Experimental studies of pore formation in surface mount solder joints

This paper reports some new experimental findings and theoretical analyses on the pore formation in surface mount solder joints fabricated using conventional infrared (IR) reflow soldering. Two major types of specimens are employed here: blank pad (with no component) and sandwiched solder joints including a gull-wing leaded assembly and shear specimen (i.e. strap specimen). It is found that pores formed in a blank pad have a critical radius which is independent of the reflow time. A pore is stable and cannot be annihilated during reheating if its radius is below the critical radius. The critical radius is enlarged and strongly correlated with the maximum radius in the sandwiched solder joints. The pore formation in sandwiched solder joints is affected greatly by the joint configuration. The maximum principal radius is normally less than 0.2 mm if the joint thickness is greater than 0.20 mm. However, it may be greater than 0.3 mm when the joint thickness is less than 0.1 mm. Pores formed in the solder joints cannot be eliminated even by prolonging the reflow time. However, the pore radius usually increases with reflow time.