Research On Thermomigration In The Interconnection Solder Joints In High Density Electronic Packaging

With the miniaturization trend and functional demand in high-density microelectronicpackaging, the thermomigration （TM） in flip chip solder joints due to the serious joule heatingbecomes a major reliability issue in recent years. This thesis presents a study of TM apparatusand specimen design, as well as the TM investigation in the eutectic SnPb solder bulk andcomposite SnPb flip chip solder bumps at both high and low ambient temperatures.In order to carry out the TM experiment separated from EM behavior in the solder layer,a novel apparatus, which consists of two temperature controlled heating rods and a Pelitercooler to serve as the heating source and heat sink respectively, has been developed. Fourtypes of sandwich specimen with Cu/Solder/Cu structure have also been designed to matchthe apparatus well. FE simulation shows that the thermal gradients across the solder layers inall these specimens can rise up to be higher than the TM threshold value1000K/cm.The investigation of TM in eutectic SnPb solder layer at high ambient temperature hasbeen performed in a Cu plate/SnPb solder layer/Cu bar specimen. FE simulation shows thatthe thermal gradients in the target solder layer are mainly in the range of1430K/cm¹579K/cm, and the highest one can reach1875K/cm. The microstructure in the eutectic SnPbsolder layer does not vary too much at the early stage of TM. But Pb phases start to migrate tothe cold side and accumulate there with anomalous shape after TM20h, which will get moreseriously with the TM duration increases. Besides, based on the atomic diffusion theory, it hasbeen demonstrated that in SnPb alloy, both Sn and Pb atoms, which migrate in Pb and Snface-centered cubic with vacancy diffusion mode, have the positive net atomic flow from hotsite to cold site under thermal gradient loading. However, the diffusion rate of each elementwill depend on the ambient temperature. Thus, it has been clarified that during TM, whichelement can be regards as the primary diffusion atom is determined by the combined effect ofboth these two factors. Moreover, the key parameter Q*of Pb atoms is calculated to be27.2KJ/mol, and the TM driving force under the thermal gradient of1430K/cm at170°C is1.48×10^-17N. The effect of TM on the composite SnPb flip chip solder bump at low ambienttemperature has been studied. During3×10⁴A/cm²current stressing at25°C, the final stabletemperatures on the Si die and the substrate can reach93.2°C and72.9°C respectively. Thethermal gradients in the unpowered bump are mainly in the range of347K/cm⁸00K/cm,and the peck value can reach2388K/cm. At the early stage of TM, the microstructure in thesolder bump does not vary too much. But Sn phases in the lower eutectic SnPb solder start tomigrate to the cold side after TM100h, and accumulate there eventually as a7.9μm thickcompact layer after TM400h. The key parameter Q*of Sn atoms is proposed to be22.1KJ/mol, and the TM driving force in the unpowered bump at low ambient temperature is9.1×10^-17N, which is much smaller than EM driving force in the powered bump of1.3×10-15N.Thus, it can be concluded that EM will definitely take lead the diffusion process at lowtemperature, rather than TM. Moreover, the digest of the IMC growth principle shows thatTM will accelerate the Cu6Sn5growth, but inhibit the Cu₃Sn development at the cold sideapparently. One reason is that after TM, the Sn atoms near the cold side can be sufficientlyprovided by the accumulated Sn rich layer; the other reason is that the thermal gradients in theIMC layer, which are in the range of900K/cm¹264K/cm, can also make a remarkableeffect on the Sn atoms to diffuse from the Sn-rich layer into the IMC, and then react with theCu atoms to generate Cu6Sn5preferentially.The effect of TM on the SnPb composite flip chip solder bump at high ambienttemperature has also been studied. During3×10⁴A/cm²current stressing at100°C, the finalstable temperatures on the Si die and the substrate can reach125.2°C and160.9°C due to theJoule heating. The thermal gradients in the unpowered bump are mainly in the range of1062K/cm²450K/cm, and the peck value can reach7309K/cm. Based on the microstructureobservation, the macroscopic TM diffusion path of Sn atoms in the solder bump can beidentified as: phase coarsenâ†'horizontal movement from the center to the edge near thesubstrate sideâ†'horizontal and vertical migration from the cold side to the hot sideâ†'accumulation at the connecting area between high-Pb and eutectic SnPb solder bulk. Moreimportantly, the analysis has been stated to explain two divergences of unique Sn and Pb phases’ redistribution in our study. Firstly, for no obvious void or micro crack at the interfaceof the hot side after TM at high ambient temperature, FE simulation proves that the existingunderfill layer in our commercial product will reduce the thermal gradient across the solderbump effectively, which will bring in a much gentler atomic migration even when the wholecomponent is stressed by such high density current. Secondly, for the accumulation of Sn-richphases at the connecting area between high-Pb and eutectic SnPb solder bulk, it can beexplained that as the thermal gradients in the high-Pb and eutectic SnPb solder bulk arenon-uniform, the thermal energy change need Sn atoms to overcome in the high-Pb solder islarger than that in the eutectic SnPb solder. Thus, when the stress gradient driving force isconstant, the Sn atoms tend to diffuse in the eutectic SnPb solder much easier than in the highPb solder bulk, which will lead to the final accumulation at the connecting area after TM.