The Dimensional Effect Of Thermal Fatigue Performance For SAC305 Leadfree BGA Solder Ball/Solder Joint

With the gradual promotion and application of high-power devices IGBT and LED,the density of packages is getting higher,the size of solder joints is getting smaller and smaller,and gradually increasing heat,force and electrical load are absorbed by the solder joints.The reduction of solder joint size means the reduction of solid-liquid and solid-solid reaction interface,and the microstructure of bulk solder,IMC composition,morphology and size of solder joint interface are changed.The mechanical properties of solder joints are also changed as the size decreases,which will inevitably bring about a series of reliability problems of solder joints.Because of the unique service conditions of high-power devices,the rapid thermal fatigue test device was self-made in this project,which can truly simulate the changes in microstructure and mechanical properties of micro solder joints of different sizes during service of high-power devices.The influence of solder ball size on the surface morphology,internal and external cracks of solder balls and bumps under extreme conditions has been studied;Under different temperature change rates,the influence of solder ball size on the IMC and mechanical properties of the solder bump interface has been studied.The main conclusions are as follows:(1)No cracks were found on the surface of solder balls with four diameters at 5500cycles;As the size of solder ball increases,The surface of the solder ball became rough and the oxide film became thicker.After conventional and rapid thermal fatigue,as the size of solder ball increases,the wrinkle cycle of the solder bump surface was earlier,but no cracks were found on the solder bump surface.As the number of thermal fatigue cycles increases,the solder bump surface became rougher and darker.Solder bumps of conventional thermal fatigue became more rougher and darker due to longer high temperature oxidation.(2)As the size of the solder ball decreases,the dendritic crystals in the soalder ball appeared earlier,and the solder ball had a smaller number of crystal grains,and the directionality of the crystal grains was more obvious.At 2500 cycles,the crystal grains inside the ?0.3mm and ?0.4mm solder balls began to have directionality;The ?0.5mm solder ball only had obvious dendrites at the bottom,and the crystal grain direction inside the solder ball was disordered;There was no obvious change in the grain size of the ?0.6mm solder ball,there was no dendrites inside the solder ball,and the grain direction was disordered.At3500 cycles,the smaller the size of the solder ball meant the larger the size of the dendrites inside the solder ball.The size of dendrites inside the ?0.3mm solder ball was the largest,and the grain growth had obvious directionality.?0.4mm and ?0.5mm solder balls were found to have obvious directionality at 4500 cycles,and ?0.6mm solder balls were found to have obvious dendrites on the bottom at 5500 cycles.(3)As the size of the solder bump increases,the cycle of internal cracks appeared earlier,the width of the cracks was wider,and the length was longer.The faster temperature changes of rapid thermal fatigue led to the cycles of internal cracks appearing earlier than conventional thermal fatigue.No cracks were found in ?300?m solder bumps after conventional and rapid thermal fatigue.No cracks were found in ?400?m solder bumps after conventional thermal fatigue,but at the 4500 cycles of rapid thermal fatigue,there were microcracks on the left and right sides of the solder bumps.Cracks were found at the corners of the solder bumps after 5500 cycles of conventional thermal fatigue;At 4500 cycles of rapid thermal fatigue,there were deep cracks at the corners of solder bumps.?600?m solder bumps will have cracks in the corners of the solder bumps when the conventional thermal fatigue was 4500 cycles;when the rapid thermal fatigue was 1500 cycles,there will be micro-cracks near the interface.(4)As the size of the sodler bump increases,the thickness of the interfacial IMC continued to decrease,and the interfacial IMC of the ?0.3mm solder bump was the thickest.In the initial stage of conventional thermal fatigue,the growth rate of the interfacial IMC was the fastest,and then the growth rate of the interfacial IMC slowed down.At 5500 cycles,the conventional thermal fatigue was about 2?m higher than the initial interfacial IMC thickness.The thickness difference of the interfacial IMC was almost all within 1?m after rapid thermal fatigue.As the size of the solder bump increases,the interfacial IMC of solder bump became flatter,and rapid thermal fatigue changed faster and flatter than conventional thermal fatigue.At 5500 cycles,the shear strengths of conventional thermal fatigue solder bumps(?0.3mm,?0.4mm,?0.5mm,?0.6mm)were 41.07 Mpa,40.8 Mpa,37.78 Mpa,36.78 Mpa,which were reduced by 49.64%,40.02%,41.95%,40.14% respectively;The shear strengths of rapid thermal fatigue solder bumps(?0.3mm,?0.4mm,?0.5mm,?0.6mm)were49.75 Mpa,50.43 Mpa,48.68 Mpa,12.52 Mpa,which were reduced by 38.99%,25.86%,25.20%,79.62% respectively.With the increase of the conventional thermal fatigue cycle,the depth and width of the dimples became smaller and shallower,and tend to be flat.After shearing,the fracture position was found at the solder,and the fracture mode was mainly ductile fracture;As the size of the solder bumps decreases,the dimples of the bumps tended to flatten faster.As the number of rapid thermal fatigue cycles increases,the depth and width of dimples of ?300?m,?400?m and ?500?m solder bumps became smaller and shallower,and tended to be flat.The fracture mode was mainly ductile fracture;as the number of cycles increases,the fracture position of ?600?m solder bumps shifted from the inside of the solder to the interfacial IMC /solder,and the fracture mode changed from ductile fracture to brittle fracture.