Solder Paste

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 through reflow, melts and coalesces into finished solder joints. It is the medium that makes surface-mount assembly and rework practical: it puts a precise, repeatable amount of solder — and the flux to make it wet — exactly where each joint needs it, on hundreds of pads at once.

What's in the syringe

Paste is specified by alloy and by particle size. The common lead-free alloy SAC305 (96.5% tin, 3% silver, 0.5% copper) melts around 217°C, while traditional leaded Sn63/Pb37 melts at 183°C — a difference that sets the entire reflow temperature budget and explains why lead-free work is less forgiving of marginal heating. Particle sizes are graded by type: Type 3 and Type 4 cover general SMT work, with finer grades reserved for very fine-pitch printing. The flux system (rosin, water-soluble, or no-clean) determines cleaning requirements after reflow. For hashboard rework, a no-clean lead-free paste matching the board's original alloy is the sensible default — mixing alloys on one joint produces unpredictable metallurgy.

Application and reflow

Paste is dispensed by syringe for one-off bench repairs or printed through an SMT stencil for many pads at once. After parts are placed, the assembly is heated through a controlled temperature curve — the reflow profile: a ramp, a soak that activates the flux and evaporates solvents, a spike above the alloy's melting point, and a controlled cool-down. During the liquid phase, surface tension pulls each component into alignment with its pads — the small self-centering magic that makes SMT viable. This reflow step, whether in a reflow oven or under hot air, is how fine-pitch and BGA parts are attached without ever touching a pad with an iron; paste printed on the pads is likewise how BGA rework and reballing restore array packages.

Handling discipline

Paste is a perishable chemical product, not a shelf commodity. It is refrigerated in storage, brought fully to room temperature before opening (condensation causes solder balling), and discarded when past its life — over-aged paste prints badly and produces grainy, weak joints. Quantity control is the other discipline: too much paste bridges adjacent pads into a solder bridge, too little starves the joint, and uneven deposits on a two-terminal part invite tombstoning as one end wets first.

For ASIC hashboard repair, controlled paste deposition is what turns replacing a fine-pitch component from a gamble into a repeatable process. Paired with a stereo microscope for inspection, it lets a bench restore joints to factory quality instead of patching them — the standard D-Central holds its own repair work to. A last word on quantity for the home bench: buy small. A 30-gram syringe outlasts a year of hobby rework, while a bargain 500-gram jar will expire long before it empties — and expired paste quietly sabotages joints in ways that only show up as intermittent faults months later. Fresh consumables are the cheapest reliability money can buy in electronics repair, and paste is the most perishable consumable on the bench. Storage habits matter as much as purchase size: log the open date on the syringe, keep it capped and refrigerated between sessions, and never return warmed, part-used paste to stock alongside fresh material. When a run of joints suddenly looks dull or grainy despite unchanged technique and profile, suspect the paste before you suspect yourself — it is the variable that degrades silently while everything else on the bench stays constant.