Tin-Lead (SnPb) Solder

Section Overview

Volume 2 introduced solder as a lab material — leaded versus lead-free, the forms, and how to choose. Chapter 2 goes deeper, into the chemistry and metallurgy of each solder family and how to pick one, and it starts with the classic: tin-lead (SnPb) solder. Tin-lead is an alloy of two metals with a clear division of labor: the tin does the wetting and bonding (it's the metal that reacts with copper to form the intermetallic bond — Section 1.4), and the lead lowers the melting point and makes the joint ductile (able to flex without cracking). The ratio defines the alloy — Sn63/Pb37 means sixty-three percent tin, thirty-seven percent lead. The star of the family is the eutectic, Sn63/Pb37: it melts at a single, sharp temperature (about 183 degrees Celsius) and passes straight from solid to liquid — and back — with no pasty in-between. To understand why that matters, you need two words: solidus (the temperature where melting begins) and liquidus (the temperature where it's fully molten). For the eutectic, solidus equals liquidus — one temperature. For off-eutectic alloys (Sn60/Pb40, Sn50/Pb50), the liquidus sits above the solidus, and the gap between them is the plastic range — a band where the alloy is partly solid, partly liquid, and where a disturbed cooling joint goes grainy (Section 1.5). This section covers the common alloys and their melting behavior (including high-lead solders for step soldering), why tin-lead — especially eutectic — is the benchmark for hand soldering, and why lead's toxicity restricts it (RoHS) even as it remains the standard for repair and exempt work. This is solder chemistry made concrete.

Why This Matters

Understanding tin-lead is understanding the yardstick every other solder is measured against. When someone says a lead-free alloy is "harder to work" or "less forgiving," they mean compared to eutectic tin-lead — so you can't understand solder selection (the rest of this chapter) without first understanding the benchmark. The chemistry here has direct, practical consequences. Knowing that the eutectic has no plastic range explains why Sn63/Pb37 is so forgiving: there's no pasty window in which a slight movement wrecks the joint — it snaps from liquid to solid instantly. Knowing that Sn60/Pb40 has a small plastic range explains why it's slightly less forgiving (and cheaper) — and why holding the joint still while it cools matters more with it. Knowing solidus and liquidus gives you the vocabulary to read any solder's datasheet and understand its melting behavior at a glance — a skill you'll use for every alloy in this chapter and beyond. Knowing about high-lead alloys and step soldering explains a whole class of assembly (how a part can be soldered, then the board reflowed later without the first joint remelting). And understanding why tin-lead is ductile and fatigue-resistant connects back to Section 1.5 — it's part of why leaded joints often survive thermal cycling better than some lead-free ones. Finally, the lead-toxicity and RoHS facts are practical knowledge: when you may use it (repair, hobby, exempt) and when you may not (new commercial products). Master the benchmark alloy and its chemistry, and every solder decision that follows has a firm reference point.

Required Prerequisites

• Solder Selection — Alloys and Form Factors — that Volume 2 section introduced solder as a material (leaded versus lead-free, eutectic, forms) at the lab-overview level. This section deepens the tin-lead half — the phase chemistry, the alloys, and why it's the benchmark — so read that first for the overview.

Recommended Consumables

• Eutectic tin-lead (Sn63/Pb37) flux-cored wire — the benchmark to learn on and compare everything to (where its use is allowed)
• Sn60/Pb40 wire (optional) — to feel the small plastic range difference against Sn63/Pb37
• Flux (Volume 2, Section 10.2) — even the best solder needs clean, fluxed metal to wet
• Soap and water (for the lead hygiene) and fume extraction (for the flux fumes)

Recommended Practice Hardware

• A temperature-controlled soldering iron/station (Volume 2, Chapter 5) — to feel how eutectic solder melts and freezes sharply
• A scrap board to make joints with Sn63/Pb37 and observe the sharp freeze (and, with Sn60/Pb40, the brief pasty moment)
• A magnifier (Volume 2, Chapter 9) to see the smooth, shiny fillet eutectic gives
• Ventilation and hand-washing facilities

