Sn3.0Ag0.5Cu/Cu无铅焊点剪切断裂行为的体积效应

VOLUME EFFECT OF SHEAR FRACTURE BEHAVIOR OF Sn3.0Ag0.5Cu/Cu LEAD-FREE SOLDER JOINTS

采用直径范围为200—600μm的Sn3.0Ag0.5Cu无铅钎料球在Cu焊盘上制作热风重熔焊点,将重熔焊点在150℃下进行老化,并对重熔和老化焊点进行剪切测试.结果表明:重熔和老化后焊点的剪切强度都随体积的增大而减小,表现出显著的体积效应.SEM断面观察显示:较小体积焊点剪切断裂发生在钎料块体内部,表现出较好韧性;较大体积焊点则发生在近焊盘的界面处,呈现脆性断裂特征.焊盘界面处和钎料内部微观组织SEM观察表明:小体积焊点内部Ag_3Sn化合物以小颗粒状弥散分布,起到强化作用;而大体积焊点内部Ag_3Sn化合物为树枝网状分布,表现出硬脆性.金属间化合物(如Ag_3Sn和Cu_6Sn_5)的形貌和分布对焊点的断裂行为有显著的影响,是焊点剪切断裂行为体积效应的内在原因.

To meet the urgent demands of future electronic packages,the solder joints need to be increasingly miniaturized.Although the size of solder joints has surpassed 50—100μm range. further decrease is still necessary.Mechanical property of solder joints is a key factor that influences the reliability of electronic packages and assembly products.As a consequence,it is very important to understand the fracture behavior of solder joints,which can better predict the reliability of solder joints in electronic interconnections.Compared to the large solder joints,the mechanics behavior for the small solder joints is very different,resulting in a series of reliability issues.In this paper,shear test of the as-reflowed and aged Sn3.0Ag0.5Cu solder joints with the diameters of 200—600μm on Cu pads was conducted,and fracture behavior was observed using SEM.The results show that the shear strengths of both as-reflowed and aged solder joints decrease with increasing the solder joints volumes.At the initial stage of aging process,the shear strength of solder joints decreases remarkably, and will not decrease much with increasing aging time.For the large solder joints,the fracture occurs close to the interface,and the solder joint shows strong brittleness.Whereas,for the small solder joints,the fracture occurs within the bulk solder,and the solder joint shows ductility.SEM images at the interface of solder joints and solder bulk show that the Ag3Sn intermetallic compounds within the bulk solder and Cu6Sn5 at the interface region have a prominent effect on the shear property and the propagation of the fracture,which is the key factor for the volume effect of the fracture behavior of the solder joints.The Ag3Sn phase inside the small solder joints has fine particle-like morphology and dispersively distributes in the bulk solder,which strengthens the solder joints,however,the Ag3Sn phase inside the large solder joints has feather-like and dendritic morphologies and makes the joint become brittle.