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Effect of Technology on Reliability of Lead-free Solder Joints
SMT and MCM process flow problems. Including improper storage temperature of solder, insufficient solder pad, improper setting of reflow temperature curve, etc. As far as lead-free welding is concerned, the optimization of temperature curve in reflow welding process is the key to ensure a high reliable welding connection while maintaining the lowest peak temperature as possible.
At present, except Japan, consumer electronics companies in other countries seem to accept Sn-Ag-Cu alloy series. The proportion of silver in the alloy is 3,0%-4,7%, and that of copper is 0,5%-3,0%. The melting point of the alloys with different compositions is almost the same, which varies from 2 L7 to 22 L (?) C. The liquid temperature of Sn-Pb alloys is 183 (?) C, and the difference between them is 34 (?). Therefore, it is very important to monitor this stage closely at the beginning, aiming at the key variables in reflow process, such as peak temperature, time higher than liquid temperature, impregnation time, diffuse temperature and slope rate caused by the selection of flux and solder paste, in order to ensure that reflow process keeps 1,33 or higher than 1,33 CPK. In addition, void is a common defect of interconnect solder joints in reflow welding, especially in BGA / CSP components.
In lead-free soldering, voids are still a necessary concern, because the surface tension of Sn/Ag/Cu alloy is larger than that of Sn-Pb alloy in melting state, and the increase of surface tension will make it more difficult for gas to spill out during cooling stage, which will increase the void ratio. This has been confirmed in the development of lead-free solder paste. The results show that the number of voids in solder joints using lead-free solder paste is more than that of solder joints using tin-lead solder paste. Large voids and some small spherical voids are caused by flux volatilization. The proportion of flux in solder paste is the most direct factor affecting solder joint voids, so lead-free solder paste still has a lot of room for improvement. As a new generation of lead-free solder paste products, Multicore (96SC LF320 AGS88) has increased the activity of flux at high temperature, achieved a great leap in technology, and reduced the void level of lead-free solder joints to 7,5%. The second generation of general solder paste formed by material progress in recent two years not only has wider process window, easier application and better appearance, but also solves the problem of void.
