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Comparison of thermal fatigue reliability between SAC and SnPb solders under various stress range conditions

Authors Yang, Chaoran
Issue Date 2009
Summary A general idea of thermal reliability of the tin-lead (SnPb) solder and the tin-silver-copper (SAC) solder family is that the SAC solders give a better performance under thermal cycling conditions due to its higher creep resistance. Some researchers argue that the priority will be reversed in a more constraint situation. This statement is derived by the material properties of SnPb and SAC solders. Various creep constitutive models proposed by different researchers show that although the SAC solders have a smaller steady state creep strain rate than the SnPb solders when the stress is low, but their creep strain rate increases more sharply when the stress level increases. This results in a higher creep rate for SAC solders after the stress reaches to a certain level. This reversed result implies that if the condition at the solder joint becomes more and more constrained, the Accelerated Temperature Cycling (ATC) test performances of SAC and SnPb solders should get closer and closer, and finally intersect each other after a certain point. Meanwhile, some results show that in some package configurations in which the solder joint have a severe condition, such as CSP, chip resistor, and Flip Chip, ATC test do exhibit a better results for SnPb solders. But in these cases, the outperformance of SnPb solders mostly happened after they changed the temperature profile, for example, by enlarging the temperature excursion range. However, the temperature factor which will also influence the creep property was involved. Therefore, there is no direct evidence to connect the stress level and the ATC reliability performance of SAC and SnPb solders. And this is where the objective of our research work presented in this paper lies. In the present study, Sn4.0wt.%Ag0.5wt.%Cu (SAC405) and eutectic Sn37wt.%Pb (SnPb) solders were used. A specific ATC test vehicle was introduced. The structure of the proposed package was one-dimensional in nature. Different Distances-from-the-Neutral-Point (DNPs) of the solder joint could be created by simply changing the solder joint spacing, thus different stress conditions can be produced at the solder joint. Based on this concept, two kinds of packages, plastic dummy packages and ceramic dummy packages, with various solder joint spacings were designed and manufactured. In addition, the plastic dummy package included two batches of samples with different solder joint pitches. After the assembly, the initial condition inspection was carried out. The selected temperature cycling profile was from -40℃ to 125℃ with one hour cycle time. From the ATC results, it is found that, for the ceramic dummy package, the reliability performance of solder joint decreases with the increasing of solder joint spacing. For the 4x pitch solder joint spacing samples, the SAC samples survived longer than the SnPb samples, while for the 12x pitch solder joint spacing samples, the SnPb solder scores higher. However, for the plastic dummy packages, no obvious DNP effect is found, as the failure life does not always decrease when the solder joint spacing increases. This is due to the warpage induced by the coefficient of thermal expansion (CTE) mismatch between the silicon layer and the BT substrate. The warpage changed with the temperature during the ATC test, thus impose an addition mechanical loading on the solder joints. For the ceramic dummy package, no warpage issue was involved. Therefore, the DNP effect was clearly observed. The finite element analysis of thermal fatigue reliability of both plastic dummy package and ceramic dummy package was conducted. The one-dimensional nature of ATC sample was first investigated by comparing the four-ball model with the two-ball model and six-ball model. It can found that the solder joint equivalent creep strains of the three models are similar to each other. Therefore, the deformation in the transverse direction of four-ball model can be neglected. The equivalent creep strains of both the plastic dummy package models and ceramic dummy package models with different solder joint spacings were also compared by using finite element simulation. Similar results that the cyclic equivalent creep strain increases with the increasing of the solder joint spacings, but with different rates, can be observed. However, after converting the cyclic creep strain into thermal fatigue life using empirical life prediction equations, discrepancies between the accumulated creep strain and ATC reliability performance are found. These discrepancies are due to the uncertainties in the curve fitting parameters used in the life prediction equations.
Note Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2009
Language English
Format Thesis
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