slightly better with the 60-minute dwell. The surprising finding was that SAC305, when tested with the 84CTBGA, exhibits no dwell time effect with the −55/125 °C test condition. All the alloys exhibited thermal fatigue failures in the bulk solder, but most of the high reliability alloys also exhibited interfacial or mixed mode failures, which complicates interpretation of the data.