Interfacial Behavior Between Tin Solders And Different Types Of Copper Pads

Miniaturization of electronic products leads to the shrinkage of joint size,resulting in a higher volume fraction of intermetallic compounds(IMCs)generated at the Sn/C u interface.The interfacial behaviors of solder joints will greatly affect the electronics reliability.In the electronic packaging industry,to restrain costs,most pads are electroplated(EP)Cu,whose micro-structure is different from that of high purity(HP)C u.However,to date,the interfacial behaviors of Sn/EP C u have not been clearly revealed yet.Also,the introduction of impurities by electroplating not only influences interfacial behavior of solder joints but also environmental issues.Many packaging companies consider using vacuum-sputtering(VS)method to prepare Cu substrates(because it can eliminate the introduction of impurities).For these reasons,in this study,the interfacial behaviors(e.g.,morphological evolution,coarsening behaviors,growth kinetics and KV behavior)of Sn/HP Cu,Sn/EP C u and Sn/VS Cu were investigated.The results from this study can contribute to both the choice of bonding pads and the improvement of the solder joints reliability.Main conclusions are as follows,1.The micro-structure of Cu pad is key to nucleation and growth of IMCs.The morphology of C u6Sn5 grains is scallop-shaped formed at Sn/HP C u interface,which is ball-shaped formed at Sn/EP Cu interface and short-rod-shaped formed at Sn/VS Cu interface.In grain ripening stage,The growth of C u6Sn5 grains is K Sn/HP Cu > K Sn/EP Cu> K Sn/VS Cu,in merge of grains stage,it is K Sn/VS Cu> K Sn/EP Cu >K Sn/HP Cu.The growth of C u6Sn5 is controlled by grain boundary diffusion at early stage and followed by volume diffusion,whereas the growth of Cu3 Sn is only volume-diffusion-controlled.2.The evolution of interfacial IMCs layer(Cu6Sn5 and Cu3Sn)formed at Sn/HP C u,Sn/EPCu and Sn/VS Cu during solid-state aging are investigated.The growth of IMCs layer follows the parabola growth model where K Sn/VS Cu> KSn/EP Cu >K Sn/HP Cu.3.The formation of K irkendall voids(KV)in Sn/HP C u,Sn/EP Cu and Sn/VS C u joints are studied.KV were hardly found in the Sn/HP C u joint even after isothermally aging at 180? for 30 days,while a large number of voids were observed both in the Sn/EP Cu,and the Sn/VS Cu joints after short-time aging.The size of voids formed in the Sn/VS Cu was relatively smaller than that in the Sn/EP Cu.The result indicated that the variation in KV can be attributed to three factors,including(i)different grain sizes from HP C u to VS C u,the finest grained VS C u was expected to promote the unbalanced diffusion,generat ing much larger vacancy flux,and further induc ing the K irkendall voiding.(ii)Different impurity from HP C u to VS C u,there are many impurities(S,C l and PGE)in EP C u,while there is nearly no impurity incorporating into the VS Cu and HP Cu.(iii)Different properties from HP C u to VS C u,the grain size of EP C u and VS C u is very small,and a large amount of energy is stored in the grain boundary.Part of the stored energy can be released into the interface during the reaction,accelerating the formation of KV.4.The micro-mechanism of the effect of Sn grain orientation and electrical field intensity(F)on the diffusion of M atom(= Cu,N i,Ag,Au or In)in M/Sn solder joints was investigated employing the density functional theory.The adsorption energy o f M atom on Sn(100)and(001)surfaces,the energy of M atom penetrating from the Sn surface to Sn body(penetration energy,Epe)and the energy of M atom diffusing in the bulk Sn(diffusion energy)were calculated,as well as the corresponding electronic structures for different steps.It is found that the M atoms were energetically favorable at the hollow sites of Sn(100)and(001)surfaces.Moreover,the M atom was more strongly adsorbed to Sn(001)than to Sn(100).The Epe for each M atom along Sn[100] was greater than that along Sn[001].The Epe increased gradually with the M atom switching from N i to In.In addition,Epe decreased with the increase of F.Electronic structure analysis shows that the Epe change can be attributed to four factors,including different atomic radius from N i to In,difference in interatomic repulsive resistance between a and c-axis,different hybrid orbitals between M 3d and Sn 2p from N i to In and different charge transfer and electro n redistribution of Sn grains under the F.