Solder Types Explained

Choosing the right solder isn't just about picking leaded or lead-free. Alloy composition, form factor, flux chemistry, and powder type all affect joint quality, process yield, and compliance. This guide breaks down the main solder types and when to use each. For a complete resource on solder materials, flux, conformal coating, and rework, visit our Soldering, Desoldering & Rework hub.

Leaded vs. Lead-Free Solder

Leaded solder (Sn63Pb37) — 63% tin, 37% lead — is a eutectic alloy, meaning it transitions directly from liquid to solid at a single temperature (183°C) with no plastic phase. This makes it highly forgiving in hand soldering and rework, producing bright, reliable joints with excellent wettability. It remains the standard for military, aerospace, and high-reliability applications exempt from RoHS.

Lead-free solder (SAC305) — 96.5% tin, 3% silver, 0.5% copper — is the dominant RoHS-compliant alloy for commercial electronics. It has a higher melting point (217–220°C), requires tighter process control, and produces duller joint appearances that can be mistaken for cold joints by inspectors unfamiliar with lead-free characteristics. Other lead-free alloys (SAC105, SN100C, bismuth-based) are used for specific applications including low-temperature reflow and step soldering.

Solder Forms

Solder paste is a mixture of solder powder, flux, and rheological agents printed onto PCB pads through a stencil before reflow. Paste is specified by alloy, powder type (Type 3 through Type 8 — finer powder for smaller features), and flux chemistry (no-clean, water-soluble, RMA). It's the standard form for SMT assembly.

Solder wire is used for hand soldering, wave soldering, and selective soldering. Wire diameter and flux core percentage affect how much flux is delivered per joint. Common diameters range from 0.015" to 0.062". Flux core options include no-clean rosin, water-soluble, and halide-free formulations.

Solder preforms are precision-stamped shapes (rings, discs, washers) used where a controlled volume of solder is needed — common in power electronics, RF assemblies, and hermetic sealing. They eliminate the variability of manual solder application.

Bar solder is used in wave solder machines and selective solder pots. It's melted into the solder bath and replenished as solder is consumed by the process.

Flux Core Types in Solder Wire

No-clean flux leaves minimal, non-corrosive residue that doesn't require removal in most applications. It's the most common choice for commercial electronics where post-solder cleaning isn't part of the process. Note: “no-clean” doesn't mean the residue is invisible — it means it's been validated as safe to leave in place under normal conditions.

Water-soluble flux is highly active and produces excellent wetting, but residues must be removed with DI water after soldering. Required when cleanliness specifications demand ionic residue below a defined threshold, or when conformal coating adhesion requires a clean surface.

Rosin flux (R, RMA, RA) ranges from mildly active (R) to fully activated (RA). Rosin-based fluxes are well-established in high-reliability and military applications. Residues may require cleaning depending on activity level and end-use requirements. See our Rosin 101 guide for a deep dive into rosin chemistry.

How to Choose the Right Solder

Start with these four questions:

• RoHS compliance required? — Lead-free alloy (SAC305 or equivalent)
• High-reliability / military / aerospace? — Leaded Sn63Pb37, with appropriate exemption documentation
• Process type? — Paste for reflow, wire for hand/wave/selective, preforms for controlled-volume applications
• Post-solder cleaning? — No-clean if skipping wash; water-soluble if cleaning is part of the process or conformal coating follows

See our Solder Solutions guide for a deeper look at paste vs. wire selection, and our Wave vs. Selective vs. Reflow guide for process selection guidance.

Featured Solder Brands at MTE Solutions

MTE Solutions carries solder materials from Indium Corporation and Kester — two of the most trusted names in electronics assembly chemistry:

• Indium Corporation Solder & Flux
• Kester Solder & Flux
• Shop All Solder Wire & Paste

Not sure which solder is right for your process? Contact our team with your alloy requirements, process type, and compliance needs and we'll recommend the right product.