Solder Types and Flux

Solder is not just "metal glue." The popular description of soldering as sticking components in place with melted metal misses almost everything important about the process. Solder is a precision-engineered alloy that forms a genuine metallurgical bond — an actual atomic-level joining — with the copper surfaces it contacts. The properties of that alloy: its melting point, its plastic range, its wetting characteristics, and whether it contains lead or not, determine how easy it is to use, how reliable the resulting joint is, and what temperature your iron needs to be. Understanding these properties from the ground up will make you a better solderer and help you diagnose and avoid many of the most common problems.

Flux is equally misunderstood. Many beginners think flux is an optional extra — something you add only to difficult joints. In fact, flux is chemically essential to every soldering operation. Most solder wire contains flux built into its core, which is why casual use works at all. But understanding what flux actually does at the chemical level explains why flux application is the solution to so many stubborn soldering problems, and why certain types of flux can destroy a PCB if left in place.

What Solder Is — The Metallurgy

To a physicist, soldering is a form of brazing — the joining of two base metals with a filler metal whose melting point is lower than either base metal. The filler (solder) does not simply sit between the surfaces the way glue does. It chemically reacts with the surface copper to form a new compound at the interface — an intermetallic layer — and this intermetallic layer is what gives a solder joint its strength and electrical conductivity.

The intermetallic compound formed at a standard tin-lead/copper joint is primarily copper-tin: Cu₆Sn₅ (copper hexatinide) and a smaller proportion of Cu₃Sn. These compounds are hard and brittle in thick layers, which is why you do not want a very thick intermetallic layer — it would make the joint brittle. In a correctly made solder joint, the intermetallic layer is extremely thin (typically 1–3 micrometers) and is sandwiched between the bulk solder and the copper. This thin layer is what makes the joint genuinely "part of the metal" rather than just adhesion.

The key property that makes solder work on copper is wetting. A liquid wets a solid surface when the liquid spreads out across it, attracted by surface energy. Molten solder wets clean copper easily because copper and tin have compatible surface energies and the tin reacts with the copper to form those intermetallic compounds. If you have ever watched solder flow up the lead of a through-hole component (against gravity), this is wetting in action — surface energy drawing the liquid solder toward the copper.

Wetting fails when the copper surface is oxidized. Copper forms a thin layer of copper oxide (CuO and Cu₂O) at room temperature within hours of exposure to air. This oxide layer has completely different surface energy characteristics from clean copper, and molten solder simply cannot react with it — the solder cannot form intermetallic compounds with an oxide. This is why flux is not optional: it removes the oxide layer just before the solder arrives. Without flux (or with inadequate flux), you get exactly the "balling up" and "no-stick" behavior that frustrates beginners, regardless of how hot the iron is.

Melting point comparison for common solder alloys. The bar for each alloy shows its solidus temperature (where melting begins) and liquidus temperature (fully liquid), with the plastic range highlighted between them. The 63/37 eutectic alloy is unique in having no plastic range — it transitions sharply at exactly 183°C. SAC305 requires a higher iron temperature than leaded solders because of its 217°C melting point.

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Tin-Lead Solder Alloys

63/37 Eutectic Tin-Lead

The 63/37 alloy — 63% tin and 37% lead by weight — is called the eutectic composition, and this is its most important property. "Eutectic" is a Greek-derived term meaning "good melting," and it describes the one specific mixture ratio at which a two-component alloy melts and solidifies at a single sharp temperature rather than over a range. For 63/37 tin-lead, that temperature is exactly 183°C. Below 183°C the solder is solid. At exactly 183°C it transitions directly to liquid (and back to solid when cooling through the same temperature). There is no "mushy" phase in between.

This single-point melting behavior is enormously valuable for hand soldering. It means a joint is either solid or liquid — there is no dangerous intermediate state where the solder is semi-solid but the joint is not yet fully set. If you disturb a joint in a non-eutectic alloy during the pasty phase, you get a disturbed or fractured joint; with 63/37 you simply wait half a second more after removing the iron and the joint is fully solid. The result is fewer cold joints and more consistent workmanship.

