Failure Mechanism Of The Interface Between Ball Grid Array Solder And Cu Substrate Induced By Thermal/Current/Vibration Loading

Electronic technology progresses towards higher levels of integration, higherperformance and increasing functionality with the rapid development of automotiveelectronic device and other electric products. Electronic packaging technology hasalready accessed to high-dengsity and small-pitch era. The functions of an electronicpackage are to protect, power, and cool the microelectronic chips or components andprovide electrical and mechanical connection between the microelectronic part and theoutside world. The intermetallic compounds (IMCs), which form by the reactionbetween the solder alloys and the base meatl during reflowing, is an essential for thebondability of solder joints. However, too thick IMC layer may weaken the solder jointsdue to the brittle nature and coefficient of thermal expansion (CTE) mismatch betweenthe substrates and solder alloys. Electronic assemblies in field use are exposed to a widerange of environmental loads. Avionics, automotive control units, and militaryequipment are often used in harsh environments combining thermal shock, vibration,current loading or the interaction of combined loadings. Consequently, failure behaviorsof Sn-Cu intermetallic compounds layer in ball grid array induced by thermal shock,mechanical vibration and the interaction of high density current load and vibration hasto be considered in this study. The ANSYS13.0finite element simulation software toolis utilized to describe the distribution of temperature and the stress/strain behavior ofball grid array (BGA) solder joint under the thermal shock, vibration, current or theinteraction of combined loadings. The main research contents and conclusions are asbelow:Thermal shock (TS) reliability testing and finite-element analysis (FEA) of solderjoints between ball grid array components and printed circuit boards with Cu pads wereused to investigate the failure mechanism of solder interconnections. The results showthat solder joints were subjected to shear and tensile stresses during TS due to thedifferences in the CTE values between discontinuous phases. With continuousdeformation cycles, the increasing strain incompatibility between the solder and IMClayers lead to a severe stress concentration in the solder-IMC interface and theappearance of cracks after TS for300h. In the initial period, cracks occurred in thesolder matrix but in a very slow propagation rate. Then, most cracks initiated in thesolder matrix near the IMC layer, and the propagation path was approximately parallel to the solder–Cu interface. For longer times, the cracks nucleated and propagated withinthe Cu6Sn5layer, and the propagation mechanism was both intergranular andtransgranular. The scallop-type Cu6Sn5phase was changed to planar-type Cu6Sn5/Cu3Snwith TS, resulting in decreased crack growth resistance. The grain size in the solderdecreased with TS, which could promote atom diffusion and thereby contributed to thehigher IMC growth rate and surface planarity.Solders with different thickness and morphologies of IMC layers were gained byaltering the reflow heat factor. Then the sinusoidal vibration fatigue test wasimplemented to investigate the effects of the morphologies and thickness of Sn-Cuintermetallic compounds (IMC) at the interface of Sn3.0Ag0.5Cu lead-free solder alloyand Cu substrates on the vibration reliability of BGA solder joints. The results show thatthe IMC grew rapidly with the increase of heating factor. The vibration fatigue life ofsolders increased gradually at first and then dropped rapidly with the increase of heatingfactor, reaching the maximum when the thickness of IMC layer was about1.5μm. Thecracks caused by vibration originated mostly from the solder matrix near the bottleneckof solder joints or macroscopic void and the propagation path was approximatelyparallel to Cu interface when the thickness of IMC layer was about1~2μm. Thefractographic morphology was seen to be mostly flat and there was the dimple-likestructure on the fracture surface. Accordingly, when the thickness of IMC layer laybetween approximately2~3μm, fracture tended to propagate along the interface ofsolder/Cu6Sn5and the fine grains of Cu6Sn5were found on the fracture surface. Whenthe total interfacial IMC layer thickness was more than approximately3μm, fracturepropagated along the interface of solder/Cu6Sn5, but that fracture of the joint under thiscondition caused some of the small protruding Cu6Sn5tips to become broken and thecleavage fracture was seen very rough. When the total interfacial IMC layer thicknesswas much more than approximately4μm, the fracture mechanism changed to fracturewithin Cu6Sn5layer and the fractographic morphology transformed into smoothness.The simulation software was utilized to describe the distribution of temperature andthe stress/strain behavior of solder joint under the vibration and different current densityloadings and generated the S-N (stress-life) curve. Then the effect of current andtemperature rising caused by current crowding on fracture behavior of solder joints wasanalyzed. The results show that the board level components of flip chip ball grid array(FCBGA) generated flexural deformation under vibration, and the maximum Von Misesstress appeared in the connection of solder and bismaleimide triazine (BT) substrate. The fatigue life of vibration dropped sharply with the increase of the current density.When the current was3A, the current crowding (about1.0104A/cm2) that appearedin the corner of the connecting region between the solder and Cu increased thetemperature to110.34oC. While the current reached15A, the current densityapproached to3.2104A/cm2and the temperature raised rapidly to209.68C. With nocurrent loading, the cracks caused by vibration originated mostly from the solder matrixnear the bottleneck of solder joints and the propagation mechanism was transcrystallinefracture and the protruding Cu6Sn5tips became broken. When the current was3A, thecracks also originated mostly from the solder matrix near the bottleneck of solder jointsbut closing IMC layer and then propagated into the solder with an angle ofapproximately45owith respect to the solder/Cu6Sn5interface. When the current reachedup to15A, the cracks occurred in the solder matrix and propagated in the solder matrixdirectly.