The Influence Of Interfacial Reaction And Kirkendall Void On The Copper Pillar Reliability In Three-demensional Electronic Packaging

With the miniaturization of electronic products,the demands for higher integration increased rapidly.Flip-chip and 3D chip-stacking packaging using bump connection becomes one of the best candidates in the industry.As a novel connection technology,copper pillar bump combines excellent electrical and thermal performance for ultra-fine pitch connection.However,as the package size decreasing,thermal and electrical as well as mechanical loading on a single copper pillar bump increases dramatically.How to effectively inhibit the growth of the interfacial IMC so as to improve the reliability of copper pillar bump in 3D packaging has draw much attention.In this paper,with multi-layer plating method,we prepared Cu/Sn/Ni copper pillar bump sandwich structure with different diameter and solder cap thickness,to study the influence of size and solder cap thickness on the reliability of copper pillar bump.The main conclusions are as follows:The effect of size of copper pillar bump on the growth of interfacial IMC is significant.In the Cu/5?m Sn/Ni system,campared to?100 bump,solder in ?20 bump comsumes faster and the transition time form solder layer to full IMC layer is shorter.Besides,the Cu₃Sn in ?20 bump is thicker and the reaction constant is greater than that in?100 bump.As the Cu₃Sn growing,Kirkendall voids can be seen in?20 and?100 bump,but disappear as the aging time prlonged.In the Cu/5?m Sn/Ni system,the grain zise of?Cu,Ni?₆Sn₅ on Cu side in?20 bump is greater and the neddle-like?Cu,Ni?₆Sn₅ on Ni side is coarser.While in?100 bump,no bulk?Cu,Ni?₆Sn₅ grains appear on Cu side,the?Cu,Ni?₆Sn₅ on Ni side is flat and the grain is finer and thinner.Also it can be observed that during the aging time,solder in?20 bump comsumes faster,and micro-cracks generate at the Cu₃Sn/Cu interface,the Cu₃Sn where the micro-cracks is thinner,and the root of?Cu,Ni?₆Sn₅ on Ni side get coarse with laminarization.While in?100 bump,?Cu,Ni?₆Sn₅ on Ni side is neddle-like and grows slower and thinner.During the aging time,Cu₃Sn and?Cu,Ni?₆Sn₅in?20 bump are thicker than that in?100 bump.For Cu/20?m Sn/Ni system,we proposed a void growth model in copper pillar bump.In this model,Cu and Sn atoms interdiffuse and react with each other to form 1^st?Cu,Ni?₆Sn₅ and 1^st Cu₃Sn,and get thicker as aging time increases.Kirkendall voids nucleate and propagate to form micro-crack at the1^st Cu₃Sn/Cu interface,which in turn inhibits the Cu diffusion.So the Sn flux relatively increases,and promote the formation of 2^st?Cu,Ni?₆Sn₅ until the 1^stt Cu₃Sn is exhausted.At this time,the 1^st Cu₃Sn/Cu interface is repalced by the2^st?Cu,Ni?₆Sn₅/Cu interface.As aging time prolonged,the Cu react with 2^st?Cu,Ni?₆Sn₅ to form 2^st Cu₃Sn,then the 2^st Cu₃Sn/Cu interface replace the prior 2^st?Cu,Ni?₆Sn₅/Cu interface and the new 2^st Cu₃Sn grow thicker.Besides,there is a significant difference of IMC and Kirkendall voids growth behavior in the depression on bump,where the growth of Cu₃Sn is promoted and Kirkendall voids propagate faster.IMC morphology and thickness significantly affected by the solder cap thickness.Compare to limited solder volume system,?Cu,Ni?₆Sn₅ on Ni side in Cu/20?m Sn/Ni system is loose and neddle-like with vimineous morphology interspersing in the solder layer.Scalloped?Cu,Ni?₆Sn₅ on Cu side is flat.While in the Cu/5?m Sn/Ni and Cu/10?m Sn/Ni system,because of limited solder volume,the?Cu,Ni?₆Sn₅ on Ni side is dense with thick and short rod-like morphology.And the scalloped?Cu,Ni?₆Sn₅ on Cu side is more protrude.The Cu₃Sn in Cu/5?m Sn/Ni system is much more thicker that that in Cu/10?m Sn/Ni system while thinner that that in Cu/20?m Sn/Ni system.As for the formation of?Cu,Ni?₆Sn₅,it grows thicker as solder layer thickness increasing.While the volume ratio of whole IMC increases with the drease of solder layer thickness.