WLCSP器件结构优化模拟及无铅焊点可靠性

Optimum simulation and soldered joints reliability of WLCSP device

构建WLCSP144器件四分之一模型,研究无铅焊点阵列的应力-应变响应.结果表明,焊点应力-应变大小和焊点的位置有密切关系,中心焊点的应力-应变最小,拐角焊点的应力-应变最大,应力和焊点位置的关系为σ(x,y)=1.78x+1.78y+0.33,焊点蠕变应变和焊点位置的关系为ε(x,y)=0.006x+0.006y+0.009.同时发现焊点可靠性与器件结构有明显关系,在结构中选择聚合物作为封装材料对应的焊点可靠性较低,应该选择与PCB线膨胀系数较为匹配的材料最为适宜.焊点阵列数(焊点间距相同)的增加会降低焊点的可靠性,这主要是和阵列拐角焊点与芯片中心的距离有明显的关系.

A quarter of WLCSP device was selected for finite element modelling and the stress-strain response of lead-free soldered joints was investigated systematically.The stress of soldered joints is variational with different positions in WLCSP array,the lowest stress of soldered joints appears in the center of array,the largest stress can be found in the outmost solder joints.The relationship between stress and position of soldered joints is as follows:σ(x,y)=1.78x+1.78y+0.33,for creep strain,the correlation is:ε(x,y)=0.006x+0.006y+0.009.Moreover,it is found that the reliability of soldered joints can be influenced by the materials in WLCSP structure,the lower reliability of soldered joints can be found in WLCSP device with polymers than that without it,so it is suitable to select the materials with matched CTE to be utilized in those devices.The reliability of WLCSP device soldered joints decreases when the pin count increases.The higher pin count means larger distance from the outmost soldered joint to the chip center at constant pitch.