Microstructure Of Lead-free And Tin-lead Mixed Solder Bump Research

With more and more kinds of electronic products，the forms of electronic packaging arebecome variety. Lead-free solder has gradually replaced the lead components, But before thelead-free solder technology become mature, it is inevitable for lead-free solder mixed withlead solder. Especially in the field of high reliability and performance requirements, Thestructure of the conventional process and microstructure analysis of solder joints become themost important research.The introduction of tin-lead solder in the reflow process reduce the melting temperatureof lead-free solder and Increase the solder melting temperature range. When the weightpercentage of tin-lead is10%the Mushy range of solder bump is the biggest. And more likelyto cause solder joint imbalance by solidification at solder ball interface with chip formation ofshrinkage. Rich Pb phase with the reduce of tin-lead in mixed solder bump change from longstrip to grid type and eventually equally distribute with the form of granular type. The Ag3Snis more likely with the type of granular in the eutectic organization of chip side, the smallamount of Ag3Sn gathered and grow up.UBM layer of Ni reaction with Sn3.0Ag0.5Culead-free solder at the process of flip chip bumping produce hexagonal of (CuNi)6Sn5. Therich Pb layer forms at the interface of chip and solder ball restrict the growth of (CuNi)6Sn5when chip is mount to PCB, and (CuNi)3Sn generate between (CuNi)6Sn5and Ni layer. Thereaction of Electronless Ni and with Sn3.0Ag0.5Cu and tin-lead mixed solder consumes a lotof Ni, due to lack of Ni cause mass of P which is not participate in reaction. And form a layerof rich P between the (CuNi)6Sn5and electronless Ni. That layer reduce the strength of solderjoint. The Ni diffusion from chip side to PCB replaced the location of Cu in (CuNi)6Sn5whenSn3.0Ag0.5Cu and tin-lead mixed solder react with Cu pad. The interface reaction of PCBand solder will effect the reaction between chip and solder when they react at the same time.The effect cause (CuNi)6Sn5growth with the type of needl-like.