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Solder 101

Solder 101

Types of Solder

Soldering is the process of joining pieces of metal (called the BASE) with a different metal of lower melting temperature (called the FILL). It requires a FLUX, which is a substance that helps the fill to flow properly, and reduces slag or impurities.

If the fill has the same or higher melting temperature as the base, it's not soldering but welding.

Different fill metal alloys melt at different temperatures. Some require a lot of heat, some require relatively little. (An alloy is a mixture of two or more metals.)


AT the top of the temperature list is BRAZING, which uses a brass (hence BRAZing) alloy of copper, zinc, and small amounts of other things. It requires a temperature up in the 1650° F. neighborhood or higher. Takes an oxy/acetylene or oxy/something else to get up that hot. Uses borax for flux. Not a whole not of horological applications for brazing.


Next down is one of the two that cause terminological headaches. Some call it SILVER SOLDERING, or HARD SOLDERING; others call it SILVER BRAZING. It uses an alloy with a goodly percentage of silver (ranging from 20% to 50%) mixed with copper and other metals. It's not strictly speaking "brazing", since the fill isn't brass. It has a melting temperature in the 1200°F. territory. Silver Solder (its rightful name) comes in wire form and sheet form. It's a hard alloy, and makes a very strong joint. Requires a dedicated silver-solder flux, which usually looks like a grainy white paste. Some silver soldering can be done with a butane torch.


Down near the bottom of the temperature scale is good old familiar lead/tin solder. Usually comes in wire form on a spool. Sometimes has a flux core, either rosin or acid. Otherwise uses flux that looks like brown grease, usually called "soldering paste". Solders for different purposes have different percentages of the two metals. The most popular alloy is 60/40 (tin/lead). Has a melting point in the 375°F. range, which means you can do it with a soldering gun or soldering iron, as well as a torch. Lead-tin solders are among the SOFT solders...the fill is a softer alloy than brazing or silver solder. But soft solders vary in their softness. Some are quite hard. In general, any alloy with a melting temperature below 500°F. is counted as a soft solder.

In between silver solder and lead/tin solder, temperature-wise, is the other terminological headache. It uses an alloy consisting of a small amount of silver, and the rest tin. A usual ratio is 5/95. Because it contains silver instead of lead, it is also called "silver solder", which as we know generates confusions every time the topic comes up. Big time solder manufacturers, who produce dozens of different fill materials, don't call it "silver solder". They list it as LEAD-FREE SILVER-BEARING SOLDER. If we could teach ourselves to call it that, or just SBS, confusions would be greatly reduced. But good luck with that. SBS has a melting point in the 430°F. neighborhood, still manageable with soldering gun or iron. Uses the same flux as lead/tin. SBS is harder than lead/tin, but is still a soft solder.

Environmental laws to reduce lead in the environment have led to much-reduced use of lead/tin solder. Its replacement is a variety of tin-based alloys with melting temperatures in the neighborhood of that of SBS —roughly 400° to 465° F. The most widely-used alloys are tin/copper or tin/silver/copper. These have the highest melting points. Other elements, added to lower melting temperatures, include bismuth, indium, antimony, and even aluminum. These lead-free solders generally require a different flux from SBS or lead/tin.


These are a special class, using exotic metals like iridium, indium, and bismuth in their alloys, to get their melting temperature down. The best known, and most easily available one, is TIX. After a great deal of searching, I was able to find what it's made of: 93% tin, 4.6% indium, and 2.3% lead. This interesting blend produces the hardest of the soft solders, with a melting point of just 275°F. It uses a special liquid TIX flux. Excellent for producing a strong, hard joint with minimal heat.


An alloy is eutectic when it solidifies and liquifies at the same temperature, with no "mushy" in-between. As soon as it quits being liquid, it's solid and holds like it's meant to. As soon as it quits being solid, it's liquid and flows nicely like it's supposed to. 63/37 tin/lead is eutectic; 60/40 isn't. 3.5/96.5 SBS is eutectic. 5/95, like Stay-Brite, isn't. TIX is eutectic. Each type of solder has its uses. Non-eutectic is needed when the solder is to be manipulated in semi-solid state.



In order for solder to adhere properly, it must be melted by heat in the metals being joined. The function of the heat source (iron, torch, gun) is not to melt the solder; it's to get the target metal hot enough to melt the solder. If the target metal isn't hot enough, the solder won't hold.


Solder is a glue, not a structural material. The proper place for solder is between the pieces being joined, not piled up on the outside. Any solder outside the joint is excess, and does no work.


Surfaces to be soldered must be clean and bright. Fine sandpaper and steel wool are your friends. Flux won't overcome an unclean surface.

