Wetting Analysis of Leadless Chips in Surface Mount Technology

It has been found that in wave soldering, when a chip entered the solder bath with its longitudinal axis parallel to the soldering direction, the resulting shape of the wetted surface of the incoming joint was concave, and the outgoing joint was convex. The difference in wetting of these two joints was analysed from the capillary viewpoints. Solder pads of various lengths were used to control the wetting conditions in infrared reflow soldering. The results show that the amount of tombstoning and joint opening are closely related to the pad length. The relationship between tombstoning, or open joint, and wetting was analysed based on theory, and it was found that the surface tension and the contact angle of the molten solder are the significant factors in the formation of the tombstoning or open joint defect.