The Effect Of Structure And Coating On Electromigration Performance Of Sn3Ag0.5Cu Lead Free Solder Interface

In general, an electronic device during service is subjected to high temperature andcurrent stress. Driven by the high-performance, multi-function and miniaturization ofelectronic devices, High-density packing technology has a rapid development in the pastdecade. The I/O number on the single chip has increased dramatically, the currentdensity reached104A/cm2. Electromigration-induced failure in the solder bumpsbecomes an unavoidable issue. Thus, in this paper the effects of current density,temperature, Under Bump Metallization (UBM) materials and welding structure on thegrowth of intermetallic compounds (IMC) have been studied. In addition, the processand reasons of solder bumps failure were also been investigated. The conclusions weregiven as follows:The growth of intermetallic compounds at the interfaces of the solder and Cu linesunder current stressing were studied. The result shows that the thickness of IMC at thecathode and anode increased with the current time. The morphology of IMC at thecathode turned from the scallop-type to flat-type. The greater the current density, thefaster the growth rate of IMC at the anode. The time index at the anode were close to1.The growth of IMC of flip chip solder joints under thermal aging and current stresswere studied. During thermal aging, Cu6Sn5grew with the aging time. Its thickness waslinear relation with the square root of time. Under current stress, IMC at the anodeincreases rapidly with time, the thickness of IMC at the anode were almost twice thanthe aging samples during the same time. And the thickness of IMC at the cathodebasically remained unchanged then slowly increased. The particularity structure of theflip chip resulted in the current crowding effect and joule heating effect. The simulatedresult of ANSYS shows that the current density of the entrance and exit of electron flowwere3times than the average density. The temperature of solder balls was about20°C～40°C higher than the ambient temperature. A large number of Cu atoms at theentrance of electron flow migrated into the solder bumps under the force of electronicwind. The dissolved Cu regions were backfilled with solder. In the end, the backfillspeed was slower than the speed of moving Cu atoms, which resulted in the failure ofthe solder ball. When the temperature was the same, the greater the current density, theshorter the failure time. When the current density were the same, the higher thetemperature, the shorter the shorter the failure time. The effect of current stressing on the growth of intermetallic compound at theinterface of Sn3Ag0.5Cu/Cu and Sn3.0Ag0.5Cu/Ni were studied. After reflow, theshort clavite (Cu, Ni)6Sn5formed at Ni/Sn3Ag0.5Cu interfaces and a layer of flatCu6Sn5formed at Cu/Sn3Ag0.5Cu interfaces. Under the stress of6A, the IMC growthbehavior was different when the Ni UBM acted as cathode and the Cu UBM acted ascathode. If Cu UBM acted as the cathode, the dissolution rate of Cu atoms were faster,leading the cathode Cu UBM and Cu wire to be partly dissolved and solder joint to failfinally. At the same time, solder balls that Ni UBM acted as cathode did not fail. This isbecause Ni UBM as barrier layer prevented the dissolution of Cu. So Ni UBM canimprove the electromigration resistance of solder joints.