Real-World Applications

Tin-lead is the working solder of repair, and its chemistry shows up daily. A repair technician — free of RoHS constraints on repair work — reaches for eutectic Sn63/Pb37 because it wets beautifully, flows, and freezes sharply, making clean joints with the least fuss; it's what most hand-soldering is done with where it's allowed. A hobbyist learning to solder starts with Sn63/Pb37 for the same reason — the no-plastic-range forgiveness means fewer cold, grainy joints. A technician who grabs a spool of Sn60/Pb40 (common and cheap) notices joints can go slightly grainy if bumped while cooling — the small plastic range at work — and learns to hold still. A manufacturer building a multi-step assembly uses a high-lead solder (like Sn5/Pb95, melting around 300 degrees Celsius) for a first, high-temperature joint — a die attach or a connector — so that a later, lower-temperature reflow of the board won't remelt it (step soldering). And a reliability-focused shop working on military or aerospace gear (RoHS-exempt) deliberately chooses tin-lead for its ductility and proven thermal-fatigue behavior (Section 1.5). Meanwhile, the careful ones wash their hands and ventilate, because lead is toxic. The thread through all of it: knowing the alloy's melting behavior and properties lets you pick and use it well — and tin-lead, especially the eutectic, is the one to know first.

Common Challenges

• Grainy joints with Sn60/Pb40. Its small plastic range means a joint bumped while cooling can go grainy — use Sn63/Pb37 (no plastic range) or hold the joint still until fully solid.
• "Which alloy is which?" The numbers are the percentages — Sn63/Pb37 is sixty-three tin, thirty-seven lead (the eutectic); higher lead means higher melting and more plastic range.
• Forgetting lead hygiene. Leaded solder is toxic — not washing hands or eating at the bench risks ingestion; make the habits automatic (Volume 2, Section 10.1).

Safety Notes

Risk Level: Low. Tin-lead is safe to work with using simple habits — but lead is genuinely toxic, so the health cautions (carried from Volume 2, Section 10.1) are real.

Professional Tips Before Starting

• Learn on the eutectic. Sn63/Pb37 is the most forgiving solder there is (no plastic range), so it's the best alloy to build good technique on — and the benchmark you'll compare every other solder to.
• Read the alloy's two temperatures. Solidus and liquidus tell you when it starts and finishes melting; if they're the same, it's eutectic (sharp); if liquidus is higher, there's a plastic range to respect while cooling.
• Make the lead habits reflex. Wash your hands, never eat at the bench, and run the fume extractor — every time you work with leaded solder.

Tin, Lead, and the Eutectic — the Chemistry of SnPb Solder

What Tin-Lead Solder Is

Tin-lead solder — written SnPb (from the chemical symbols Sn for tin and Pb for lead) — is an alloy of two metals, tin and lead, melted together in a specific ratio. Each metal has a job. Tin is the active, bonding metal: it wets the copper and reacts with it to form the intermetallic bond (Section 1.4) that makes the joint — tin is why solder sticks. Lead is the modifier: on its own, pure tin melts at about 232 degrees Celsius and is less ideal to work with; adding lead lowers the melting point, makes the alloy flow more smoothly, and makes the solidified joint ductile — able to deform slightly under stress rather than cracking (which helps it survive thermal cycling, Section 1.5). The ratio is written as the percentages: Sn63/Pb37 is sixty-three percent tin and thirty-seven percent lead; Sn60/Pb40 is sixty/forty; and so on. Changing the ratio changes the melting behavior and properties — which is the whole story of this section. Tin does the bonding; lead lowers the melting point and adds ductility; the ratio sets the character.

