Solder Selection — Alloys and Form Factors

Section Overview

This chapter turns from the tools to the materials you feed them, and it starts with the most fundamental: solder, the metal that makes the joint. Solder is a fusible alloy — a metal that melts at a low temperature, wets the metal surfaces of a pad and a component lead, and solidifies into a joint that is both an electrical connection and a mechanical bond. The central choice is the alloy. Leaded solder — tin-lead, especially the eutectic Sn63/Pb37 — melts lower (around 183 degrees Celsius), wets and flows beautifully, and is easy and forgiving to hand-solder, the traditional favorite for hand rework; but lead is toxic, and RoHS restricts it for most new commercial products (though it's still legal and common for repair, hobby, and RoHS-exempt work). Lead-free solder — mostly tin with a little silver and copper (SAC alloys, like SAC305) — melts higher (around 217 to 220 degrees Celsius), wets less easily, needs more heat and good flux, and is less forgiving; it's required by RoHS for new commercial products and is what most modern boards are built with. You'll learn what eutectic means (a single sharp melting point with no pasty range — a cleaner, more forgiving joint), how to match or rework a board's alloy, and the form factors: flux-cored wire (the hand-soldering form, in thin diameters for SMD and thicker for through-hole) and solder paste (powder in flux, for SMD reflow). And — because lead is toxic — you'll learn to handle it safely: wash your hands, don't eat at the bench, and ventilate. Choosing the right solder is the first material decision of any repair.

Why This Matters

Solder is the joint — get the alloy or form wrong and every joint you make fights you, or fails. The leaded-versus-lead-free choice shapes the whole experience: leaded Sn63/Pb37 melts lower and flows so willingly that it's what beginners should learn on — joints form easily and look clean; lead-free needs a hotter iron, better flux, and more patience, and punishes poor technique with dull, poorly-wetted joints. Knowing which a board uses — and why — saves you from fighting a cold iron against stubborn lead-free solder or wondering why a joint won't flow. Eutectic matters because a single sharp melting point means the joint goes straight from liquid to solid — no pasty, plastic range in which a tiny movement while it cools leaves a grainy, weak, cold joint; the eutectic Sn63/Pb37 is forgiving precisely because there's no pasty window to ruin. Form factor matters because the wrong wire diameter makes work miserable: thick wire floods a tiny SMD pad, thin wire takes forever on a big through-hole joint — matching diameter to the work is a small choice with a big payoff. And solder paste is a different tool entirely — for reflowing SMD parts, not hand soldering. Above all, lead is a genuine health hazard: it's toxic if ingested, so the habits — washing your hands, keeping it away from food, ventilating the fumes — matter for your health, not just your joints. Understanding solder is the foundation of every soldering task to come; this section makes the first, most consequential material choice an informed one.

Required Prerequisites

Soldering Iron Selection — Beginner to Professional — solder and the iron are a pair: the alloy you choose sets the temperature your iron must reach (lead-free needs more heat than leaded), so knowing your heat source frames the solder choice, and vice versa.

Recommended Consumables

Flux-cored solder wire — an eutectic leaded (Sn63/Pb37) spool to learn on (where allowed), and a lead-free (SAC) spool for lead-free work; in a couple of diameters (a thin one around 0.3 to 0.5 millimeters for SMD, a thicker one around 0.8 to 1.0 millimeters for through-hole)
Solder paste (leaded or lead-free) if you do SMD reflow — stored refrigerated, minding its shelf life
Soap and water (for washing hands after handling lead) and fume extraction (Chapter 1)

Recommended Practice Hardware

A soldering iron/station (Chapter 5) capable of the temperatures your alloys need (higher for lead-free)
A scrap board with through-hole and SMD pads to feel the difference between alloys, diameters, and (if you have it) paste
Fume extraction or ventilation (Chapter 1) for the flux fumes