The 63/37 finish is also cosmetically bright and shiny, which makes visual joint inspection easier. A correctly made 63/37 joint has a smooth, shiny, slightly concave meniscus shape. This visual characteristic is directly useful for quality control — a dull, grainy, or lumpy finish on a 63/37 joint indicates a cold joint or disturbed joint and must be re-done.

At the iron, 63/37 solders well with a tip temperature of 315–325°C on a 50W or larger station. This is comfortably above the 183°C melting point, allowing rapid wetting without being so far above it that thermal damage to the board or components becomes a risk. For a ham radio builder, 63/37 in 0.8mm diameter with a rosin or no-clean flux core is the ideal all-around solder for kit building, connector assembly, and most repair work.

60/40 Tin-Lead

The 60/40 alloy — 60% tin, 40% lead — is slightly off the eutectic point and consequently has a small plastic range of 183–188°C. In practice this 5-degree range is almost imperceptible during hand soldering and 60/40 performs very similarly to 63/37. It is marginally more affordable because it contains slightly less tin (the more expensive metal), and it is widely available in larger spools for production use. The joint finish is slightly less shiny than 63/37 but still inspectable. Many ham radio builders use 60/40 without noticing any practical difference. Both are acceptable choices; 63/37 is technically superior for the reasons described.

Health, Safety and Regulatory Notes

Lead is a toxic heavy metal. The relevant exposure route for solderers is not ingestion of solid solder (leaded solder is not unusually hazardous to handle briefly) but absorption through the skin and especially contamination of food. The practical rules are straightforward: wash your hands thoroughly with soap and water after soldering, before eating or touching your face. Do not eat or drink while soldering. Avoid handling solder wire bare-handed for extended periods.

Regulatory context: the European Union's RoHS (Restriction of Hazardous Substances) Directive, and similar legislation in many other countries, requires that new electronic products manufactured for sale use lead-free solder. However, these regulations apply to manufacturers, not to individuals repairing, modifying, or building their own equipment. In the United States, hobbyists and amateur radio operators can legally use leaded solder for personal use. Many professional repair technicians also continue to use leaded solder for repair work, where the original board may already contain leaded solder. When in doubt, check the regulations in your jurisdiction — but for home workshop ham radio construction work, leaded 63/37 remains a practical and legal choice.

Lead-Free Solder Alloys

SAC305 — The Standard Lead-Free Alloy

SAC305 (96.5% tin, 3% silver, 0.5% copper) is the dominant lead-free alloy in commercial electronics manufacturing worldwide. It was selected as the industry standard when RoHS required the elimination of lead from consumer electronics, based on its balance of mechanical properties, electrical conductivity, and solderability. Understanding SAC305 properties is important for any ham radio technician because every piece of commercially manufactured equipment produced in the last 15–20 years was assembled with this alloy or a close relative.

SAC305 melts at 217–221°C, giving it a 4°C plastic range. This is a true plastic range — during cooling through those 4 degrees, the solder is in a semi-solid state. Disturbing a SAC305 joint during cooling through this window creates a disturbed joint that will have poor mechanical and electrical reliability. This is why lead-free hand soldering requires more care and a slightly longer undisturbed cooling time than 63/37.

Because SAC305 melts 34°C higher than 63/37, it requires a higher iron tip temperature. Set your station to 340–360°C when working with SAC305. The higher temperature compensates for the higher melting point and ensures the joint wets properly. At correct temperature, SAC305 flows well and forms reliable joints. The finish, however, is characteristically different from 63/37 — SAC305 joints look duller and more matte. This is normal and does not indicate a bad joint; it is simply the natural appearance of the alloy. Inspecting lead-free joints by their finish requires recalibrating your expectations: a dull SAC305 joint may be perfectly formed, while a bright-looking joint that was disturbed during the plastic phase may be defective.

SAC305 also contains silver, which marginally raises the cost of the alloy compared to plain tin-copper compositions. The silver content improves electromechanical fatigue resistance — the joint's ability to withstand repeated flexing and thermal cycling without cracking — which is one reason it was chosen for commercial manufacturing where products must survive years of use. For ham radio repair work involving re-soldering joints on commercial boards, using SAC305 for the repair is correct practice if you want to match the original alloy type.

