Effect Of Cooling Rate On Quality Of Solder Joints In Lead-Free Reflow Soldering

The legislation to ban the use of Pb-based solders will become effective im-mediately, which provide a driving force for enterprises to accelerate Pb-free process. It's found that reflow soldering plays an important role in Surface Mount-ing Technology ,moreover, cooling rate in reflow soldering profile is getting more and more attention after the use of high-melting-point solders.The melting point of Pb-free solder is 30℃~40℃higher than Sn-Pb eutectic solder. The increase of temperature in reflower becomes a challenge of Print Circuit Board (PCB) and components. As a result, the Time Above Liquid (TAL) of solder joints becomes longer, therefore, fast cooling in reflow soldering is used for controlling the PCBA temperature , improving the microstructure of joints and decreasing the thickness of intermetallic compound , consequently, high quality products can be obtained. How cooling rate affects the quality of soldering joints in lead-free process was studied in this paper. The experiments were based on practical industrial production and it focused on the effect of cooling rate on microstructure and mechanical properties.When cooled at -4~-6.5℃/S, the microstructure of joints were refined, the IMC of Ag3Sn and Cu6Sn5 phases dispersed in eutectic network in joints which presented spherical particles. The fracture of these joints after tensile failure presented dimple mode. Furthermore, the thickness of IMC was thin and it presented gentle incline morphology , as a result, it got high mechanical properties. Actually, the strength of IMC on the interface was higher than the inside of joints under fast cooling condition, so the inside of joints crack first ; On the contrary ; joints cooled at a speed less than -1.5℃/S got rather coarse microstructure, including coarse Ag3Sn phase and IMC on the interface. These joints behaved relatively weak mechanical properties.In a word, the thickness and morphology of intermetallic compound is the determinant factor of strength of joints which explains how cooling rate affects the mechanical properties of solder joints.It should pay much attention to the distance between the last heating zone and the first cooling zone of lead-free reflower. It should decrease the distance as...