Real-World Applications

Choosing solder is the first move of countless repairs. A hobbyist learning to solder starts with eutectic leaded Sn63/Pb37 flux-cored wire — because it melts low, flows easily, and forgives — and finds joints forming cleanly where lead-free would have frustrated them. A technician repairing a modern, RoHS-compliant product uses lead-free SAC wire (and a hotter iron) to stay compliant and match the board. Reworking a stubborn lead-free joint, they add a little leaded solder to lower its melting point and make it flow — a common trick, while knowing they've now mixed alloys (fine for a repair, not for a compliant rebuild). Hand-soldering a fine-pitch SMD part, they reach for thin wire (around 0.3 to 0.5 millimeters) for control; soldering a chunky through-hole connector, they switch to thicker wire to deliver solder fast. Placing and reflowing a board of SMD parts, they don't use wire at all — they stencil solder paste onto the pads and reflow it with hot air or a reflow oven. And throughout, the careful ones wash their hands after handling leaded solder, never eat at the bench, and run the fume extractor — because lead is toxic and flux fumes irritate. The failures this prevents are the daily ones: a cold, grainy joint from a disturbed non-eutectic alloy; lead-free that won't flow on a too-cool iron; a flooded SMD pad from thick wire; and — worst, long-term — lead exposure from careless handling. The right solder, well chosen and safely handled, is where good soldering starts.

Common Challenges

Lead-free won't flow like leaded. Lead-free melts higher and wets less — it needs a hotter iron and good flux, and punishes a cool iron with dull, poor joints; it's not leaded, and expecting it to behave the same frustrates beginners.
A grainy, cold joint. Disturbing a non-eutectic alloy while it cools (in its pasty range) leaves a weak, grainy joint — a eutectic alloy (no pasty range) avoids this, as does holding the joint still while it solidifies.
The wrong wire diameter. Thick wire floods a tiny SMD pad and thin wire is slow on big joints — matching diameter to the work is an easy win people overlook.

Safety Notes

Risk Level: Low. Solder is safe to work with using simple habits — but lead is genuinely toxic, so the health cautions below are real, not boilerplate.

Lead is toxic — wash your hands, keep it away from food, and ventilate the fumes

Lead is a genuine health hazard. Leaded solder is toxic if ingested, and lead accumulates in the body, so treat it with respect: wash your hands with soap and water after handling leaded solder (and before eating), do not eat, drink, or touch your face at the bench, and keep leaded solder and its residue away from food and children. This is not about fume — at normal soldering temperatures the lead does not vaporize meaningfully; the risk is ingestion from hands and surfaces, which hand-washing controls. The fumes you see are mostly vaporized flux, not lead — but they are a respiratory sensitizer and irritant (rosin flux fume is a recognized cause of occupational asthma), so ventilate or use fume extraction (Chapter 1's ventilation guidance) regardless of alloy. A separate caution: lead-free solder needs higher temperatures (around 217 to 220 degrees Celsius and up), so the iron and board run hotter — mind the burn risk. And dispose of solder dross and lead-containing waste responsibly (it's an environmental hazard, not household trash). None of this makes soldering dangerous — with hand-washing, ventilation, and burn awareness, it's a low-risk activity — but the lead habits are worth making automatic. (Powered/mains work is still governed by Sections 3.1 and 3.2.)

Professional Tips Before Starting

Learn on eutectic leaded, if you're allowed to. Sn63/Pb37 melts low, flows easily, and forgives — it's the best alloy to learn good technique on; move to lead-free for compliant work (and expect it to be harder).
Keep two diameters on hand. A thin spool (around 0.3 to 0.5 millimeters) for SMD and fine work and a thicker one (around 0.8 to 1.0 millimeters) for through-hole and big joints — matching the wire to the job transforms the experience.
Make the lead habits automatic. Wash your hands after handling leaded solder, never eat at the bench, and run the fume extractor — do it every time, so it becomes reflex.

Choosing Solder — Alloys and Forms

What Solder Is

Solder is a fusible metal alloy — an alloy formulated to melt at a relatively low temperature — used to join electronic components to a circuit board. When you heat a pad and a component lead and apply solder, the solder melts, wets the metal surfaces (flowing over and bonding to them), and — as it cools — solidifies into a joint. That joint does two jobs at once: it's an electrical connection (carrying current between the component and the board) and a mechanical bond (physically holding the component in place). Good solder wets well — it spreads and adheres to clean, fluxed metal, forming a smooth, shiny, concave fillet; poor wetting leaves a balled-up, dull joint that may be weak or intermittent. Everything about choosing solder — the alloy and the form — is about getting good wetting and a sound joint for your work, safely. Solder is the material at the heart of every repair, so understanding it is where the chapter rightly begins.

