Accumulated creep strain and energy density based thermal fatigue life prediction models for SnAgCu solder joints

Abstract

Pb free solder is fast becoming a reality in electronic manufacturing due to marketing and legislative pressures. The industry has pretty much concluded that various version of SnAgCu solder alloy offer the best alternative for eutectic SnPb solder currently in use. With the current trend of cheaper, faster, and better electronic equipment, it has become increasingly important to evaluate the package and system performance very early in the design cycle using simulation tools. This requires life prediction models for new solder alloy systems so that the package-to-board interconnect reliability can be predicted for various environmental and field conditions. This paper describes in detail the life prediction models foi SnAgCu solder joints. The models are based on published coiistitutive equations for this alloy and thermal cycle fatigue dai a on actual components. The approach uses advance finite element modeling and analysis techniques and is based on mechanics of deformation. Both accumulated creep strain and creep strain energy density based models are developed. The model has been correlated with a number of data points and predicts life within 25% in most cases. The framework of modeling and prediction methodology described here is fully coinpatible with the framework used for SnPb solder previously. In1 roductionThe reliability of solder joints is one of the most important factors when selecting a package for a particular application. Thle CTE and stiffness mismatch between the package and the board results in thermal stresses in solder joints during temperature and power cycling. The damage caused by these stresses accumulates as the electronic assembly is subjected to multiple cycles, ultimately causing failures of solder joints. This is a very well documented failure mode for electronic assemblies and a wealth of data is available in the literature for SnPb solder. Due to this reason, the reliability of Pb free sobier joints is an important factor for selecting the proper replacement of SnPb solder. Based on various studied conducted, the industry as a whole has converged towards SnAgCu solder alloy (with different compositions) to replace SnPb solder from electronic assemblies.The reliability of SnAgCu solder joints has been a subject of major research in electronic industry and a number of researchers have published data [l, 2, 31 showing SnAgCu performs better or worse than SnPb solder, depending on the conaponents tested and test conditions employed. While more test data is being gathered under accelerated test conditions, it is also becoming apparent that this will not be enough due to rapid implementation of this soldering system. Today, electronic industry uses electronic components using lead frame or laminate technology with countless number of packages in various lead counts, leadhall pitches, and die sizes. Since every component has a different interconnect reliability behavior, it is unrealistic and cost prohibitive to generate test data for every case. The interconnect reliability also dep...

A review of photovoltaic module technologies for increased performance in tropical climate

“…Consequently, the interconnection surfers delamination, crack initiation, propagation and rupture [126], [137]- [142]. Interconnection degradation has been reported by [117][143], [144] as causing over 40% of PV failures and this figure is greater in the tropical climate. Thus, there is the need to mitigate this challenge by investigating interconnection degradation mechanism to increase prediction accuracy of lifecycle of solder interconnections in various environments and climates.…”

“…More and more lead-free solders are being developed to substitute lead based solders [141], [142], [150]- [152]. The tin-silver-copper (SnAgCu or SAC) solder alloys have been reported to be the best alternative for eutectic SnPb solder in the PV industry [144], [153]. Section 4.1.2 focuses on the thermo-mechanical reliability of mainstream SAC solder alloys which include SAC305, SAC405 and SAC387.…”

“…In the PV industry, lead based alloys were used for various interconnection technologies [147]. Currently, there is a shift from lead-based alloys to lead-free alloys because of the environmental impacts of lead, globally [144], [65], [148], [149]. More and more lead-free solders are being developed to substitute lead based solders [141], [142], [150]- [152].…”

Creep damage study at powercycling of lead-free surface mount device

“…where N f is the number of cycles to failure (fatigue life), C'= 0.0468 [5], ε acc is the accumulated inelastic strain range per cycle. Fatigue life for both cases is calculated based on the strain range in the fifth power cycle (see Table 5).…”

“…One among the most commonly used is the strain-range based Coffin-Manson method. The following CoffinManson equation [5] is used to calculate the fatigue life based on the inelastic strain range obtained from the creep analysis:…”