Solder Paste for SMT: Types, Storage, and How to Print It Right

Solder paste does most of the work in SMT assembly before reflow even starts. It holds components in place during placement and becomes the solder joint in the oven. Choosing the right paste, storing it correctly, and printing it inside its working window are decisions that show up directly in yield.

What Solder Paste Is

Solder paste is a suspension of solder alloy powder in a flux vehicle. The flux removes oxides and prepares the surfaces; the powder forms the joint when it melts. The ratio of powder to flux, called metal loading, is typically around 88 to 90 percent by weight.

Alloy Choice

The most common lead-free alloy is SAC305 (96.5% tin, 3.0% silver, 0.5% copper). It is a general-purpose choice with good thermal fatigue performance. Lower-silver alloys such as SAC105 reduce cost but trade away some reliability margin. Leaded alloys such as 63/37 are still used where the application is exempt from RoHS. The target market decides the alloy, not the assembler.

Flux Types

Flux falls into a few families. Rosin and resin fluxes are the traditional types; water-soluble fluxes require cleaning; no-clean fluxes are the standard for SMT because they leave a residue that is safe to leave on the board. Some assemblies still specify cleaning, which adds a process step and cost.

Powder Size

Solder powder is graded by type number: the higher the number, the finer the powder. Type 3 is a common general-purpose grade. Type 4 and Type 5 are used for fine-pitch and small components where the aperture is too small for coarser powder. The stencil and paste must be matched to the smallest pitch on the board.

Storage and Handling

Solder paste is temperature-sensitive. It is stored refrigerated (typically 2 to 10 °C), allowed to warm to room temperature before opening, and used within the manufacturer's shelf life and working life. Paste left out too long dries and prints poorly. Used paste should never be returned to the jar. A defined storage and rotation system prevents most paste-related defects.

Printing

Printing is the step where most SMT defects begin. The stencil, squeegee, and printer settings control how much paste lands on each pad. After printing, solder paste inspection (SPI) measures paste height and volume on every board and flags deviations before placement — catching defects at the earliest point, before they become rework.

FAQ

Q: What solder paste is used for lead-free SMT assembly?

A: SAC305 (tin-silver-copper) is the common general-purpose lead-free alloy; lower-silver options exist when cost matters more than margin.

Q: Does solder paste need to be refrigerated?

A: Yes, it is stored refrigerated and warmed to room temperature before use to keep its printing properties stable.

Q: What does no-clean flux mean?

A: It leaves a residue that is safe to leave on the board, so no post-reflow cleaning step is required.

Q: What is solder paste inspection (SPI)?

A: SPI measures paste height and volume right after printing, catching defects before placement and reflow.

Shenzhen 1943 Technology Co., Ltd. controls paste selection, storage, and print quality with SPI as part of its SMT process for NPI and small-batch assembly.