The Microstructure And Evolution Of Interconnect Interface In Flexible LED Package Module

Flexible LED packaging modules have great development potential because oftheir flexibility. To solve the reliability problems caused by the bending platesurface and the bad thermal management, this thesis carried out high temperatureaging test on the solder bump between the flexible Ni coated Cu wire and LEDcomponent. The interface compounds （IMC） between the Ni coated Cu wire andsolder was studied, especially its relationship with the plating of the LEDcomponent and the solder amount. Moreover, the thermal shock properties of anovel high-temperature lead-free solder named Innolot used in chip interconnectionwere researched by analyzing its microstructure, crack propagation andrecrystallization.By analyzing the variety, thickness, tensile property of the IMC at the interfacebetween the Cu wire and the solder, it was found that although the Cu wire did notcontact directly with the LED component, the plating on the component could affectthe evolution of the mentioned IMC. If the Ni coated Cu wire was soldered with theCu/Ag plated component, the Cu in the plating could make Cu in the solder move tothe other side and led to the generation of （Cu,Ni）₆Sn₅and resulted in the Ni₃Sn₄turned to be （Cu,Ni）₆Sn₅during the aging time.（Cu,Ni）₆Sn₅was also found to besensitive to the solder amount at the same time. Soldered with Ni plated component,the IMC was proved to be Ni₃Sn₄, and the Cu only came from the solder to create（Ni,Cu）₃Sn₄, which was not sensitive to the solder amount. If the solder couldprovide enough Cu,（Cu,Ni）₆Sn₅was produced right after solder process, and thegrow speed was faster than the one relying on the plating on the component offeringCu. The process of turning Ni₃Sn₄to （Cu,Ni）₆Sn₅+（Ni,Cu）₃Sn₄would cause aserious tensile property reduce, while only the thickening process of Cu-Ni-Snwould not cause the reduction.Nondestructive testing, polarizing observation, EBSD and fracture observationwere used to realize that the Innolot solder （SnAg3.8Cu0.7Ni0.12Sb1.5Bi3） ownedfine thermal shock resistant ability. Solution strengthening was offered by Sb and Bi,and dispersion-strengthening was provided by Ni （by creating （Ni,Cu）₃Sn₄with Snand Cu）. The shapes of the solder bumps affected the distribution of the stress, andthen the compounds coarsening process under thermal cycles and the crackpropagation in turn. Comparing with SnAg3.8Cu0.7the Innolot solder recrystallizedmuch more seriously and helped to consume the energy brought by the crack toblock its propagation. Stress was the necessary condition of recrystallization, meaning more stress, more recrystallization. Sometimes the turning of therecrystallized grains could change the orientation from “hard” to “soft” to allow thecrack propagation continuing in the primary direction without consuming too muchenergy. Trans-granular fracture and dimple fracture with bad plasticity caused byIMC were found in Innolot solder. When the stress were relatively lower, wiredrawing phenomenon could be found.