Effects of Isothermal Aging on the Thermal Expansion of Several Sn-Based Lead-Free Solder Alloys
- PDF / 2,242,753 Bytes
- 7 Pages / 593.972 x 792 pts Page_size
- 44 Downloads / 154 Views
JMEPEG https://doi.org/10.1007/s11665-018-3235-8
Effects of Isothermal Aging on the Thermal Expansion of Several Sn-Based Lead-Free Solder Alloys M. Hasnine and M.J. Bozack (Submitted July 8, 2017; in revised form January 14, 2018) In this paper, effects of high-temperature aging on the thermal expansion behavior of several lead-free alloys SAC305, SAC387, Sn-3.5Ag, SnCu, SN100C (SnCu-Ni-Ge) and SnCu-0.01Ge have been explored. The coefficients of thermal expansion (CTEs) of the alloys have been experimentally determined over the temperature range 30-150 °C after isothermal aging at 125 °C for up to 30 days (720 h). The CTE values of SAC305, SAC387 and Sn-3.5Ag increase by 8-16% after 30 days of aging, while the CTE values of SnCu, SnCu-Ge and SN100C solders increase by only 3-6%. The CTE evolution of lead-free solders can be explained by microstructural changes observed during isothermal aging, which causes coarsening of various phases of the solder. As the phases coarsen, dislocation movement proceeds with a consequent increase in the average interparticle distance. The observation of CTE increases during isothermal aging suggests potential reliability problems for lead-free solder joints subjected to long-term aging exposures at high temperatures. Keywords
aging, CTE, lead-free solder, microstructure, reliability, thermal expansion
1. Introduction In microelectronic packaging, components often encounter temperature fluctuations which impose cyclic shear strains on the solder joint interconnect due to the coefficient of thermal expansion (CTE) differences between the printed wire board, solder and components (Fig. 1). The CTE measures the fractional change in size per degree change in temperature at a constant pressure. This leads to thermal fatigue and eventual failure of solder joints. As the CTE mismatch increases, the consequent shear strain decreases the thermal cycle lifetime of board components by weakening the solder joints at regions of high stress concentration where cracks typically start and propagate (Ref 1). CTE mismatch is the general cause of fatigue failure that results from cyclic loading. A measure of the linear CTE mismatch is given by the difference in the linear coefficient of thermal expansion of the contacting materials, the difference in temperature and the physical length of the component, expressed by: Du ¼ De L DT
ðEq 1Þ
Here Du is the thermal mismatch, De is the difference in CTE between the materials, L is the linear dimension of the component, and DT is the temperature change. To minimize solder joint cracks, it is necessary to minimize the thermal mismatch, which can be done by decreasing the difference in CTE, minimizing the temperature delta or reducing the size of the component. For high reliability, any mismatch of CTE must be accommodated by the solder joint (Ref 2, 3). This
article will focus on CTE changes observed in selected, contemporary lead-free solder materials used in microelectronics during long-term isothermal aging. The microstructure and mechanical behavior o
Data Loading...
