Study On Microstructure And Mechanical Properties Of Interconnection Between SAC305 Solder And Cu Column

As a common microelectronic packaging device,CuCGA is used in many fields.Traditional CuCGA devices realize positioning interconnection with the help of planting column mold.In this paper,a new connection method is proposed,in which the rotating friction planting column connection of welded column in CuCGA devices is realized by using high precision micro table drill on the basis of the good alignment effect between pure copper column and SAC305 solder ball.At the same time,the structure and mechanical properties of solder joints during the isothermal aging process were investigated.In this experiment,SAC305 solder with diameter of 2.4 mm,pure copper column with diameter of 0.8 mm and PCB plate were used as materials to make solder joint under the parameters of rotating speed 2400 r/min,post-planting time13 s and feed speed 0.2 mm/s.The microstructure and mechanical properties of the post-planting solder joint and the post-planting solder joints after aging and reflowing were analyzed.The results of the study show that:1. Under the combined effect of thermal and mechanical coupling,the solder structure was divided into three different areas according to the difference in structure morphology,which were:Area ? fine microstructure with a distance of0?80?m from interface,area ? elongated microstructure with a distance of80?180?m from interface,and area ? approximate reflux microstructure beyond the interface of 180?m.After aging,the IMC particles in areas ?,?,and ? were obviously coarsened,and the size was coarsened from less than 1?m before aging to3?5?m.A certain degree of coarsening had occurred after aging in various districts.2. Under the effect of the rotating friction of the copper column,the solder joint formed an interface layer with a width of about 0.3?m at the interface between the solder and the pure Cu column.At the aging temperature of 90? for 36 days,only Cu₆Sn₅ layer was generated at the interface,and the thickness was 0.98?m for 9days aging increased to 2.01?m for 36 days aging.Cu₃Sn layer was formed at the aging temperature of 120? for 9 days,and its thickness was increased from 1.03?m for 9 days aging to 2.89?m for 36 days aging.After 9/16/25/36 days,the total IMC layers were 3.01?m,4.51?m,7.51?m and 15.03?m.The solder joints of the post were interconnected.With the occurrence of aging,the thickness of the interface layer increased.3. The hardness of the solder in different areas was tested using a nano-indenter.The results show that:the average indentation hardness in area ? was the smallest,about 200 MPa;the average hardness of the indentation hardness in area ? was significantly increased,reaching 250 MPa.The average indentation hardness of ?-Sn matrix in area ? was about 240 MPa;the average indentation hardness of eutectic structure in area ? was about 300 MPa.After aging,the average indentation hardness of area ? increased from 200 MPa under no aging conditions to230?270 MPa;the average indentation hardness after aging in areas ? and ? decreased from 250 MPa and 270 MPa under no aging conditions to about 160 MPa and 180 MPa.The hardness of the tissue in area ? improved significantly and the property was restored.4. The creep rate sensitivity index m of each area was calculated under the experimental conditions with a holding time of 180 s:the m value of the area ? structure was 0.2556,the m value of the area ? structure was 0.1195,the m value of the eutectic structure in area ? was 0.0694,and the m value of the?-Sn matrix in the area ? was 0.1275.After aging,the value of m in each area changed significantly.The value of m in area ? decreased to about 0.1,and the values of m in areas ? and ? were about 0.25 and 0.1,respectively.After aging,the creep resistance of the tissue in area ? was improved,which reached the creep resistance of the solder base metal.5. The average pull-off load of the solder joint before aging was 32.59 N.After aging,the joint strength of the solder joint at both aging temperatures increased first and then decreased with the extension of the aging time.The solder joints of the post had a certain interconnection strength,and the connection strength was enhanced after aging.