Research On Solderability And Temperature Cycling Reliability Of SAC305 Lead-free Solder Joints Fabricated In Protective Reflow Atmospheres

With the enhancement of environmental awareness and the enforced enactment of lead-free regulations,surface mount technology?SMT?has entered the era of lead-free.But the high melting point and poor wetting performance of lead-free solder will bring a big challenge to its assembly process.On the other hand,with the development of electronic products in their directions of miniaturization and light weight,the use of tiny components has further increased the difficulties of SMT assembly process.In this paper,the effects of reflow process on the solderability of SAC305 lead-free solder joints were systematically studied,especially for the N₂ protection reflow.The appearance quality,microstructure,interfacial intermetallic compounds?IMC?and mechanical strength of the solder joints were investigated.Temperature cycling reliability of the solder joints was also studied by temperature cycling tests and finite element simulations.The main conclusions are as follows:?1?Reflow atmosphere had a significant effect on the wetting performance of the lead-free solder.In the case of air reflow,the wetting property of the solder joint was decreased due to the severe oxidation of the pad and solder.Under the condition of N₂protection,the spreading performance of the solder and the filling ability of the terminals were improved.With decrease of the oxygen concentration in N₂,this improvement became more and more effective.To increase the reflow temperature could also enhance the wettability of the solder,but the improvement was not significant for the air reflow system.?2?Compared to the air condition,N₂ protection reflow enhanced the solderability of lead-free solder joints.The appearance quality of the solder joints was improved and the void ratio of the solder joints was reduced,resulting in the increase of shear strength for the solder joints.Such a enhancement benefited from the N₂ protection reflow,which reduced the oxidation of the solder joints and thereby promoting the solder wetting performance.However,the reflow atmosphere had no significant effect on the interfacial IMC morphology of the solder joints.?3?Scallop-type Cu₆Sn₅ IMC was formed between the interface of SAC305 solder and Cu pad.As the IMC formation was mainly controlled by the diffusion process of the elements,reflow temperature had a significant influence on the interfacial IMC morphology of the lead-free solder joints.With the increase of peak reflow temperature,the thickness of interfacial IMC layer gradually increased,and the shear strength of the solder joint was also improved.?4?Although N₂ protection reflow could improve the solderability of the solder joints,experimental results indicated that large-size components such as 1206 and SOIC-8fabricated in the air condition could also form the qualified solder joints.However,the N₂-based protective atmosphere was necessary for the assembly process of tiny 01005component,but a very low oxygen concentration in the N₂ atmosphere could increase the risk of tombstoning defect for the tiny components.When fine-pitch BGA device was reflowed in the air condition,the collapse of the solder joints was relatively low,which increased the risk of void and insufficient solder defects.Thus,N₂ protective atmosphere was also suggested for the assembly process of fine-pitch BGA device.Considering the cost and protection efficiency,the N₂-based protective atmosphere containing 1000 ppm O₂ is recommended and acceptable for SMT assembly.?5?During the temperature cycling tests?TCT?,the failure mechanism of the solder joints was attributed to fatigue and creep under the combined effects of periodic stress and high temperature.With the increase of TCT cycles,the thickness of IMC layer also increased significantly.The morphology of IMC layer changed from undulating scallop-type to uniform straight-type,and a thin Cu₃Sn phase layer was generated between the interface of Cu₆Sn₅ phase and Cu substrate.Kirkendall voids also formed on account of the unbalanced diffusion,which reduced the long-term reliability of the solder joints.The results of finite element simulations indicated that the shapes of the solder joints had a significant influence on the temperature cycling reliability.Larger stand-off thickness and plumper solder filling were beneficial to improve the temperature cycling reliability of solder joints for the chip components,the solder balls with a lanky shape also contributed to improve the reliability of BGA solder joints.