SAC387

SAC387 (95.5% tin, 3.8% silver, 0.7% copper) is a close relative of SAC305 with slightly higher silver content. Properties are similar — melting range 217–220°C — and it is used in some commercial board production processes. For hand soldering purposes, SAC387 performs identically to SAC305 and the two are interchangeable in amateur radio repair and construction contexts.

Sn99Cu1 (Tin-Copper)

Sn99Cu1 is 99% tin with 1% copper and no silver content. It is inexpensive compared to SAC alloys. Its melting point is higher (around 227°C) and it has a wider plastic range, making it more difficult to hand solder well. Critically, it has a known problem with copper dissolution — the high proportion of tin will dissolve copper from PCB pads over time during soldering operations, accelerating pad lifting and trace erosion. It is not recommended for repair work. Sn99Cu1 finds use in wave-soldering production processes where the solder bath temperature is tightly controlled and contact time is brief. Avoid it for hand soldering applications.

Practical Recommendation for Ham Radio Builders

For new construction projects on your own PCBs and kits, leaded 63/37 is the practical choice: easier to use, lower iron temperature required, better joint appearance for inspection, and no regulatory restriction on individual hobby use. For repairing commercially manufactured equipment (transceivers, amplifiers, accessories) that was assembled with lead-free solder, consider using SAC305 so the repair alloy is compatible with the existing board chemistry. Mixing leaded and lead-free solders on the same board is not ideal (it shifts the effective alloy composition unpredictably) but is generally acceptable for single-joint repairs where the entire board cannot be reflowed.

Specialty Solders

Silver Solder (Tin-Silver, High Silver Content)

The term "silver solder" is used in electronics to describe tin-silver alloys with higher silver content than SAC305 — typically 95/5 (95% tin, 5% silver, melting around 221–240°C) or even higher silver alloys. In ham radio contexts, silver solder is specified for some RF connector terminations — particularly N-type and SMA connectors used at UHF and microwave frequencies — where lower contact resistance and superior mechanical strength at the connector-cable junction are important. The silver content increases the joint's electrical conductivity slightly and its mechanical hardness considerably.

Note that the "silver solder" used by jewelers and for brazing stainless steel is a completely different material — a silver-copper-zinc alloy with much higher silver content (typically 40–70% silver) and a melting point well above 600°C. This requires a propane or oxygen-acetylene torch and has no place in electronics work. When an electronics application calls for "silver solder," it means the tin-base, electronics-grade alloy, not the jewelry variety.

Bismuth Low-Melt Alloys (Sn42/Bi58)

The Sn42/Bi58 alloy (42% tin, 58% bismuth) melts at approximately 138°C — 45°C lower than 63/37. This extremely low melting point makes it useful for attaching heat-sensitive components: certain polymer components, sensors, or LCD assemblies that cannot tolerate even the brief 183°C exposure of a standard solder joint. The iron temperature for this alloy is set to around 200–220°C.

The significant trade-off is that bismuth makes joints brittle over time. Bismuth intermetallics have poor ductility and will crack under repeated thermal cycling or mechanical stress. Bismuth alloys should be used only where genuinely required by the component sensitivity, not as a shortcut to avoid learning to solder quickly. Additionally, bismuth and lead interact to form a ternary eutectic at only 96°C, meaning that if bismuth alloy is used on a board that already has leaded solder, any joints where the two alloys mix could melt at unexpectedly low temperatures. Always flag bismuth solder joints clearly in any documentation.

Indium Alloys

Indium-containing alloys have very low melting points (some below 60°C) and excellent adhesion to difficult substrates including glass and ceramic. They are used in specialized sensor, optical, and vacuum electronic applications. Unless you are working with specialized equipment such as satellite LNBs or cryogenic components, you are unlikely to encounter them. Indium is extremely expensive, further limiting its use to high-value specialized applications.

