Research On The Thermal Fatigue Reliability And Interfacial Reactions Of Plastic Ball Grid Solder Joints

Driven by competition and consumers’ demand, the electronics industry continues to use smaller size components for efficient printed circuit board (PCB) space utilization. As a result of the demand, the surface mount soldering technology (SMT) has become one of the most common solderings. Surface mount technology offers opportunities for significant cost reduction due to efficient production line automation. This technology uses surface mount components which are soldered directly to the solder pad of a printed circuit board (PCB). Consequently, the solder interconnect is the only mechanical means of connecting the components to the PCB. As the integration of electronic products continues to increase, more and more electronic devices are packaged in a small space, and the stress-strain and heat flux density in the interconnecting solder joints are rising, and the performance of solder joints have strongly mechanical size effects with reducing the size of the joints. As a result, the reliability of SMT solder joints has become a major concern. Insight into the mechanical behavior of solder interconnects during macufacturing processes and service is essential to product design optimization, component selection and refining manufacturing process refinement. In this study, the reliability analysis of PBGA Sn-3.8Ag-0.7Cu solder joints was investigated.Interfacial reactions on PBGA Cu-SnAgCu-Ni(P) solder joints were intestigated under the accelerated thermal cycling test. The results showed that a coupling effect between the two different interfaces was discovered in the solder balls. The metal atoms of Cu from OSP Cu layer tended to diffuse to the opposite interface of Ni(P) layer, increasing the growth rate of ternary intermetallic compounds. However, the growth of Cu6Sn5and the formation on the Cu interface were greatly suppressed as the result of the diffusion of Ni atoms from ENIG Ni(P) layer. The results also showed that the weakest area of solder interconnects was interfacial layer near PBGA side. The solder joints at corner positions were more vulnerable to crack failure under the thermal cycling test.Finite element method using the ANSYS transient thermal-structural coupling function and the element "life and death" technology based on the thermal elastic-plastic theory was performed to study the mechanical behaviors of PBGA solder interconnects during cooling process of reflow soldering. The soldering stresses and deformation were introduced by the non-uniform temperature field due to the material differences in the thermal properties. The results showed that the soldering crack most likely occured after the initial solidification of solder balls during the cooling process of the reflow soldering.A computational simulation using a unified Anand’s viscoplastic constitutive model was employed to investigate the thermal fatigue performance of the solder interconnects under thermal cycle loading. The results showed that the interfaces between solder and PBGA pad were the highest stress concentration regions under cyclic temperature loading. The corner solder joints in the PBGA module under thermal cyclic loading suffered greater stress and strain. The peak stress placed at the beginning of lower temperature dwell time, and peak strain placed at the end of higher temperature dwell time per each cycle. The equivalent cyclic stresses versus time have obviously accumulative effective because of existing residual stress. The intermetallic layer played a critical role in the overall thermal fatigue life of BGA solder joints due to the brittle property of interfacial intermetallic compounds (IMCs). The equivalent plastic strain of the solder joints near the IMC layer was larger significantly than non-IMC solder joints because of the properties of the brittle intermetallic. It can be found that the fatigue life of IMC solder joints was shorter than that of non-IMC solder joints.In this paper, a methodology was presented to include key uncertainties in the analysis of lead-free solder reliability. Based on the widely used Engelmaier C-M (Coffin-Manson) model, a probabilistic approach for predicting the thermal fatigue life was developed with consideration of solder joint geometric parameters and temperature. The effect of each key uncertainty due to the manufacturing and users’ diversities to the thermal fatigue life in the probabilistic analysis has been formulated. This information can be used on implementing an overall optimizations including:product design, component selection, process criteria, etc. The method provided the information for understanding how uncertainties impact the thermal reliability in electronic packaging with ball grid array interconnections.