Solder Solutions: A Guide to Choosing the Right Paste or Wire for Your Application
Solder Solutions: A Guide to Choosing the Right Paste or Wire for Your Application
The choice between solder paste and solder wire is determined by the assembly process, not personal preference — solder paste is engineered for stencil printing and reflow in SMT production, while solder wire is the correct choice for hand soldering, through-hole assembly, and rework. Within each format, alloy composition (leaded vs. lead-free, melting point, reliability profile), flux type (no-clean vs. water-washable), and particle size or wire diameter must all be matched to the specific application. MTE Solutions is an ISO 9001:2015 and ISO 13485:2016 certified distributor carrying the full solder lines from Indium Corporation, Kester, and Chip Quik. For a full overview of solder materials, flux, and rework tools, visit our Electronics Assembly Chemicals & Coatings hub.
Solder Paste vs. Solder Wire: Selection Guide
| Attribute | Solder Paste | Solder Wire |
|---|---|---|
| Primary Process | SMT stencil printing + reflow oven | Hand soldering, through-hole, rework |
| Best For | High-volume production, fine-pitch SMT, mass pad printing | Manual assembly, repair, prototyping, low-volume, rework of individual joints |
| Alloy Options | Sn63/Pb37 (leaded, 183°C eutectic), SAC305 (lead-free, ~217°C), specialty alloys | Sn63/Pb37, SAC305, SN995, and other lead-free alloys for RoHS compliance |
| Flux Type | No-clean (residue stays on board), water-washable (must clean after reflow), RMA | Flux-core wire (no-clean or rosin core); flux activates during soldering |
| Size Specification | Powder type (Type 3–8); finer powder for smaller apertures and fine-pitch components | Wire diameter (0.5mm for fine-pitch; 1.0mm+ for larger joints and through-hole) |
| Storage | Refrigerated storage required; warm to room temperature before use; track open-jar life | Room temperature storage; no special handling required |
| Key Brands at MTE | Indium, Kester, Chip Quik | Indium, Kester, Chip Quik |
Frequently Asked Questions
What is the difference between Sn63/Pb37 and SAC305, and how do I choose?
Sn63/Pb37 is a eutectic tin-lead alloy that melts at 183°C — its eutectic composition means it transitions directly from solid to liquid without a plastic range, producing bright, reliable joints and making it forgiving for hand soldering. SAC305 (96.5% tin, 3% silver, 0.5% copper) is the dominant lead-free alloy for RoHS-compliant assembly, melting at approximately 217°C. SAC305 requires higher process temperatures, is less forgiving in hand soldering, and produces duller joint appearance. Use Sn63/Pb37 for mil/aero/medical applications with RoHS exemptions or for rework on existing leaded assemblies; use SAC305 for RoHS-compliant production. Never mix leaded and lead-free solder in the same assembly without engineering review.
When should I use no-clean flux vs. water-washable flux in solder paste?
No-clean paste leaves a benign flux residue designed to remain on the board without causing reliability issues — it is the dominant choice in modern SMT because it eliminates the cleaning step. Water-washable paste uses higher-activity organic flux that must be cleaned with DI water after reflow; it is preferred for difficult-to-solder surfaces or when conformal coating requires a completely clean substrate. Never leave water-washable flux residue on a board — it is hygroscopic and will cause corrosion. Match your flux type to your post-reflow process before committing to a paste specification.
What solder wire diameter should I use for different applications?
Use thinner wire (0.5–0.6mm) for fine-pitch SMT rework, 0402/0603 passives, and QFN/BGA reballing where precise solder volume control is critical. Use medium diameter (0.8mm) for general through-hole and standard SMT rework. Use thicker wire (1.0mm+) for larger through-hole components, connector pins, and wire terminations where more solder volume is needed per joint. Using wire that is too thick for a fine-pitch joint makes it difficult to control solder volume and increases the risk of bridging; using wire that is too thin for a large joint requires multiple feeds and increases dwell time, risking pad or component damage.
Does solder paste need to be refrigerated, and what happens if it isn’t?
Yes. Solder paste must be stored refrigerated (typically 0–10°C / 32–50°F) to slow flux degradation and prevent premature reaction of the activators. Paste stored at room temperature has a significantly shorter shelf life — flux activity degrades, viscosity changes, and print performance suffers. Before use, paste must be warmed to room temperature in the sealed container (typically 2–4 hours) to prevent condensation from forming inside the jar, which would contaminate the paste. Track open-jar life per the manufacturer’s specification — most no-clean pastes have a 24–48 hour open-jar life once removed from refrigeration.
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