Failure Performance And Mechanism Of Board-level Sn-Ag-Cu/Cu Solder Interconnects Under The Coupling Of Multifields Loading

As automotive electronics, mobile electronics and high-power integration electronic equipments are towards the high density, minaturization and multifuction, solder interconnect within board level packaging component are under more and more harsh condition. Solder interconnects often experience thermal,mechanical,electrical loading simultaneously during the actual service. The coupling loadings not only affect PCB responsesfeatures but also solder failure mechanism. And the PCB responses also influence the solder interconnects failure. Therefore, the lead-free Sn-Ag-Cu solder failure mechanism and PCB responses features are studied under the thermal-mechanicaland thermalmechanical-electrical coupling loadingin this paper.The strain, frequency, acceleration data of PCB are measured under thermal, electricity, mechanical coupling condition. The PCB frequency and strain response features under the coupling of temperature-vibrationelectricity(T-V-E) loadings are acquired. Results indicate that the 1st eigenfrequency of PCBdecreases with temperature increase from 25 oC to 105 oC, during which the peak strain amplitude is almost the same under the temperaturevibration(T-V) coupling loading.Under the coupling of T-V-Eloadings, electricity loading changs PCB temperature distribution, which decreases not only the 1st eigenfrequency of PCB but also the the peak strain amplitude from 0.001 to 0.00078. The results reveal the PCB loading complexity under multi-fields coupling condition.The solder interconnects life, failure modes and mechanism under the coupling of T-V loadings are studied using statistical analysis methods. Results show that solder joint life subjected to vibration loading is increased with the temperature increase from 25 oC to 100 oC at the power spectral density(PSD) of 1.55(m/s2)2/Hz due to the improvement of solder plasticity. The statistic analysis of failure modes indicate that the crack propagation site gradually changes from the interface of intermetallic compound(IMC)/Cu pad to the bulk solder with temperature rise. Fracture mechanism is transformed from brittle fracture to ductile frature. The T-V coupling test with different excitation levels is designed. Effect of excitation level on solder joint life and failure mode is studied. Results indicate that the tendecy of effect of temperature on solder life and failure mode becomes insensitive graudallywith the increase of PSD(2.34(m/s2)2/Hz,3.35(m/s2)2/Hz,4.55(m/s2)2/Hz). At the PSD of 2.34(m/s2)2/Hz, soler joints life decreass with temperature increase. At the PSD of 3.35(m/s2)2/Hz and 4.55(m/s2)2/Hz, effect of temperature on solder life becomes not obvoius.The statistical analysis of failure mode indicate that the primary failure modes are the mixed crack(crack generates in the IMC/Cu and bulk soler) and the bulk solder crack(crack generates in the bulk solder) under the T-V(PSD=2.34(m/s2)2/Hz,3.35(m/s2)2/Hz,4.55(m/s2)2/Hz) coupling loading. The effect of temperature on solder failure mode becomes also not obvoiusdue to the overstress. The vibration loading strength dominates the solder interconnects failure.The failure features under the real-time and sequence T-V coupling loading are compared. Results indicate that the mechanical strength of bulk solder caused by microstructure coarensing after thermal shock aging decreases, which leads to the bulk solder crack initiation and the reduction of solder joint life. The thermal shock loading accelerates the solder interconnects failure. For the T-V real-time coupling test, the improvement of bulk solder plasticity leads to the decrease of interface stress level resulting in the bulk solder crack initiation. The solder joint failure mechanism under the sequence couping loading are remarkably different from that under the real-time coupling loading.The solder joint failure tests are designed under the T-V-E coupling loading. The effect of three fields coupling factors on solder joint failure performance is studied. Results indicate that electricity current loading causes the PCB strain to decrease resulting in the reduction of vibration loading strength, which improves the solder interconnects life. The failure modes under the T-V-E loading are maily the bulk solder crack and mixed crack, exhibiting the ductile fracture performance. The electric field loading strength is enhanced with the increase of current density from 0.7×104A/cm2 to 0.85×104A/cm2 to analyze solder joint failure performance. Results indicated that solder joint failure does not ocurre in an hour. The obvoius electro-migration phenomenon occurs within the solder joint. The decrease of PCB strain and the liquid state of solder joint dominate the solder joint failure performance. And the decrease of ambient temperature makes the 1st natural frequecny of PCB increase resulting in the increase of PCB srain, which leads to solder joint failure.And the life prediction models under the the coupling of T-Vand T-V-E loadings are built respectively. The life subjected to vibraiton loading and temperature exhibts the linear function relationship, while the life under the coupling of T-V-Eloadings and temperature exhibits quadratic function relationship.