Solder Paste Guide: Alloys, Particle Sizes, Flux Types & Storage

What Is Solder Paste?

Solder paste is a mixture of tiny solder alloy particles suspended in a flux medium. It is the primary joining material used in surface mount technology (SMT) assembly, applied to PCB pads via stencil printing before components are placed and reflowed in an oven.

Solder Paste Alloys

The alloy determines the melting point, mechanical strength, and reliability of the solder joint. The most common alloys are:

  • SAC305 (Sn96.5/Ag3.0/Cu0.5) — The industry-standard lead-free alloy. Melting point 217°C. Excellent mechanical strength and reliability for most SMT applications.
  • SAC387 (Sn95.5/Ag3.8/Cu0.7) — Slightly higher silver content. Good for fine-pitch and high-reliability applications.
  • Sn63/Pb37 (Eutectic) — The classic leaded alloy with a sharp melting point of 183°C. Still used in military, aerospace, and medical applications exempt from RoHS.
  • Low-Temperature Alloys (Bi-based) — Bismuth-tin alloys with melting points below 140°C. Used for temperature-sensitive components and step soldering.
  • High-Temperature Alloys — Used in power electronics and applications requiring joints that survive high operating temperatures.

Flux Types in Solder Paste

The flux activates metal surfaces, removes oxides, and promotes wetting. Solder paste flux types include:

  • No-Clean (ROL0, ROM0, REL0) — Leaves minimal, non-corrosive residue that does not require cleaning. The most widely used flux type in commercial electronics.
  • Water-Soluble (ORL0, ORM0) — More aggressive activation for difficult-to-solder surfaces. Residues must be cleaned with DI water after reflow.
  • Rosin/Resin (RMA) — Traditional flux chemistry. Residues are non-corrosive but may require cleaning for cosmetic or conformal coating applications.

See our Flux Types Explained guide for a deeper comparison.

Particle Size (Type)

Solder paste is classified by powder particle size, which determines the minimum feature size it can print reliably:

  • Type 3 (25–45 μm) — Standard for most SMT applications with 0402 components and larger.
  • Type 4 (20–38 μm) — For fine-pitch components (0201, QFP, BGA).
  • Type 5 (15–25 μm) — For ultra-fine pitch and micro-BGA applications.
  • Type 6 & 7 — For advanced packaging and jet printing applications.

A general rule: the stencil aperture should be at least 5x the maximum particle diameter to ensure consistent paste release.

Viscosity & Slump

Solder paste viscosity affects printability and slump resistance. Higher viscosity pastes hold their shape better after printing (less slump), which is critical for fine-pitch work. Viscosity is measured in Pascal-seconds (Pa·s) and should be matched to your printer speed and stencil design.

Solder Paste Storage & Handling

  • Store solder paste refrigerated at 0–10°C (32–50°F) to maximize shelf life (typically 6 months from manufacture).
  • Allow paste to reach room temperature before opening — typically 2–4 hours. Never heat to accelerate warm-up.
  • Once opened, use within the manufacturer’s recommended stencil life (typically 8–24 hours).
  • Track lot numbers and expiration dates. Never use expired paste.
  • Unused paste returned to the jar should be used first (FIFO).

Choosing the Right Solder Paste

Match your paste selection to your process requirements:

  • Component pitch — Finer pitch requires smaller particle size (Type 4 or 5).
  • Cleaning process — No-clean if no post-reflow cleaning; water-soluble if cleaning is required.
  • RoHS compliance — Lead-free (SAC305) for most commercial products; leaded for exempt applications.
  • Reflow profile — Match alloy melting point to your oven capability and component temperature ratings.

Shop solder paste at KEM-TRON from AIM, Kester, and Indium in a full range of alloys, flux types, and jar sizes.