Microstructure Evolution And Reliability Analysis Of Cu/(Ni,Sn)/Cu,Ni/(Ni,Sn)/Cu,Ni/(Ni,Sn)/Ni And Si/(Ni,Sn)/Cu Joints During TLP Bonding

In recent years,electronic systems have been confronted with great challenges due to the high operating temperature for power devices.SiC and GaN,as wide band gap semiconductors,are enable to operate at 300?or even higher and are wildly used in aerospace,nuclear and other applications due to the perfect physical and chemical properties.However,an important issue is whether the die attachment materials utilized in these semiconductor power devices can stand such high operating temperature.Conventional solders such as lead-free eutectic alloys solder and nanoparticle materials both achieved limited success due to their disadvantages.Transient liquid phase?TLP?bonding technology that producing solder joint composed of high melting temperature intermetallic compound?IMC?while its processing temperature is relatively low has great advantage.To date,there are few study focuses on the correlation between microstructure evolution and reliability of Cu/solder/Cu,Ni/solder/Ni and Cu/solder/Ni joints that commonly form in flip-chip structure.In this study,TLP bonding technology was used to investigate the correlation between microstructure evolution and reliability of the above three joints,and the effect of Cu atom concentration on it was studied furtherly.In addition,the microstructure evolution and reliability of Ni-Sn joint in practical Si/?Ni,Sn?/Cu packaging structure was analyzed.?1?The change of Cu concentration and its effect on the microstructure evolution of Cu/?Ni,Sn?/Cu joint when sintering at 350?for 3 h to 5 h was investigated.It was found that after sintering for 3 h,the average Cu concentration at solder-up Cu substrate and solder-bottom Cu substrate interface were 38.66 at%and 61.77 at%,respectively.A sawtooth layer of Cu₃Sn formed at each interface and intruded into Cu substrate.The diffusion reaction among Cu,Ni and Sn atoms firstly occurred at interfaces which resulted in volume shrinkage in the central of joint and the formation of voids and unreacted residual Sn.The average Cu concentration in the central of joint was in the range of 23.89 at%³3.42 at%and?Cu,Ni?₆Sn₅formed.After sintering for 5 h,the average Cu concentration at up and bottom interface significantely decreased to 4.77 at%and 4.62 at%.Joint was completely composed of?Ni,Cu?₃Sn₄ with average Cu concentration in the range of 3.55 at%⁴.14 at%.Sawtooth IMC layers at interfaces became straight and smooth and the amount of voids in the joint decreased a lot.?2?The change of Cu concentration and its effect on the microstructure evolution of Ni/?Ni,Sn?/Cu joint when sintering at 350?for 3 h to 5 h was investigated.It was found that after sintering for 3 h,the average Cu concentration at solder-bottom Cu substrate was 7.35at%,and the sawtooth?Cu,Ni?₆Sn₅ layer formed that intruded into Cu substrate.The central of joint was composed of?Ni,Cu?₃Sn₄ with average Cu concentration in the range of 2.91at%³.35 at%and few obvious voids and fractures formed.After sintering for 5 h,the average Cu concentration at solder-Cu substrate interface decreased to 5.80 at%and?Cu,Ni?₆Sn₅ transformed to?Ni,Cu?₃Sn₄.Sawtooth IMC layer at interface transformed to straight and smooth.The central of joint was composed of?Ni,Cu?₃Sn₄ with average Cu concentration in the range of 2.57 at%³.85 at%.During the sintering,negligible change occurred in the microstructure?voids and fractures,etc.?of joint.?3?The influence rule of Cu concentration for the microstructure of joint was investigated.It was found that when Cu concentration below 7.35 at%,?Ni,Cu?₃Sn₄ formed and the interface exhibited straight and smooth;when Cu concentration in the range of 7.35at%³8.66 at%,?Cu,Ni?₆Sn₅ formed and the interface exhibited sawtooth that intruded into Cu substrate;when Cu concentration beyond 38.66 at%,Cu₃Sn formed at the interface where the interreaction among Cu,Ni and Sn atoms was fierce and the sawtooth layer intruded into Cu substrate seriously.?4?The microstructure evolution of Ni/?Ni,Sn?/Ni joint when sintering at 350?for 2 h to 5 h was observed.It was found that after sintering for 3⁵ h,negligible change occurred in the microstructure of joint that was completely composed of Ni₃Sn₄ and few voids formed.?5?The reliability of three joints was compared.It was found that after sintering at 350?for 3 h,Cu/?Ni,Sn?/Cu joint achieved the highest shear strength of 20.33 MPa compared to Ni/?Ni,Sn?/Ni and Ni/?Ni,Sn?/Cu joint with shear strength of 13 MPa and 10.33 MPa,respectively.The deformability of Ni/?Ni,Sn?/Cu joint was higher than that of Ni/?Ni,Sn?/Ni joint.After thermal aging for 96 h,shear strength of Cu/?Ni,Sn?/Cu joint decreased slightly and after 100 cycles of TCT?Thermal cycling test?,it decreased from 20.33 MPa to 15.92MPa.Cu/?Ni,Sn?/Cu joint presented high reliability after thermal aging treatment and TCT.?6?The microstructure evolution and reliability of Ni-Sn solder in practical Si/?Ni,Sn?/Cu packaging system was evaluated.After sintering at 350?for 3 h,the shear strength of joint achieved 15.4 MPa,which decreased to 10 MPa after thermal aging at 250?for 72 h and exhibited negligible change in the extended aging time.After 150 cycles of TCT,the shear strength of joint decreased to 8.7 MPa.Compared to commercial nano-Ag paste interconnection in SiC/nano-Ag/Cu packaging system,the as fabricated Ni-Sn joint remained a relatively high bonding strength after 150 cycles.