Abstract
The criteria of mechanical reliability in solder joints can be identified and described by comparative evaluation via drop test and high speed pendulum impact test. Systematic samples of assembly and attachment joints with various Pd additions were employed and investigated in this study. The statistical values of mechanical performances were calculated and compared. Better high speed impact performance of SAC305/ENEPIG attachment joints with 0.06 μm Pd layers was confirmed owing to the single Cu6Sn5 phase growth. However, the comparative measurement of the better performance on drop testing exhibited in ENEPIG/SAC305/immersion Sn assembly joints with 0.1 μm Pd layers deposit resulted from the thinner and layer-type IMC growth. It is expected that through comparison between impact and drop test in mechanical reliability, a criterion of joints reliability can be established. Besides, the optimal Pd layer deposit for the ENEPIG surface finish in the attachment and assembly solder joints was demonstrated and confirmed. Most failure regions existed at interface near PCB (Cu) side during mechanical drop testing due to the stress accumulation. The correlation between the cracks generation and Pd addition was established on the basis of the elemental X-ray color mapping via Field-Emission Electron Probe Microanalyzer (FE-EPMA). In the ENIG and ENEPIG assembly joints, cracks propagation might travel toward the weakest interface, which located at interface between Cu6Sn5 and Cu3Sn phases during drop testing. The IMC morphology and thickness might dominate the cracks initiation and propagation. Joints with layer-type and thinner IMC growth after thermal treatment revealed the longer crack propagation in ENEPIG (Pd 0.1)/SAC305/immersion Sn assembly joints. The better drop performance were confirmed in the ENEPIG (Pd 0.1) assembly joints. Besides, the crystallographic orientation of IMCs and β-Sn in the ENIG and ENEPIG assembly joints was investigated. With the aid of EBSD analysis, various grain structures and preferred growth orientation of IMCs and β-Sn were observed. The distinctive growth behaviors of intermetallic compound on the ENEPIG UBM and immersion Sn substrates were associated with the cross-interaction of minor Cu, Ni and Pd elements. To verify that the thermal and mechanical influence would affect the β-Sn grain variation, the ENIG and ENEPIG assembly joints were employed. It is noted that Pd elements might stabilize the grain orientation of β-Sn, inhibiting the grain refinements, recrystallization and deformation during the thermal and mechanical stressing impact. Especially the single β-Sn grain texture with parallel c-axis to the substrate at as-reflowed stage was probed and demonstrated. Finally, the correlation between microstructure variation and grain orientation was investigated and discussed. The possible mechanism was also proposed.