Flux — What It Does and Why It Matters

The word "flux" comes from the Latin fluxus, meaning "flow" — a hint that flux's original role was to help molten metal flow. In electronics soldering, flux serves two distinct functions that both contribute to making solder flow properly onto copper.

The primary function is chemical: flux removes copper oxide from the joint surfaces at soldering temperature. Copper forms a surface oxide layer (cupric oxide, CuO, and cuprous oxide, Cu₂O) rapidly when exposed to air. This oxide is chemically stable at room temperature but becomes reactive when heated in the presence of an acid. Flux contains a mild acid or acidic compound that reacts with the copper oxide at soldering temperature, converting it to a soluble copper salt that dissolves into the molten flux. The result is a brief window of clean copper surface — clean enough for the molten solder to wet and form intermetallic bonds. This reaction happens very quickly (in fractions of a second at soldering temperature), which is why you do not need to apply flux and wait: the flux activates instantly when the joint reaches temperature.

The secondary function is physical: flux reduces the surface tension of molten solder. Molten metals have high surface tension, which tends to cause solder to form spherical blobs rather than spreading out. Flux chemically modifies the surface tension of the molten solder, allowing it to spread more easily across the copper surface. You can observe this directly: apply a drop of liquid flux to a pad that has a small ball of solder sitting on it, then touch the heated iron. The solder ball immediately spreads and flows across the pad in a way it would not without the flux.

Flux also prevents re-oxidation during soldering. As the iron heats the joint, the flux breaks down and releases compounds that form a vapor barrier over the liquid solder, preventing oxygen in the air from immediately re-oxidizing the surfaces the flux has just cleaned. This gives the solder more time to wet properly.

The four main flux types used in electronics soldering. No-clean flux (center-right) is the most common choice for hobby and ham radio work. Water-soluble (OA) flux (right) is highly active but must be washed off within 30 minutes or it will corrode the PCB. Flux paste (far right) is used for desoldering with wick and SMD rework.

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Most rosin-core solder wire contains two or three hollow channels running the length of the wire, filled with flux. When you feed the wire into a hot joint, the solder melts from the outside in and the flux in the core activates first — cleaning the surface in the fraction of a second before the solder itself arrives. This is the primary reason that fresh, quality solder wire with a good flux core solders well even without additional flux applied to the joint. However, on older PCBs, heavily oxidized leads, or joints that have been previously soldered and re-soldered, the flux in the solder wire alone may be insufficient. In those cases, applying additional flux from a separate liquid or gel source makes a significant difference.

Flux Types

Rosin / RMA Flux

Rosin flux is the traditional electronics flux, derived from pine tree resin (colophony). It is mildly acidic at soldering temperature, providing reliable oxide removal for most applications. At room temperature rosin is an electrical insulator — the flux residue that remains after soldering is not electrically conductive and does not cause leakage between adjacent PCB pads.

RMA stands for Rosin Mildly Activated — rosin flux to which a small amount of activating agent (typically a halide compound) has been added to improve oxide removal capability. RMA is the most common type of flux used in rosin-core solder wire for electronics. The residue is a hard, amber or brown coating on the joint and surrounding copper. Electrically it is safe to leave in place indefinitely. Cosmetically, it can be cleaned with isopropyl alcohol (IPA) and a brush if you want a clean-looking board.

No-Clean Flux

No-clean flux is a synthetic organic compound formulated to leave a minimal, non-corrosive, non-conductive residue after soldering — a residue so thin and inert that it does not need to be removed for the board to function reliably. No-clean flux is now the dominant type in rosin-core solder wire sold for hobby and amateur use, and it is the type you will encounter in most commercial solder wire at electronics suppliers.

The "no-clean" designation means that the residue does not need to be washed off for electrical reasons. For cosmetic reasons or for inspection purposes, the residue can still be removed with IPA. One point of confusion: if you apply extra no-clean flux from a bottle or syringe and then solder with it, you will see slightly more residue than from the wire core alone. This is still no-clean residue and is still electrically safe. However, if you heat no-clean flux but do not complete the solder joint (e.g., you heat the flux for a desoldering operation and then let it cool without adding solder), the partially activated residue can be somewhat more difficult to remove. The practical rule: complete every joint in a single operation.

