在线测量Sn-3．5Ag焊点蠕变的电阻应变

In situ measuring electronic-resistance strains of Sn-3.5Ag solder joints creep

以焊料(Sn-3.5Ag)在薄铜基片之间制作截面积约为1 mm2,厚度分别为0.42 mm和0.10 mm的试样(其面积与电子封装中的无铅焊点面积大体相同)为对象,利用特制的电子测试系统实时、在线测量试样焊点的微电阻及其剪切应力,并通过串行接口将相关数据传输至计算机。数据经计算机处理后,拟合成实时微电阻和测试时间的关系曲线。基于经典的Griffith断裂模型,建立一个简易数学模型,从理论上论证电阻应变与焊点机械蠕变裂纹之间的关系。研究结果表明:该关系曲线反映焊点的裂纹连续生长、蠕变失效过程,变化趋势与经典结果吻合;在24 MPa应力作用下,厚度为0.42 mm的试样最大电阻应变为0.18,最大电阻应变率为2.2×10-3 s-1;在25 MPa应力作用下,厚度为0.10 mm试样的最大电阻应变为0.036,最大电阻应变率为3.5×10-3 s-1,实验持续时间较厚度为0.42 mm的试样短。

Single shear lap creep specimens with a 1 mm2 cross sectional area and thickness of 0.42 mm and 0.10 mm, respectively （similar in size to small lead-free solder joints used in electronic packaging and jointing） between thin copper strips were fabricated using lead-free solder （Sn-3.5Ag） to quantify their creep strains with in situ micro electronic-resistance measurement. The solder joints＇ micro electronic-resistance and stress were in situ measured by a tailor-made electronic testing system and recorded by a PC via serial port, then all data of micro electronic-resistance and elapsing time were displayed in curves. The results show that the relationship between microelectronic-resistance and testing time reflects the continual development of damage and fracture mechanisms. Under a shear stress of 24 MPa, the maximum value of electronic-resistance strain of specimen with thickness 0.42 mm is 0.18. The maximum rate of electronic-resistance strain measurement is 2.2×10^-3s^-1. Under a shear stress of 25 MPa, the highest electronic-resistance strain value of specimen with thickness 0.10 mm is lower, which is 0.036. The maximum rate of electronic-resistance strain measurement is 3.5×10^-3s^-1.