Leaded vs Lead-Free: Composition, Melting Point, Ease, and RoHS

The defining choice is leaded versus lead-free, and they differ in composition, melting point, ease of use, and legal status. Leaded solder is a tin-lead alloy — most importantly the eutectic Sn63/Pb37 (sixty-three percent tin, thirty-seven percent lead; the very similar Sn60/Pb40 is also common). It melts low — the eutectic around 183 degrees Celsius — wets and flows beautifully, and is easy and forgiving to hand-solder, which is why it's the traditional favorite for hand rework and the best alloy to learn on. Its drawback is lead: lead is toxic (a health and environmental hazard), and the RoHS directive restricts it in most new commercial electronics — though leaded solder remains legal and widely used for repair, hobby, prototyping, and RoHS-exempt applications. Lead-free solder is mostly tin with small amounts of silver and copper — the SAC family (tin-silver-copper), such as SAC305 (tin with about three percent silver and half a percent copper). It melts higher (around 217 to 220 degrees Celsius), wets less readily, needs more heat and good flux, and is less forgiving of technique. But it's required by RoHS for new commercial products, so it's what most modern boards are built with. The practical summary: leaded is easier (learn on it, use it where allowed); lead-free is required for compliance (and harder, needing more heat).

Eutectic vs a Plastic Range

One property deserves its own explanation, because it's why some solder is more forgiving: whether the alloy is eutectic. A eutectic alloy has a single, sharp melting point — it goes directly from solid to liquid (and, cooling, directly from liquid to solid) at one temperature, with no in-between. Sn63/Pb37 is eutectic (melting sharply at 183 degrees Celsius), and that's a big part of why it's so forgiving: as the joint cools, it snaps from liquid to solid instantly, so there's no window in which it's half-set. A non-eutectic alloy (like Sn60/Pb40, or most lead-free alloys) instead has a plastic range (also called a pasty range) — a band of temperatures over which it is partly solid and partly liquid, slushy. If the joint is moved or disturbed while it's in that plastic range — cooling through the slushy zone — the solidifying metal fractures into a grainy, dull, weak joint: a cold or disturbed joint. This is why eutectic solder is prized for hand work (there's no plastic range to ruin), and why, with non-eutectic alloys, you hold the joint perfectly still until it's fully solid. Eutectic = sharp melt, clean joint, forgiving; non-eutectic = a plastic range to respect.

Matching and Reworking Alloys

When you rework an existing joint, the alloy matters. The general rule: match — or exceed the melting point of — the board's alloy, so your solder flows into and bonds with what's there. A useful trick for reworking a stubborn lead-free joint: add a little leaded solder to it — the lead lowers the melting point of the resulting mix, making it flow at a lower temperature and become easier to work. This is common and effective for repair, but be aware you've now mixed alloys (leaded into lead-free): fine for a repair, but not for a board that must remain RoHS-compliant (a new commercial product), where you must use lead-free throughout. Conversely, use lead-free where it's required — compliance is a legal, not just technical, matter for new commercial work. For most repair and hobby, you have freedom to choose the easier alloy; for compliant manufacturing or warranty work, match the board's lead-free requirement. Know what the board is, know what the job requires, and choose accordingly.

Form Factors: Flux-Cored Wire, Solder Paste, and More

Solder comes in a few forms, each for a different process. Solder wire is the everyday hand-soldering form — a spool of wire you feed into a joint, almost always flux-cored (a core of flux runs down the center of the wire, so the flux is delivered with the solder as it melts — the flux itself is Section 10.2's topic). Its diameter matters: thin wire (around 0.3 to 0.5 millimeters) gives control for fine SMD and precise work, while thicker wire (around 0.8 to 1.0 millimeters) delivers solder quickly for through-hole and larger joints — keep both on hand. Solder paste (a previously introduced material) is different: a gray paste of tiny solder powder suspended in flux, applied to pads (via a stencil or a syringe) and then reflowed — melted all at once with hot air or in a reflow oven — the form for SMD placement and reflow work, not hand soldering; it comes in leaded and lead-free, must be stored refrigerated, and has a limited shelf life. Two brief others: solder bar (solid ingots for industrial wave/dip soldering — not a bench item), and specialty solders like low-melting-point bismuth alloys (for gentle, low-temperature removal of heat-sensitive parts or BGAs) and solder preforms (pre-shaped bits for specific joints). For the bench, the two that matter are flux-cored wire (hand work) and solder paste (reflow).