Water-Soluble (Organic Acid) Flux

Water-soluble flux — also called OA (organic acid) flux — is formulated with strong organic acids (typically citric acid, adipic acid, or similar) that provide significantly more aggressive oxide removal than rosin or no-clean flux. It is used in production environments where boards are heavily oxidized or where maximum wetting reliability is required on difficult surfaces.

The critical rule with water-soluble flux is that its residue is hygroscopic — it absorbs water vapor from the air — and remains chemically active. If left on the board after soldering, the residue will absorb moisture and become a conductive, corrosive electrolyte that will attack copper traces, cause leakage currents between pads, and eventually destroy the board. You must wash water-soluble flux residue off completely with deionized water within 30 minutes of soldering. This typically requires a board wash station, an ultrasonic cleaner, or thorough hand washing with a brush.

For most ham radio amateur work, water-soluble flux is unnecessary. Its aggressive oxide removal is valuable for production work, but the cleaning requirement is a significant burden in a home workshop and the risk of incomplete cleaning causing damage is real. Reserve water-soluble flux for situations where heavily oxidized surfaces genuinely cannot be cleaned any other way, and commit to the washing process before you reach for it.

Flux Paste (Gel Flux)

Flux paste is the same chemistry as liquid flux (available in RMA, no-clean, and water-soluble variants) but formulated as a thick gel that is dispensed from a syringe. The gel consistency keeps the flux exactly where you apply it rather than flowing to adjacent areas. This makes flux paste particularly useful for three tasks: applying flux to desoldering wick before pressing it onto a joint (the flux in the paste greatly improves the wick's ability to absorb solder), applying flux to a dense SMD component before rework with a hot-air station (the gel stays put while you heat), and pre-fluxing through-hole component leads before wave or drag soldering.

The dispensing syringe format gives excellent control over the amount applied. A tiny amount of gel flux applied to the body of a recalcitrant through-hole joint and then touched with the iron will often fix a cold joint in one pass that would otherwise require multiple attempts. Buy no-clean flux paste for general use; use RMA paste if you prefer the rosin chemistry and do not mind the amber residue.

Wire Diameter Selection

Solder wire comes in a range of diameters, typically from 0.3mm (for very fine SMD work) through 0.5mm, 0.6mm, 0.8mm, 1.0mm, and up to 1.6mm or larger for heavy industrial applications. The diameter you choose directly affects how much solder you can apply per unit length of wire fed into the joint, and therefore how precisely you can control the solder volume.

Using wire that is too thick for a small joint makes it easy to apply far more solder than the joint needs, creating a solder bridge or a blob-filled joint that buries the component lead and is difficult to inspect. Using wire that is too thin for a large joint forces you to feed in many passes of wire to fill the joint, during which time the flux in each piece of wire activates and then exhausts, potentially leaving later solder additions to land on a flux-depleted surface.

The practical guidance for ham radio work:

0.5–0.6mm: Very fine SMD work. The small diameter allows you to apply a tiny, controlled amount of solder to a single 0402 or 0603 pad without flooding adjacent pads. The flux core is proportionally small, so application technique must be good — heat the pad first, then touch the wire briefly. Suitable for fine-pitch IC lead soldering when used with flux paste and drag technique.

0.8–1.0mm: The most versatile range for ham radio kit building. This diameter works for all through-hole components — resistors, capacitors, ICs in DIP sockets, transistors — and for most SOIC-package SMD components. The flux core is substantial enough to clean most moderately oxidized surfaces without additional flux. A 500-gram reel of 0.8mm 63/37 with RMA or no-clean flux core is the single best first purchase for a ham radio bench.

1.2–1.6mm: Large through-hole joints, connector assembly, and heavy wire terminations. This diameter delivers a large amount of solder quickly, filling the barrel of a PL-259 connector or a large terminal lug efficiently. The thick wire can be awkward to control for small joints — a single touch deposits too much solder. Keep this size specifically for connector and heavy cable work.