Definition
Solder paste is a gray, putty-like mixture of microscopic solder alloy spheres suspended in flux and a binder. Deposited onto a board's pads before components are placed, it holds parts in position and then, when heated, melts and coalesces into proper solder joints. It is the medium that makes surface-mount assembly and rework practical, because it puts a precise, repeatable amount of solder exactly where each joint needs it.
Application and reflow
Paste is dispensed by syringe for one-off work or printed through an SMT stencil for many pads at once. After the part is placed, the assembly is heated through a controlled temperature curve, the reflow profile, that activates the flux, brings the alloy past its melting point, and lets surface tension pull each joint into shape. This reflow step is how fine-pitch and BGA parts are attached without touching each pad with an iron.
Handling on the bench
Solder paste has a real shelf life and is sensitive to temperature; it is typically refrigerated and brought to room temperature before use, and over-aged paste produces poor, grainy joints. Too much paste bridges adjacent pads; too little starves the joint.
For ASIC hashboard rework, controlled paste deposition is what makes replacing a fine-pitch component repeatable rather than a gamble. Paired with a stereo microscope for inspection, it lets a repair bench restore joints to factory quality instead of patching them.
In Simple Terms
Solder paste is a gray, putty-like mixture of microscopic solder alloy spheres suspended in flux and a binder. Deposited onto a board’s pads before components…