Solder joint reliability of plastic ball grid array (PBGA) and bottom-leaded plastic (BLP) assemblies.

摘要

The plastic ball grid array (PBGA) and the bottom-leaded plastic (BLP) packages have attracted substantial attention since their appearance in the electronics industry. Since the solder materials have relatively low creep resistance and are susceptible to low cycle fatigue, the life of solder joints under cyclic thermal loading is a critical issue for the reliability of PBGA and BLP packages. The present study established a finite element model for the analysis of solder joint reliability under thermal cyclic loading. An elastic-viscoplastic constitutive relation was adopted for solder materials in the modeling and analysis.; In this thesis, a 400-pin full-grid PBGA assembly was modeled to investigate the effects of variation in material properties, the lifetime analysis index and various temperature profiles on the fatigue life of solder joints subjected to cyclic thermal loading. The creep strain range per cycle and the creep strain energy density per cycle were evaluated and incorporated into the modified Coffin-Manson equation to estimate the fatigue life under thermal cycling. It is found that two indices may lead to opposite conclusions in some situations. It is also found that the least cycles to failure do not correlate to the shortest lifetime. Therefore, the testing period of thermal cycling could be optimized by choosing an appropriate temperature profile. The result of this study will help the reliability engineers to determine the most efficient testing specifications for solder joint reliability.; In this thesis, a 28-pin BLP assembly is modeled to investigate the effects of various epoxy molding compound, leadframe materials, the thickness of the PCB, the shape of solder joint and the solder pad size on solder joint reliability. The fatigue life of solder joint is estimated by the modified Coffin-Manson equation. The two coefficients in the modified Coffin-Manson equation are also determined. A new design for lead is also evaluated by using finite element analysis. Parametric studies are performed to investigate the dependence of solder joint fatigue life on various package materials, the thickness of the PCB, the shape of solder joint, and the solder pad size.; This thesis also presents a new life prediction model for solder joint reliability. This method not only considers the damage evolution in the solder, but also saves substantial computational efforts. In the present model, the evolution of damage is determined by the collapse of hysteresis loops of shear stress-strain of solder joints in the double-beam joint specimen. The agreement in results between FEM and the experiment is very encouraging. This method is used to estimate the solder joint reliability of PBGA and BLP assemblies. The proposed model is validated by experimental testing data.