Solder Paste

Solder paste (or solder cream) is used to connect the leads of surface mount integrated chip packages to attachment points (lands) in the circuit patterns on a printed circuit board. The paste is typically applied to the lands using a stencil to "print" the paste, although other methods, like dispensing from a tube, are also used. Typically, solder paste accounts for 0.05% of a circuit board's final cost.

A majority of the defects in circuit-board assembly are caused due to issues in the solder-paste printing process or due to defects in the solder paste. There are many different types of defects—too much solder, and the solder melts and connects to many wires (bridging), resulting in a short circuit. Insufficient amounts of paste result in incomplete circuits. Head-in-pillow defects, or incomplete coalescence of Ball grid array (BGA) sphere and solder paste deposit, is a failure mode that has seen increased frequency since the transition to lead-free soldering. Often missed during inspection, a head-in-pillow (HIP) defect appears like a small head resting on a pillow with a visible separation in the solder joint at the interface of the BGA sphere and paste deposit. An electronics manufacturer needs experience with the printing process, specifically the paste characteristics, to avoid costly re-work on the assemblies. The paste's physical characteristics, like viscosity and flux levels, need to be monitored periodically by performing in-house tests.

When making PCBs (printed circuit boards), manufacturers often test the solder paste deposits using SPI (solder paste inspection). SPI systems measure the volume of the solder pads before the components are applied and the solder melted. SPI systems can reduce the incidence of solder-related defects to statistically insignificant amounts. Inline systems are manufactured by Koh Young (Korea), CyberOptics (US), Parmi (Korea) and Test Research, Inc. (Taiwan). Offline systems are manufactured by VisionMaster, Inc. (US).