The Electromigration Behavior Of Cu-cored Composite Solder Joints

The ball-grid-array packaging, in which metal pins are replaced by solder joints, hasbecomethemostpopularpackagingtechnologysinceitincreasesthepackagingdensity,reduces the packaging size and promotes the packaging reliability. As the workingconditions of the solder joint are getting harsher, more excellent solder joints, such ascomposite solder joints, are drawing people’s attention.Among all the composite solderjoints, Cu-cored solder joint is considered as the next generation BGA solder joint dueto its prominent properties. Moreover, with the continuous growth of the electricalcurrent in solder joints, the reliability issue caused by electromigration has becomemoreandmoreserious.Consequently,theelectromigrationbehavior of Cu-coredsolderjoints were studied in this work.The results of current density simulation presented that current crowding occurredat the Cu core and the areas between the Cu core and the Cu wires due to the lowresistivity of Cu. Thus the Cu core performed as additional electrodes. The Cu corecould be regarded as additional anode at the cathode of the solder joint, while additionalcathode at the anode of the solder joint.Serious microstructure evolution took place at the current crowding region in Cu-cored Sn solder joint, where Sn grain rotation at the two anodes were observed. Thedriving force of Sn grain rotation was composed by two kinds of stress: the back stressinduced by Sn accumulation under electromigration and stress caused by the thermalexpansion of the intermetallic compound. These two kinds of stress were imposed onthe two sides of the Sn grain and generated a torque which forced the Sn grain to rotate.In addition, a large back stress gradient due to the limited space between the Cu coreand the Cu wire reduced Cu electromigration and thus the growth of Cu6Sn5.As additional electrodes, the Cu core in Cu-cored Sn58Bi solder joint showed aJanus-faced appearance after current stressing due to Bi electromigration. Meanwhile,the Bi-rich phase between the Cu core and the Cu wire redistributed along the currentdirection under the current gradient force induce by the resistivity discrepancy betweenSn-rich phase and Bi-rich phases.What’s more, the tensile strength of Cu-cored Sn58Bi solder joints and traditionalSn58Bi solder joints after current stressing were studied. The Cu-cored solder jointsshowed a better mechanical performance than traditional solder since the ultimatetensile strength of the Cu-cored solder decreased more slowly. There are two possiblereasons: firstly, the Cu core acted as stress buffer in Cu-cored solder joint according to the stress simulation and the max stress at the corner of the solder joint was reducedcompared with traditional solder; secondly, the electromigrationofBiinthelowcurrentdensity region was restrained and the growth of Bi-rich layer at the corner of the solderjoint was inhibited at the same time.