The mechanical properties as well as the reliability of a solder, which is the primary interconnect of electronic assemblies, are strongly influenced by its microstructure, which is controlled by its thermal history including its solidification rate and thermal exposures after solidification.  Lead free solders are common as interconnects in electronic packaging due to their relatively high melting point, attractive mechanical properties, thermal cycling reliability, and environment friendly chemical properties.  However, environmental conditions, such as, operating temperature, aging temperature, and aging time significantly affect these properties due to the microstructural evolution of the solder that occurs during aging.  Moreover, electronic devices, sometimes experience harsh environment applications including well drilling, geothermal energy, automotive power electronics, and aerospace engines, where solders are exposed to very high temperatures from T = 125-200 °C.  Mechanical properties as well as microstructural study of lead free solders at elevated temperatures are limited in literature.  Previous investigations on the microstructural evolution mainly emphasized on aging at temperatures up to 125 °C.  In addition, those studies were limited on investigating the coarsening of Ag~3~Sn IMC particles within the beta-Sn matrix.

 

In this work, the microstructural evolution of SAC305 BGA joints was investigated for different aging conditions utilizing Scanning Electron Microscopy (SEM) to better understand aging induced degradations.  In particular, our approach has been to monitor aging induced microstructural changes occurring within fixed regions in selected lead free solder joints and to create time-lapse imagery of the microstructure evolution.  Aging was performed at T = 125, 150, and 175 °C for up to 20 days, and the topography of the microstructure of a fixed region was captured using the SEM system.  This process generated several images of the microstructure as the aging progressed.  As expected, the analysis of the evolving SAC305 BGA microstructure showed a significant amount of diffusion of silver and copper in the beta-tin matrix during aging.  In addition, a very remarkable growth of copper-tin layer at solder joint and copper pad interface in the PCB side has been visualized and quantitative analysis has been performed with aging time and temperature. The Quantitative analysis of the evolving microstructure showed that the particles coalesced during aging leading to a decrease in the number of particles.  This caused an increase in the average diameter of the particles, which helped us to build a model to guide the growth of IMC particles at extreme high temperature aging.


Time-Lapse Imagery and Quantitative Analysis of Microstructural Evolution of Sac305 Bga Joints During Extreme High Temperature Aging

Division / Sponsors: 
Electronic & Photonic Packaging Division (EPPD)

Paper Proceedings: https://asmedigitalcollection.asme.org/InterPACK/InterPACK2020/volume/84041

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