Solder paste is a thick gray substance made of powdered solder alloy suspended in a flux medium. It is used to solder surface-mounted components (SMDs) to printed circuit boards (PCBs), primarily during reflow soldering. The paste serves two purposes: to make the mechanical connection and to conduct electricity between components and the board.
🧬 Types of Solder Paste
Type
Description
Usage Area
No-Clean
Leaves minimal residue, does not require cleaning
Prototyping, consumer electronics
Water Soluble
High activity, but must be thoroughly cleaned
Industrial & critical applications
RMA (Rosin)
Mildly activated, may require cleaning in some cases
Mixed assembly, rework
⚖️ Comparison of Common Solder Paste Alloys
Alloy
Composition
Melting Point
Lead-Free
Notes
Sn63/Pb37
63% Tin, 37% Lead
183°C
❌
Excellent wetting, low cost
SAC305
96.5% Sn, 3% Ag, 0.5% Cu
~217–221°C
✅
Most popular lead-free alloy
Sn42/Bi58
42% Tin, 58% Bismuth
138°C
✅
Low-temp soldering, brittle
🧰 Best Solder Pastes (Proven & Trusted)
Name
Type
Notes
Kester EP256
RMA
Great for reflow, low residue, long stencil life
Chip Quik SMD291AX
No-Clean
Smooth printing, easy reflow, consistent results
Loctite GC10
No-Clean
Long pot life, high reliability, excellent printability
Amtech NC-559-V2-TF
No-Clean
Legendary among hobbyists, stable and easy to clean
Application Method: Stencil, syringe, or screen printing?
Component Type: QFN, BGA, or large PTH pads?
Shelf Life & Storage: Refrigeration usually required ❄️
Flux Compatibility: With board finish (ENIG, HASL, etc.)
Reflow Profile: Does your oven match the melting curve?
🧼 Cleaning and Safety
Always wear gloves and avoid skin contact
Use IPA or specialized flux remover if cleaning is needed
Store refrigerated between 0–10°C
Follow MSDS for ventilation and handling
🏁 Final Verdict
For most DIY and repair users, Chip Quik SMD291AX or Amtech NC-559 will provide great results with minimal hassle. For lead-free or commercial reflow, Kester EP256 or Loctite GC10 are industry favorites. Avoid mystery syringes with no label — they can ruin boards and sanity alike.
Solder paste isn’t just gray goo — it’s what makes your entire circuit actually work.
