Study On The Influence Of Sn Solder Layer Properties On The Growth Of Cu/Sn Interface

With the shrinkage of chip feature size and innovation of package technology,the size of interconnect joint is reducing.Electroplated Cu pillar bump,with the excellence in process simplicity,electromigration resistance and heat dissipation,is regarded to be widely applied in high aspect ratio package.To improve the joint reliability,investigations on the influences of solder layer properties on the growth of Cu/Sn interfacial intermetallics and voids becomes an urgent demand.In the present work,effects of plating addtives and current density on the Sn texture and grain size were firstly investigated.Then Sn layers of different textures and grain sizes were electroplated on polycrystalline Cu substrates.The growth of intermetallics and voids at Cu/Sn interfaces in the aging process were observed from the cross section and top view.Firstly,the textures of Sn layers plated by three solutions with different addtives were(112),(112)and(501)respectively.With the current density increasing,the texture of Sn layer shift from(321)to(501).It is found that the Sn layer textures have a significant effects on the growth of intermetallics at Cu/Sn interface and voids at Cu₃Sn interface by choosing the above Sn layers with texture of(112),(501)and(321).The growth of intermetallics is controlled by diffusion process,of which the thickness has a linear relationship with the square root of aging time.(501)Sn/Cu has the fastest IMC growth rate,(112)Sn/Cu has a close growth rate,that of(321)Sn/Cu is the slowest.The numbers of voids at Cu₃Sn interface decrease from(112)to(501),then to(321).Secondly,with current density varying in a small range of 0.4 to 1.8 ASD,the Sn grain size changed slightly,whereas when current density shifted from 1 ASD to20 ASD,the Sn grain size reduced by four times.The sizes of equiaxed Sn grains plated by three solutions with differnent additives were 2?m,8?m and 16?m respectively.An evolution model is proposed when studying the influences of Sn grain size on the intermetallics growth at Sn/Cu interfaces.For small grain size(2?m)Sn layer,at early aging stage,scallop Cu₆Sn₅ grains grow preferentially at Sn grain boundaries contact each other,the size is less than 1?m.Then Cu₆Sn₅ merges to larger rod-like grains in the driving force of surface energy.For large grain size(8?m,16?m)Sn layer,at early aging stage,scallop Cu₆Sn₅ grains of large size(4?m)grow preferentially at Sn grain boundaries,with small size(1?m)Cu₆Sn₅ grains growing intragranularly.Then large Cu₆Sn₅ grains swallow small ones,grow horizontally and become granular,the size difference weakens.In addition,the growth of intermeallics is controlled by grain boundary diffusion,the thickness of intermetallics at Sn/Cu interface and the number of voids at Cu₃Sn/Cu interface increases with Sn grain size shrinking.