The Eutectic and Phase Behavior — Solidus, Liquidus, and the Plastic Range

Here is the core chemistry, and it rests on two temperatures. When an alloy is heated, it doesn't always melt all at once. The solidus is the temperature at which melting begins (below it, the alloy is fully solid); the liquidus is the temperature at which the alloy is fully molten (above it, all liquid). Between solidus and liquidus, the alloy is part solid, part liquid — a slushy, pasty state. Now the special case: a eutectic alloy is one specific composition at which solidus and liquidus are the same temperature — it melts (and freezes) at a single, sharp point, passing straight between solid and liquid with no slushy in-between. For tin-lead, the eutectic is Sn63/Pb37, melting sharply at about 183 degrees Celsius. Every other tin-lead ratio is off-eutectic: its liquidus is above its solidus, and the temperature gap between them is the plastic range (or pasty range) — the band in which the cooling joint is partly frozen, partly molten. This is where a disturbed joint goes grainy: if the joint is moved while cooling through its plastic range, the partly-solidified metal fractures into a dull, weak, grainy joint (the disturbed cold joint of Section 1.5). The eutectic's gift is that it has no plastic range — no window to ruin — which is why it's the most forgiving solder to hand-solder. Solidus, liquidus, and the plastic range between them are the language of how any solder melts.

The Common Tin-Lead Alloys

A few tin-lead alloys cover almost all use. Sn63/Pb37 — the eutectic — melts sharply at about 183 degrees Celsius with no plastic range; it's the ideal for hand soldering and the benchmark. Sn60/Pb40 is near-eutectic: it's very common and slightly cheaper, with a small plastic range (roughly 183 to 190 degrees Celsius — solidus at the eutectic 183, liquidus a few degrees higher), so it's almost as good but slightly less forgiving (a brief pasty moment while cooling). Sn50/Pb50 has a wider plastic range (more lead, higher liquidus) and is less common in electronics (more a plumbing/general solder). At the other extreme, high-lead alloys like Sn10/Pb90 or Sn5/Pb95 are mostly lead: they melt much higher (around 300 degrees Celsius) and are used for die attach and step soldering (below). There are also silver-bearing tin-lead alloys — for example Sn62/Pb36/Ag2 (two percent silver) — where a little silver is added to reduce silver leaching when soldering to silver-coated pads or components (the silver in the solder slows the solder from dissolving the part's silver). For the bench, Sn63/Pb37 is the one to reach for; Sn60/Pb40 is the common near-equivalent; the rest are specialties.

Properties and Why It Is the Benchmark

Why is eutectic tin-lead the standard every solder is judged against? Because it's excellent on every axis that matters for hand work. It wets and flows beautifully — the tin bonds readily and the alloy spreads smoothly. It's forgiving — the no-plastic-range sharp freeze means fewer cold, grainy joints and more tolerance of imperfect technique. It melts at a low temperature (183 degrees Celsius), which is gentle on components and grows a thinner, healthier intermetallic for a given process (less thermal stress, Sections 1.4 and 1.5). It's ductile — the lead makes the solidified joint able to flex, giving good resistance to thermal-cycling fatigue (Section 1.5), part of why leaded joints have a long, well-understood reliability record. And it's decades-understood — every process, flux, and technique was developed around it. All of this is why it remains the benchmark for hand soldering and the preferred choice for repair, hobby, prototyping, and high-reliability RoHS-exempt work (military, aerospace, medical) where it's allowed. Lead-free solders (Section 2.2) are measured against exactly these qualities — and generally fall a little short on ease, which is why the benchmark matters.

Lead Toxicity and RoHS Restriction

The one serious drawback of tin-lead is lead itself. Lead is toxic — a cumulative poison if ingested — and it's an environmental hazard. For health, the controls are the hygiene habits in the Safety Notes (wash hands, keep it from food, ventilate the flux fumes). For the environment and regulation, the RoHS directive (Restriction of Hazardous Substances) restricts lead in most new commercial electronic products sold in many markets — which is why new consumer electronics are built with lead-free solder (Section 2.2). But RoHS does not ban tin-lead everywhere: it remains legal and common for repair (repairing an existing product), hobby and prototyping, and RoHS-exempt categories (certain military, aerospace, and medical applications where reliability requirements justify an exemption). So the practical rule: use tin-lead where it's allowed and its ease and reliability help — repair, hobby, exempt — and use lead-free where RoHS requires it — new commercial products. Tin-lead is the better solder to work with; lead-free is the required one for new commercial goods.

