IMPACT Your Future : Integration , Efficiency & the Eco-Friendly 423 Designs of Temperature Loads on the Fatigue Life Analysis of Lead-Free Solder in IC Packages

Firstly, finite element analysis software (ANSYS) is used to investigate the accumulated creep strain range and accumulated creep stain energy density properties of lead-free solder (95.5Sn-3.9Ag0.6Cu) in IC packages under the temperature cyclic loading by the Garofalo-Arrhenius hyperbolic sine law. Then the Taguchi method is further used to analyze the effects of IC package models on the fatigue life subjected to the temperature cyclic loading. The five main factors are the high and low temperature dwells, the temperature ramp rate, and the dwell time of both high and low temperatures, respectively. Moreover, each factor has four levels. According to the L16 orthogonal arrays of the Taguchi method, three different IC packages PBGA-256, PBGA-388 and CCGA-1657 are used as the noise factors. Furthermore, the Schubert’s model is used in Taguchi method and ANOVA analysis is to find the main influence factors on the fatigue life and reliability of lead-free solders. It is thus expected to provide reference data for the setting lead-free solder temperature cycle testing criteria in the future. The results show that when applying accumulated creep strain range on fatigue life and reliability, the main influence factors are the temperature range. On the other hand, when applying accumulated creep strain energy density on fatigue life and reliability, the main influence factors are the low temperature dwell and temperature ramp rate. The main reason for this difference is that the accumulated creep strain energy density is obtained from calculating the surrounded area by the hysteresis loops of creep stress and creep strain. PREFACE In thermal fatigue analysis, experiments that use temperature cycle properties such as ramp rate, temperature range and temperature dwell time influence factors as the subject have always been discussed and studied. In 2002, by applying the micro surface mounted devices (SMD) of 18 solder joints, Pitarresi [1] used two kinds of temperature cycle loads (-25 C ~ 125 C and -40 C ~125 C ), according to the testing of mold samples, to arrange different ramp rates, high temperature dwell time and low temperature dwell time and also to compare them with the finite element method. The results showed that bigger temperature range and shorter ramp time decreases the fatigue life. In 2005, Lau [2] used five different high and low ramp rates (0.5, 1, 5, 10 and 15 minutes); for all the cases, the dwell time at peak cycling temperatures (0 C , 100 C , -25 C and 125 C ) is 15 minutes. It analyzes the PBGA mold assemblies under two different temperature ranges (0 C ~100 C and -25 C ~125 C ) which comparing the effects of thermal shock and thermal cycling on the reliability of the fatigue life of lead-free solder joints in different temperature cycles. The results showed that, under the circumstance of thermal shock, stress relaxations easy occur for the lead-free materials. Therefore, when the ramp time is shorter, the creep strain energy density resulted from the thermal shock is higher than that of the thermal cycling. In 2005, Chaparala [3] studied the reliability of solder joints of wafer level chip scale package (WLCSP) assemblies under the thermal cyclic loading and discovered that longer ramp time causes internal stress relaxations. On the other hand, shorter ramp time causes lower creep strain energy density, thus increasing the fatigue life of lead-free alloy materials. In addition, when the temperature range is widened, the fatigue life will be reduced. In 2008, Jong [4] applied the finite element analysis software and the Taguchi method to study five temperature factors which are high temperature dwell, low temperature dwell, dwell time of high temperature, dwell time of low temperature and ramp rate and compared them using different alloys as the noise factors. The results showed that, based on the shear creep strain range from those analyses, the effects of high temperature dwell is greater than that of low temperature dwell under the circumstance of temperature cyclic loads. The temperature cycle criteria of lead-free solder joints set up by IPC-SPVC-WP-006 (A White Paper Report by the Lead-Free Technical Subcommittee of the IPC Solder Products Value Council, July 2003) almost follows the information of traditional tin-lead (SnPb) solder joints. Nevertheless, in electronic micro systems assembling, Sn-Pb and lead-free solder joints have the difference in the material properties. The melting point of binary and trinary alloys Sn-Ag/Sn-Ag-Cu is about 30 C higher than that of Sn-Pb, which would cause major discrepancy in the plastic and creep strain behaviors. Therefore, in this study, the finite element software is applied to study the accumulated creep strain and the accumulated creep strain energy density characteristics of lead-free joints (95.5Sn3.9Ag-0.6Cu) of IC assemblies under temperature cyclic loading. Then, Taguchi method is applied as well to analyze the effects of the IC package models on the fatigue life of lead-free solder joints after temperature cycle being designed. Five main temperature load factors are planned, and each has four levels for the L16 orthogonal arrays in Taguchi method. Also, the fatigue life equations formulate from the experiment data by Schubert and the fatigue life number is calculated. Finally, ANOVA analysis is used to find the effect of main influence factors on the lead-free solder joints under the temperature cyclic loading. THEORY OF ANALYSIS Quality Characteristics and the Design of Parameters [5] In Taguchi method, various engineering problems have different quality measure methods, meaning that their S/N ratio will be different. The three types of quality characteristics are commonly seen: (a) Nominal-the-best, as shown in Equation (1); (b) Smallerthe-better, as shown in Equation (2); (c) Larger-the-better, meaning the bigger the assessed value, the better the quality characteristic, as shown in Equation (3). Nominal-the-best 2 2 10 log 10 S y N S NTB (1) 978-1-4244-4342-0/09/$25.00 ©2009 IEEE IMPACT Your Future: Integration, Efficiency & the Eco-Friendly 424 Smaller-the-better n i i STB y m N S 1 2 10 1 log 10 (2)