Solder Wire Guide: Alloys, Gauges & Core Types
Solder wire is the most fundamental consumable in electronics assembly, yet the number of available specifications — alloys, diameters, flux core types, flux percentages, and lead-free vs. leaded formulations — can make selection feel overwhelming. This guide covers everything you need to know to choose the right solder wire for your process.
How Solder Wire Works
Solder wire consists of a metal alloy formed into a wire, with one or more flux cores running through the center. During soldering, the wire melts and the flux is released simultaneously — cleaning the metal surfaces and enabling the molten solder to wet and bond. The quality of the joint depends on the alloy, the flux chemistry, the thermal profile, and the technique.
Leaded vs. Lead-Free Solder Wire
The most fundamental choice in solder wire selection is whether to use leaded or lead-free alloy.
Leaded solder — Traditional tin-lead alloys, most commonly Sn63/Pb37 (63% tin, 37% lead) and Sn60/Pb40. These alloys have a well-understood eutectic or near-eutectic melting behavior, excellent wetting characteristics, and decades of process data behind them. Sn63/Pb37 is the eutectic alloy — it melts and solidifies at a single temperature (183°C) rather than passing through a plastic range, making it forgiving and reliable for hand soldering.
Lead-free solder — Required for products sold in the EU under RoHS (Restriction of Hazardous Substances) directive and increasingly mandated globally. The most common lead-free alloy is SAC305 (96.5% tin, 3% silver, 0.5% copper), with a melting range of approximately 217–220°C. Lead-free alloys require higher soldering temperatures, are less forgiving in hand soldering, and demand careful tip maintenance to prevent oxidation.
Exemptions: Certain applications — including military, aerospace, medical, and some industrial electronics — are exempt from RoHS and may continue to use leaded solder where reliability data supports it. Tin-lead solder remains the preferred choice in many high-reliability programs due to its superior thermal fatigue resistance and well-characterized failure modes.
Common Solder Alloys
Sn63/Pb37 — The workhorse of leaded electronics soldering. Eutectic alloy, 183°C melting point, excellent wetting, ideal for hand soldering and wave soldering.
Sn60/Pb40 — Slightly lower tin content, small plastic range (183–190°C). Widely used and cost-effective for wave soldering.
SAC305 (Sn96.5/Ag3/Cu0.5) — The dominant lead-free alloy for electronics. Good wetting, reliable joints, compatible with most lead-free processes. Higher melting point requires adjusted soldering temperatures.
Sn99.3/Cu0.7 — Lower-cost lead-free alloy without silver. Slightly higher melting point and less forgiving than SAC305, but economical for wave soldering applications.
Solid wire (no flux core) — Used with separately applied flux, or in applications where flux contamination must be avoided. Common in specialized industrial and non-electronic soldering.
Flux Core Types
The flux core in solder wire determines the chemistry released during soldering. Core type must be matched to your cleaning process and reliability requirements — the same considerations that apply to liquid flux selection.
Rosin (R) and Rosin Mildly Activated (RMA) — The traditional standard for electronics. Reliable, well-characterized, residues are generally benign and can be left in place or cleaned with rosin-compatible solvents. RMA is the most widely used core type for general electronics hand soldering.
No Clean — Low-residue flux that does not require cleaning after soldering. Preferred for high-volume production where eliminating the cleaning step reduces cost. Residues are minimal and electrically benign under normal conditions.
Water Soluble — High-activity organic acid flux core for maximum wetting performance. Residues must be cleaned with deionized water after soldering. Used where joint quality is paramount and aqueous cleaning is available.
Acid — For non-electronic applications only. Never use acid-core solder wire on electronics.
Flux Percentage
Solder wire is available with varying flux core percentages, typically ranging from 1% to 3.3% by weight. Higher flux content provides more aggressive cleaning action and is useful for oxidized or difficult surfaces. Lower flux content produces less residue and is preferred where cleanliness is critical. For most hand soldering applications, 2% to 2.2% flux content is a good starting point.
Wire Diameter (Gauge)
Solder wire diameter should be matched to the size of the joints being soldered. Using wire that is too large for fine-pitch work makes it difficult to control the amount of solder applied; wire that is too small for large joints requires multiple passes and slows production.
0.020" (0.5mm) — Fine-pitch SMT rework, small connectors, precision hand soldering.
0.025" (0.6mm) — General SMT rework and fine hand soldering.
0.031" (0.8mm) — The most versatile diameter for general electronics hand soldering. A good default for through-hole and mixed assembly work.
0.040" (1.0mm) — Larger through-hole components, connectors, and terminals.
0.062" (1.6mm) — Heavy connectors, large terminals, chassis ground connections, and non-electronic applications.
Spool Sizes
Solder wire is available in a range of spool sizes to suit different usage volumes — from small 1 oz. spools for rework stations and low-volume use, to 1 lb. spools for production benches, to larger 5 lb. spools for high-volume operations. Selecting the right spool size reduces waste and ensures wire is used before it ages.
Solder Wire for Robotic and Automated Soldering
Robotic soldering systems — including selective soldering machines and soldering robots — have specific wire feed requirements. Wire diameter, flux percentage, and alloy must be specified to match the machine's feed mechanism and thermal profile. Consistency of wire diameter and flux distribution is critical for repeatable automated results; premium solder wire from manufacturers like Kester is manufactured to tight tolerances to ensure reliable automated feeding.
Choosing the Right Solder Wire
Start with your regulatory requirements: if your products must comply with RoHS, lead-free alloy is required. If you are in an exempt category and reliability is paramount, leaded alloy may be preferred. Match flux core type to your cleaning process: RMA or no clean if you are not cleaning, water soluble if you have aqueous cleaning capability and need maximum wetting performance. Select diameter based on the smallest joints in your assembly.
KEM-TRON stocks a comprehensive range of Kester solder wire in leaded and lead-free alloys, rosin, RMA, no clean, water soluble, and acid core types, across a full range of diameters and spool sizes.