Common Mistakes

• Treating Sn60/Pb40 like the eutectic. It has a small plastic range, so it's slightly less forgiving — hold the joint still while cooling, or use Sn63/Pb37.
• Confusing the ratio numbers. The numbers are percentages (Sn63 = sixty-three percent tin); more lead means higher melting and more plastic range, not "stronger."
• Using tin-lead on a RoHS-compliant new product. New commercial products must be lead-free — tin-lead is for repair, hobby, and exempt work.
• Skipping lead hygiene because "the fumes aren't lead." True — the risk is ingestion, not vapor — but that means hand-washing is exactly the control that matters; don't skip it.

Troubleshooting Guidance

Most tin-lead questions are about melting behavior or which alloy. If joints keep coming out dull and grainy: if you're using Sn60/Pb40 (or a higher-lead alloy), its plastic range lets a disturbed cooling joint go grainy — hold the joint still until fully solid, or switch to eutectic Sn63/Pb37 (no plastic range). If you're unsure whether an alloy is eutectic: check its solidus and liquidus — if they're equal (one melting point), it's eutectic; if the liquidus is higher, it has a plastic range. If a joint you made won't survive a later heating step: you may need a higher-melting (high-lead) solder for the first joint so a later, lower-temperature step doesn't remelt it (step soldering). If solder is dissolving a silver-plated part's coating: use a silver-bearing tin-lead (e.g. Sn62/Pb36/Ag2) to reduce silver leaching. If you must build a new commercial product: you can't use tin-lead — RoHS requires lead-free (Section 2.2). And if you're worried about lead exposure: the control is hygiene — wash your hands, keep solder from food, and ventilate (the fume is flux, not lead). The throughline: read the alloy's melting behavior (solidus/liquidus/plastic range), pick the right one for the job and its RoHS status, and handle the lead safely.

Verification & Testing Methods

Use this as a tin-lead understanding check:

• [ ] I can explain that tin-lead solder is a tin+lead alloy where tin wets and bonds and lead lowers the melting point and adds ductility.
• [ ] I can define solidus (melting begins) and liquidus (fully molten), and say that a eutectic has them equal (one sharp melting point).
• [ ] I know Sn63/Pb37 is the eutectic (about 183 degrees Celsius, no plastic range) and is the hand-soldering benchmark.
• [ ] I know off-eutectic alloys (Sn60/Pb40, Sn50/Pb50) have a plastic range where a disturbed cooling joint goes grainy.
• [ ] I know high-lead alloys melt much higher (about 300 degrees Celsius) and are used for step soldering and die attach.
• [ ] I know lead is toxic (hand-washing/ventilation) and RoHS-restricted for new commercial products but legal for repair, hobby, and exempt work.

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

Practice Exercises

1. Tin versus lead (5 minutes, reasoning). Explain what the tin and the lead each contribute to tin-lead solder, and predict what would happen to melting point and workability if you used pure tin instead.
2. Read the two temperatures (5 minutes, reasoning). Given an alloy's solidus and liquidus, explain how you'd tell whether it's eutectic and whether it has a plastic range — and what the plastic range means for how you cool the joint.
3. Pick the alloy (5 minutes, applied). For (a) everyday forgiving hand soldering, (b) a first joint that must survive a later reflow, and (c) soldering to a silver-plated pad, name the tin-lead alloy you'd choose and why.
4. The RoHS decision (5 minutes, reasoning). For a board repair, a hobby project, and a new consumer product, say whether you may use tin-lead and why, referencing RoHS.

