LGA焊点形态对焊点寿命影响的有限元分析

通过Surface Evolver软件对LGA焊点进行了三维形态预测,利用有限元数值模拟对LGA焊点在热循环条件下寿命进行了分析。研究了热循环条件下LGA焊点的应力应变分布规律,随着焊点远离元件的中心位置焊点所受到的等效应力、等效应变和塑性应变能密度逐渐增大,从而得出处于外面拐角的焊点最先发生失效的结论。基于塑性应变范围和Coffin-Manson公式计算了焊点热疲劳寿命;找出了LGA焊点形态对焊点寿命的影响规律,模板厚度一定时PCB焊盘尺寸小于上焊盘时LGA焊点的热疲劳寿命与PCB焊盘尺寸成正比,大于上焊盘时成反比,大约相等时焊点寿命最大。当PCB焊盘和模板开孔尺寸固定时,通过增大模板厚度来增加焊料体积在一定程度上可提高LGA焊点的热疲劳寿命,但是模板厚度增大到一定值时LGA焊点寿命会逐渐降低。

FEM Analysis of Effect of LGA Solder Joint Shape on Lifetime of Solder Joints

Predict the 3D shape of LGA solder joints with Surface Evolver software, and analyze the lifetime of LGA solder joints under thermal cycling though the fnite element numerical simulation. Also study the stress and strain distribution law of LGA solder joints during thermal cycling. So it can conclude that the equivalent stress, equivalent strain and plastic strain energy density of solder joints increase gradually with the solder joints far away from the center of component. Then it fnds that the failure mostly happen at the out edges of solder joints. Based on the plastic strain range and Coffn-Manson formula, it calculates the thermal fatigue life of solder joints. It also fnds the infuence rule of LGA solder joint shape to the lifetime. Keeping the stencil thickness invariant, when the bonding pad size is smaller than the upper pad, the thermal fatigue life of LGA solder joints is proportional to PCB bonding pad size, when the bonding pad size is bigger than the upper pad, the thermal fatigue life of LGA solder joints is inversely proportional to PCB bonding pad size when they are equal, the fatigue life will reach the longest. Keeping PCB bonding pad size invariant, increasing the stencil thickness can improve the fatigue life in some way. But stencil thickness cannot exceed a fxed value, or the fatigue life will reduce slowly.