Low temperature soldering

Low temperature soldering may reduce the cost for surface mount electronic assembly by using low cost electronic components and substrate materials. It also provides options for step soldering, and reduces the risk of thermally induced damages. A vast amount of Sn-Pb-Bi-In alloys are available for any given melting temperature (liquidus) between 50°C and 183°C. Currently available water clean or no-clean fluxes, however, are not suitable for low temperature soldering, because they are activated at temperature higher than 150°C. Two solderability test methods were used to evaluate the newly developed fluxes for low temperature soldering: (a) spreading test to compare the relative strength of the fluxes, and (b) wetting balance to determine the activation temperature of the fluxes. The mechanical properties (shear strength, creep, isothermal and thermal fatigue) of several common low temperature solders were determined. Prototypes of PCBs assembled with several low temperature solders were successfully built and passed typical product qualification tests. Potential problems for low temperature soldering are the poor adhesion strength of the low temperature solders to Alloy 42 leaded components, and exceptional grain growth and early thermal failure when 58Bi-42Sn was used to solder on Sn-Pb surface