Solder paste is a suspension of tiny solder spheres, graded into powder types such as type 3 and type 4 by particle size, in a flux vehicle that cleans the surfaces and holds parts until reflow. Common alloys are SAC305 for lead-free work and Sn63Pb37 for leaded. It is sensitive to temperature and humidity, stored cold and brought up to room temperature before opening, and printed volume is often checked by solder paste inspection before placement.
Switch to type 4 solder paste for the 0.4 mm pitch parts, type 3 keeps clogging the small apertures.
Also heard as
- paste
- cream solder
Related terms
Stencil
Thin laser-cut metal sheet used to print solder paste onto board pads before parts are placed.
Reflow
Soldering surface mount parts by melting printed solder paste in an oven that follows a set temperature profile.
Voiding
Gas pockets trapped inside solder joints, seen as light spots on X-ray, which weaken them and block heat.
Head-in-pillow
BGA defect where a solder ball rests in melted paste but never fuses with it, leaving a hidden weak joint.