Effect Of Cooling Rate On The Solidification Microstructure Of Cu/Sn-3.5Ag/Cu Solder Joints

Sn-based solders are widely used in electronic packaging technology.Eutectic Sn-3.5Ag solder has been broadly applied in Ball Grid Array（BGA）,Chip Scale Packaging（CSP）and some other packaging structures due to its good mechanical and wetting properties.In Cu/Sn-3.5Ag/Cu solder joints,the number of Sn grains is always limited,and sometimes there even contains only one Sn grain in a joint.The structure of Sn crystal is body-centered tetragonal,and its physical and mechanical properties vary greatly along different crystal orientations,which will inevitably cause anisotropy in the performance of solder joints and then seriously affect the reliability of components.However,the service life of the component is determined by the weakest solder joint.In this paper,the solidification microstructure of Cu/Sn-3.5Ag/Cu solder joints with different pitch（300,100,50μm）solidified under different cooling rate（1.5,0.5,0.1 ℃/s）was studied.The quantity and orientation of Sn grains,as well as the characteristics of intermetallic compound（IMC）were analyzed.The main results show:1.Both the solder joint pitch and the cooling rate affect the number of Sn grains in the solder joints.In general,the number of Sn grains decreases as the solder joint pitch decreases.When the solder joint pitch is 300μm or 100μm,the number of Sn grains shows a minimum value under a cooling rate of 0.5 ℃/s.However,when the solder joint pitch is 50μm,the number of Sn grains is barely affected by the cooling rate,and it tends to form only one Sn grain in one solder joint.2.In the solder joints with different pitch solidified under different cooling rate,theθangles（the angle between the c-axis of Sn grain and the length direction of the solder joint）of the Sn grains present different distribution.In the 300μm pitch solder joints,theθangles in the joints solidified under different cooling rates are mostly distributed in the range of 50°to 90°.In the solder joints with 100μm pitch,theθangles of solder joints solidified under different cooling rates are mostly distributed in the range of 40°to 90°.And in the 50μm pitch solder joints,the distribution ofθangles is relatively random,among which,theθangles are mostly distributed above 40°in the joints solidified under a cooling rate of 0.1 ℃/s,while the distribution ofθangles shows some uncertainty under the other two cooling rates.3.Solder joint pitch and cooling rate all affect the fromation of IMC.Excessive growth of large Ag₃Sn IMC may occur in the solder joints of 300,100,and 50μm pitches which solidified under a cooling rate of 0.1 ℃/s,especially in the 50μm fine pitch solder joints.The large Ag₃Sn IMC can precipitate throughout the upper and lower interfaces of the whole solder joint along the length direction.The throughout grain boundaries（connected the two interfaces）and the large Ag₃Sn IMCs will inevitably degrade the reliability of the solder joints.