Choosing Solder for the Job

Pulling it together: choose by what you're doing. For hand rework, use flux-cored wire — in the right diameter (thin for SMD, thick for through-hole) and the right alloy (leaded for ease where it's allowed; lead-free where it's required for compliance). If you're learning, start with eutectic leaded Sn63/Pb37 — it's the easiest to hand-solder and builds good technique. For SMD placement and reflow, use solder paste (matched leaded/lead-free to your needs). When reworking, match or exceed the board's alloy (and remember the leaded-into-lead-free trick eases stubborn joints but mixes alloys). And whatever you choose, handle lead safely — wash your hands, keep it from food, ventilate. The decision is small but foundational: the right solder, in the right form and diameter, well chosen and safely handled, makes every joint that follows easier and sounder.

Common Mistakes

Expecting lead-free to behave like leaded. Lead-free melts higher and wets less — use a hotter iron and good flux, and don't expect leaded ease.
Disturbing a non-eutectic joint as it cools. Movement in the plastic range makes a grainy, cold joint — use eutectic solder, or hold the joint still until fully solid.
Using one wire diameter for everything. Thick floods SMD; thin is slow on through-hole — keep both and match the diameter to the work.
Mixing alloys on a compliance-critical board. Adding leaded to lead-free eases a repair but breaks RoHS compliance — keep it lead-free throughout where required.
Careless lead handling. Not washing hands, eating at the bench, or leaving residue near food risks lead ingestion — make the hygiene habits automatic.

Troubleshooting Guidance

Most solder problems trace to alloy, form, or technique. If the solder won't flow or the joint is dull: the iron may be too cool for the alloy — lead-free needs more heat (and good flux) than leaded; raise the temperature and ensure fresh flux. If a joint is grainy, dull, and weak (a cold joint): the alloy was likely disturbed in its plastic range while cooling — use eutectic solder (no plastic range) and hold the joint still until fully solidified; reflow the bad joint to fix it. If you're flooding a tiny SMD pad: your wire is too thick — switch to a thin (around 0.3 to 0.5 millimeters) diameter for control. If a big through-hole joint takes forever: your wire is too thin — use a thicker one to deliver solder faster. If a rework joint won't flow with lead-free: try adding a little leaded solder to lower the melting point (a repair trick — but you've mixed alloys, so not for a compliant board). If your solder paste is dried out or performing poorly: it may be past its shelf life or wasn't stored cold — paste is perishable. And if you're ever unsure whether a board needs lead-free: a new commercial product likely does (RoHS); a repair/hobby board usually gives you freedom. The throughline: match the alloy and heat, use eutectic and hold still for clean joints, pick the wire diameter to the work — and handle lead safely.

Verification & Testing Methods

Use this as a choosing-solder checklist — confirm these before you solder:

[ ] I've chosen leaded or lead-free to suit the job and any compliance need (lead-free where RoHS requires it; leaded is fine for repair/hobby where allowed).
[ ] For hand work I'm using flux-cored wire; for SMD reflow I'm using solder paste (stored cold, within shelf life).
[ ] The wire diameter fits the work — thin (around 0.3 to 0.5 millimeters) for SMD, thicker (around 0.8 to 1.0 millimeters) for through-hole.
[ ] If I want the most forgiving hand soldering, I'm using a eutectic alloy (Sn63/Pb37) so there's no plastic range to ruin the joint.
[ ] My iron reaches the temperature the alloy needs (hotter for lead-free, ~217 to 220 degrees Celsius and up).
[ ] I follow the lead-safety habits: wash hands after handling leaded solder, no food/drink at the bench, and ventilate/extract the fumes (Chapter 1).

Then try the practice exercises below — solder-choosing reasoning; scenarios differ from the quiz.

Practice Exercises

Choose the alloy and form (5 minutes, reasoning). For (a) learning to hand-solder, (b) repairing a modern RoHS-compliant product, and (c) reflowing a board of SMD parts, say what solder alloy and form you'd choose and why.
Why eutectic is easier (5 minutes, reasoning). Explain what a plastic (pasty) range is, what a eutectic alloy does differently, and why that makes Sn63/Pb37 forgiving for hand soldering.
Pick the diameter (5 minutes, reasoning). For a fine-pitch SMD part and for a chunky through-hole connector, say which wire diameter you'd use and why the wrong one hurts.
The lead-safety routine (5 minutes, applied). List the habits you'd make automatic when working with leaded solder, and explain what each protects against (ingestion versus fumes).

