Effects of Element Partition on the Fatigue Life Predictions of Lead-Free Solder Joints
On the complicated manufacturing processes of the highdensity packages, the reliability of solder bumped IC chip on substrate and/or PCB has been confirmed by many researchers through thermal cycling tests, mechanical tests and computational modeling 1-5. In this study, the highdensity package, 256-pin plastic ball grid array (PBGA) on printed circuit board (PCB) subjected to temperature cycling with the effects of element spacing ratio on the thermalfatigue life of lead-free (95.5Sn-3.9Ag-0.6Cu) solder joints are investigated by using the validated finite-element analyses. This research studies the determination of thermalfatigue life, creep responses and creep strain energy density per cycle with the various spacing ratio (-1, -5, -10, and -15)of the meshed lead-free solder joint for a range of cycling temperatures. Temperature loads are applied to the models that represent the cycling environment of chamber. After starting at room temperature, the model is subject of 5 cycles consisting of 10-minute ramps and 10- minute holds at each of 0℃ and 100℃. Five cycles are executed in order to confirm the stabilization of the creep responses. For all the cases, the lead-free solder is assumed to obey the GarofaloArrhenius creep constitutive law.As shown in Figures 1 and 20, the 256-pin PBGA package has a 27 × 27 × 0.36 mm3 substrate with a 10 × 10 × 0.3 mm3 silicon IC chip and a 1.17 mm thick overmold. The package has 0.635 mm diameter pads with a thickness of 0.008 mm on the substrate and the assembled lead-free solder joints are assumed to have a height of 0.5 mm and a maximum diameter of 0.9 mm. The PCB thickness is 1.6 mm. Figure 3 shows the 3-D finite element strip model that captures the construction along a diagonal strip from the 256-pin PBGA lead-free assemblys geometric center to a corner. It also provides a close-up view of the solder joint having the fine mesh along with the other coarser mesh due to its location further away from the chip. Due to the relatively high chip stiffness and the flexibility of the substrate, the solder joint closest to the chip will likely fail first in PBGA packages 2. As such, mesh refinement was applied to the solder joint that is predicted to fail first. The finite element strip model has 6172 elements, of which 1920 elements are in the refined ball and 24 elements are in each of the other coarse balls. The Solid185 in ANSYS 6 is used to construct the 3-D solid models where each node has three degrees of freedom (translations in the nodal x-, y-, and zdirections).For investigating the influence of the element partition on the fatigue life predictions of lead-free solder joints, as shown in Figure 4, the four spacing ratios (-1, -5, -10, and -15) of the meshed solder joint are used. For example, the difference of the shear stress time-history, creep shear strains, shear stress vs. creep shear strain hysteresis loops and creep strain energy density are shown in Figures 5-8 respectively among the four spacing ratios under five temperature cyclic loadings. These results should be useful for a better understanding of the thermal-mechanical behaviors of the lead-free solder joints,due to the thorough studies of various spacing ratio of element meshing in a high-density package assembly. The effects of the various spacing ratio of the refined solder joint on the reliability assessment are studied under the temperature cyclic loading further 4,5.
Wen-Ren Jong Hsin-Chun Tsai Ching-Tzu Huang
Chung Yuan Christian University, Dept.of Mechanical Engineering, Chung-Li, Taiwan, China;R&D Center Minghsin University of Science and Technology, Dept.of Mechanical Engineering, Hsin-Chu, Taiwan, Chi Chung Yuan Christian University, Dept.of Mechanical Engineering, Chung-Li, Taiwan, China
国际会议
第八届电子封装技术国际会议(2007 8th International Conference on Electronics Packaging Technology ICEPT2007)
上海
英文
617-618
2007-08-14（万方平台首次上网日期，不代表论文的发表时间）