Solder Types Explained
Quick Answer
The main solder types are leaded (Sn63Pb37) and lead-free (SAC305), available in paste, wire, preform, and bar form. Leaded solder melts at 183°C and is preferred for high-reliability, military, and aerospace applications. Lead-free SAC305 melts at 217–220°C and is required for RoHS-compliant commercial electronics. Flux chemistry — no-clean, water-soluble, or rosin — determines whether post-solder cleaning is required. The right solder depends on your compliance requirements, process type, and whether cleaning is part of your workflow.
In this guide: Leaded vs. lead-free alloys, solder forms (paste, wire, preforms, bar), flux core types, and how to choose the right solder for your process.
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
Frequently Asked Questions
What is the difference between leaded and lead-free solder?
Leaded solder (Sn63Pb37) is a eutectic alloy that melts at 183°C and produces bright, reliable joints with excellent wettability. It is preferred for military, aerospace, and high-reliability applications exempt from RoHS. Lead-free solder (SAC305) melts at 217–220°C, requires tighter process control, and is required for RoHS-compliant commercial electronics. Lead-free joints appear duller than leaded joints, which can be mistaken for cold joints by inspectors unfamiliar with lead-free characteristics.
What is solder paste and when is it used?
Solder paste is a mixture of solder powder, flux, and rheological agents that is printed onto PCB pads through a stencil before reflow soldering. It is the standard solder form for SMT (surface mount technology) assembly. Paste is specified by alloy (e.g., SAC305), powder type (Type 3–8, with finer powder for smaller features), and flux chemistry (no-clean, water-soluble, or RMA). After printing, the board goes through a reflow oven where the paste melts and forms solder joints.
What is no-clean solder flux?
No-clean flux is a flux chemistry that leaves minimal, non-corrosive residue after soldering that does not require removal in most applications. It is the most common flux type for commercial electronics where post-solder cleaning is not part of the process. "No-clean" does not mean the residue is invisible — it means the residue has been validated as safe to leave in place under normal operating conditions. If conformal coating will be applied after soldering, verify that the no-clean residue is compatible with the coating adhesion requirements.
What is SAC305 solder?
SAC305 is the most widely used lead-free solder alloy, composed of 96.5% tin, 3% silver, and 0.5% copper. It is the dominant RoHS-compliant alloy for commercial electronics assembly. SAC305 has a melting point of 217–220°C — significantly higher than leaded Sn63Pb37 at 183°C — and requires tighter process control in reflow and wave soldering. It is available in paste, wire, bar, and preform formats.
When should I use water-soluble flux instead of no-clean flux?
Water-soluble flux should be used when cleanliness specifications require ionic residue below a defined threshold, when conformal coating adhesion requires a clean, residue-free surface, or when the end-use application demands the highest level of surface cleanliness. Water-soluble flux is highly active and produces excellent wetting, but residues must be removed with DI water after soldering — leaving water-soluble flux residue in place will cause corrosion over time.
What solder brands does MTE Solutions carry?
MTE Solutions carries solder materials from Indium Corporation and Kester — two of the most trusted names in electronics assembly chemistry. Both brands offer a full range of solder alloys, paste formulations, wire, flux, and specialty products for commercial, industrial, and high-reliability applications.
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:
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.