These core ideas — what solder is, leaded versus lead-free (composition, melting temperature, ease, RoHS), eutectic versus a plastic range, the form factors (flux-cored wire and diameters, solder paste), and handling lead safely — are tested in the Chapter Quiz at the end of this chapter, where a score of 80% is required to continue.

Key Takeaways

Solder is a fusible metal alloy that melts, wets the metal, and solidifies into a joint that is both an electrical connection and a mechanical bond.
Leaded solder (tin-lead, especially the eutectic Sn63/Pb37, melting around 183 degrees Celsius) melts lower, wets and flows easily, and is forgiving — the best to learn on — but lead is toxic and RoHS-restricted for new commercial products (still fine for repair/hobby).
Lead-free solder (SAC tin-silver-copper, melting around 217 to 220 degrees Celsius) melts higher, wets less, needs more heat and flux, and is less forgiving — but is required by RoHS for new commercial products and is what most modern boards use.
A eutectic alloy has a single sharp melting point (no plastic/pasty range), giving a clean, forgiving joint; a non-eutectic alloy has a plastic range where a disturbed cooling joint goes grainy (a cold joint).
Form factors: flux-cored wire for hand work (thin ~0.3 to 0.5 millimeters for SMD, thicker ~0.8 to 1.0 millimeters for through-hole); solder paste (powder in flux, stored cold) for SMD reflow; plus bar (industrial) and specialty (low-melt bismuth) forms. Match or exceed the board's alloy when reworking (leaded-into-lead-free eases rework but mixes alloys).
Safety: lead is toxic — wash your hands, keep it from food, and don't eat at the bench (the risk is ingestion, not vapor); ventilate the flux fumes; and mind the higher burn temperatures of lead-free.

Skills Learned

You can now choose leaded or lead-free solder for a repair and its compliance needs.
You can now explain why a eutectic alloy is easier to hand-solder.
You can now pick a wire diameter for SMD versus through-hole work.
You can now handle leaded solder safely with hand-washing and ventilation.
You can now match the right solder form (wire or paste) to your process.

Glossary Additions

solder — a fusible metal alloy used to join electronic components to a circuit board: heated at a pad and lead, it melts, wets (flows over and bonds to) the metal surfaces, and solidifies into a joint that serves as both an electrical connection and a mechanical bond. Solder is chosen by alloy (leaded or lead-free) and form (flux-cored wire for hand soldering, solder paste for reflow), and a good joint shows smooth, shiny wetting rather than a dull, balled-up shape.
eutectic — describing an alloy with a single, sharp melting point at which it passes directly between solid and liquid, with no intermediate pasty (plastic) range; the eutectic tin-lead solder Sn63/Pb37 (melting around 183 degrees Celsius) is prized for hand soldering because, with no plastic range, a cooling joint solidifies instantly and cannot be spoiled into a grainy cold joint by slight movement — making it more forgiving than non-eutectic alloys, which have a temperature band where they are partly solid and partly liquid.
leaded solder — a tin-lead soldering alloy, most commonly the eutectic Sn63/Pb37 (or the similar Sn60/Pb40), that melts at a relatively low temperature (the eutectic around 183 degrees Celsius), wets and flows easily, and is forgiving to hand-solder, making it the traditional choice for hand rework and for learning; however, lead is toxic and an environmental hazard, and the RoHS directive restricts leaded solder in most new commercial products, though it remains legal and widely used for repair, hobby, prototyping, and RoHS-exempt work. It must be handled with hand-washing and kept away from food.
lead-free solder — a soldering alloy containing no lead, most commonly a SAC alloy of tin with small amounts of silver and copper (such as SAC305, tin with about three percent silver and half a percent copper); it melts at a higher temperature than leaded solder (around 217 to 220 degrees Celsius), wets less readily, and needs more heat and good flux, making it less forgiving to hand-solder, but it is required by the RoHS directive for new commercial electronic products and is what most modern boards are built with.

Suggested Next Sections

Must read next:

Flux — Types, Selection, and Application — the partner to solder: flux, the chemical that cleans the metal and lets solder wet and flow. What the flux core in your wire is, the types (rosin, no-clean, water-soluble), when to add extra flux, and how flux choice affects the joint and its cleanup.

Recommended:

Soldering Iron Selection — Beginner to Professional — the iron that melts the solder; the alloy you choose sets the temperature it must reach (lead-free runs hotter).
Ventilation and Fume Extraction — the flux fumes solder produces are a respiratory irritant; the ventilation and extraction that keep the air safe.