These core ideas — what tin-lead solder is, the eutectic and solidus/liquidus/plastic range, the common alloys and their melting behavior, why eutectic tin-lead is the benchmark, and lead toxicity/RoHS — are tested in the Chapter Quiz at the end of this chapter, where a score of 80% is required to continue.

Key Takeaways

• Tin-lead (SnPb) solder is an alloy where tin wets and bonds (forming the intermetallic, Section 1.4) and lead lowers the melting point and adds ductility; the ratio (e.g. Sn63/Pb37 = sixty-three percent tin) sets its character.
• The solidus is where melting begins, the liquidus where it's fully molten; a eutectic alloy has solidus = liquidus — a single, sharp melting point.
• Sn63/Pb37 is the eutectic (about 183 degrees Celsius, no plastic range) — the most forgiving solder and the hand-soldering benchmark.
• Off-eutectic alloys (Sn60/Pb40, Sn50/Pb50) have a plastic range (liquidus above solidus) where a disturbed cooling joint goes grainy; high-lead alloys (Sn5/Pb95) melt much higher (~300 degrees Celsius) for step soldering and die attach.
• Tin-lead wets well, is ductile and fatigue-resistant, and melts low — which is why it's the benchmark and the preferred solder for repair, hobby, and RoHS-exempt work.
• Lead is toxic (wash hands, keep from food, ventilate the flux fumes) and RoHS-restricted for new commercial products — but legal and common for repair, hobby, and exempt work.

Skills Learned

• You can now explain what tin-lead solder is and what tin and lead each contribute.
• You can now explain the eutectic and read solidus, liquidus, and the plastic range.
• You can now identify the common tin-lead alloys and their melting behavior.
• You can now explain why eutectic tin-lead is the hand-soldering benchmark.
• You can now handle leaded solder safely and state its RoHS status.

Glossary Additions

• solidus — the temperature at which an alloy begins to melt: below the solidus the alloy is fully solid, and at the solidus the first liquid appears. In a eutectic alloy the solidus and liquidus are the same temperature (a single sharp melting point), while in an off-eutectic alloy the solidus is the lower bound of the plastic range, above which the alloy is partly molten.
• liquidus — the temperature at which an alloy becomes fully molten: above the liquidus the alloy is entirely liquid, and at the liquidus the last solid disappears. In a eutectic alloy the liquidus equals the solidus (one melting point); in an off-eutectic alloy the liquidus lies above the solidus, and the gap between them is the plastic range in which the alloy is part solid and part liquid.
• plastic range — the temperature band, between an alloy's solidus and liquidus, over which it is partly solid and partly liquid (a pasty, slushy state); off-eutectic solders such as Sn60/Pb40 have a plastic range, and a joint disturbed while cooling through it fractures into a dull, weak, grainy (disturbed cold) joint, whereas a eutectic solder such as Sn63/Pb37 has no plastic range and freezes sharply, which is why it is more forgiving. Also called the pasty range.
• step soldering — an assembly technique that uses solders of different melting points in sequence so that a joint made first, with a higher-melting solder, is not remelted by a later soldering step done at a lower temperature; high-lead tin-lead alloys (such as Sn5/Pb95, melting around 300 degrees Celsius) are used for the first, high-temperature joints in step soldering and in die attach, so a subsequent lower-temperature reflow leaves them intact.

Suggested Next Sections

Must read next:

• Lead-Free Solder — Alloys and Trade-offs — the other half of the solder-chemistry story: the SAC (tin-silver-copper) and other lead-free alloys, why they melt higher and wet less readily, their reliability trade-offs, and why RoHS made them the standard for new commercial electronics — all measured against the tin-lead benchmark of this section.

Recommended:

• Solder Selection — Alloys and Form Factors — the Volume 2 overview this section deepens: leaded versus lead-free, eutectic, and the form factors, at the lab-materials level.
• Intermetallic Compounds and Joint Strength — why the tin in solder is the bonding metal, and why a low melting point and ductility help joint reliability.