Analysis of elastoplastic thermal stresses and creep of surface mount solder joints
Student thesis: Master's Thesis
Solder joint integrity has been a subject of major investigation ever since the beginning of surface mount technology (SMT) in 1970s. SMT has grown enormously with the pace of silicon technology, in which materials utilization plays an importmt role an the miniaturization of electronic products. However, due to the cyclic operation of a component at a relatively high temperature compared with the low melting point of solder alloy, the joint suffers from both thermal fatigue and creep effects which severely reduce the service life of an assembly. In addition, the microstructural change of the solder due to intermetallic formation with the underlining metal substantially alters the solder properties in such a way that the normal stress-strain relationship behaves differently. The present work was set up to study the elastoplastic and creep behaviour of surface mount (SM) solder joints. To compare with the difference between leaded and leadless joints, a plastic Quad Flat Pack (PQFP) SM assembly was also analyzed. A leadless 1206 surface mount ceramic resistor was investigated using the Finite Element Method with various solder joint shapes. A thermal stress analysis incorporating the Ramberg-Osgood equation to describe the elastoplastic properties of the solder was carried out. Heating in the assembly was generated by the power supply in the resistor. The resulting stress patterns were obtained and the locations of high stress regions were correlated with the predicted failure position obtained by past investigators. A creep analysis was also carried out for the same assembly. In this analysis, a hyperbolic creep equation taking into account the temperature and grain size effects was used. It was observed that stress relaxation process took place at various locations inside the solder joint. The strain rate sensitivity of 63Snl37Pb and 60SnI40Pb solders were also compared. The latter seemed to be less sensitive to the strain rate at the same level of applied stress. Finally, a PQFP Integrated Circuit which was surface mounted on a FR-4 PCB was also investigated by an elastoplastic analysis. The assembly was found to produce a dome like deformation shape at each of the four different applied power levels. Apart from studying the effect of power level on the deformation and stress pattern in the assembly, the variation of stress with the IC leg configuration was also investigated. Using the calculated forces and moments on the IC legs, the stresses and displacements of the "G", "J" and "L" leg configurations were studied. The G- and J-legs were found to bear the smallest maximum stresses in the leg and in the solder respectively.
- Surface mount technology, Solder and soldering