The three familiar heat sources for soldering are the soldering iron, soldering gun, and soldering torch. The soldering iron has limited application in clock work, since it has a hard time producing enough heat to get the work pieces up to temperature. They are fine for electronics, and tiny bits, but not for many clock jobs.

The small butane pencil torch is an ideal soldering tool for most clock work, for one simple reason: unlike the soldering gun, it does its work without touching the pieces being joined, moving them out of position. This is especially important when the pieces can't be held or clamped in position during soldering.

The high-temperature hand-held butane torch gets hot enough for silver soldering (hard soldering). [LATER NOTE. I've found that for most hard-solder alloys, this is not true. A two-gas torch (oxy-acetylene, oxy-MAPP, etc) will be needed.]

A good work surface for soldering is a ceramic kitchen or bathroom tile from the hardware store.

A typical approach to a soldering job is to heat the pieces up to temp, then poke the solder at the joint and let it run in. But there is a better way.


Always use a flux, and use one proper for the kind of solder you're using. Flux both pieces, and arrange them as you want them to be joined. Snip off two or three small bits of solder, and position them next to the joint. Direct your torch at the metal away from the joint, not at the joint and not at the solder. Let the heat work its way down to the joint. When the two target pieces are up to temperature, the bits of solder will melt and wick into the joint. Solder follows the heat. Both pieces need to be up to temperature, in order for the solder to adhere to both of them. Use no more heat than you need to get the solder to melt and wick in. Allow the piece to cool, and the solder to harden, before touching or moving it.

When the soldering job is finished, wash the piece thoroughly under hot running water, to remove all traces of flux. Some fluxes can be quite corrosive if left in place. See About Fluxes below.

In general, it's better to locate the solder where you want it before applying heat, rather than applying heat and then applying solder. If a relatively broad piece is being attached, put a snippet or two of solder between the two fluxed pieces, as well as at the edges. That will adhere in the middle of the piece.


A common use for a soldering gun is for tinning pieces to be joined. Tinning is a process of applying a thin skin of solder to the piece, to assure proper adhesion in the joint. Clean brass and copper seldom require prior tinning (they seem to tin themselves when the solder is applied —but prior tinning never hurts). Ferrous metals always require tinning.

Flux the item to be tinned. Pick up a droplet of solder on the tip of the soldering gun, and rub it on the fluxed item to apply the tinning. That's all there is to it.

The soldering gun is also useful for very thin pieces, such as the inner brass ring of a bezel, where positioning isn't likely to be a problem.

For really small jobs, like watch bits, use a fine-pointed soldering iron instead of a gun or torch. Too much heat can damage them.


The function of a soldering flux is (a) to clean the metal and prevent oxides from forming, (b) to help the solder flow instead of beading up, and (c) to enable the solder "wet" the surfaces being joined.


Soldering flux for "soft" solders comes in liquid and paste forms, and as a core in wire solder. With solder on spools, flux core is the norm. Except for small jobs, such as electronics or electrical wiring, the amount in the core usually isn't enough to do a thorough fluxing job.

Soldering fluxes come in two types: rosin-based and non-rosin based.

The most widely used fluxing agents in non-rosin fluxes are zinc chloride and ammonium chloride (sal ammoniac). Because of their corrosive nature, these are known as "acid fluxes".

Liquid fluxes are usually solutions of zinc chloride, with or without added ammonium chloride. Thus, all are basically the same, and work the same, differing only in the strength of the solution.

Soldering paste flux, the familiar brown goo, is a mixture of zinc chloride/ammonium chloride and petrolatum ("Vaseline"). It isn't water soluble, and can't be washed off under running hot water.

Some pastes use a different vehicle, and are water soluble. If it doesn't say "water soluble" on the can, it isn't.

Zinc chloride flux is corrosive, and leaves a corrosive residue after soldering that needs to be removed. Except for petrolatum pastes, it can be washed off under hot running water. Petrolatum pastes first need a good swishing in naptha or acetone to remove the grease, followed by hot water wash.

A good all-purpose flux for most clock work is TIX flux, which is a zinc chloride solution. It works for all soft solders. Just brush it on. Wash off the residue.

Rosin-based fluxes have pine rosin as their main active ingredient. They come in several forms, including pure chunks of rosin of the kind used on violin bows. Rosin is a less vigorous flux than zinc chloride. It is also non-corrosive. The residue it leaves is not water soluble. To get it off you need a commercial flux remover...or, I suppose, turpentine to dissolve the rosin. If left alone it hurts nothing except appearance.


Metal tinned before soldering gives a surer bond.Tinning fluxes are paste fluxes that contain finely powdered solder. They are brushed on the heated metal (or brushed on and then heated) and supposedly leave a thin skin of solder on the metal. The ones I've tried haven't worked very well or at all (but I haven't yet tried them all).

A different product, sometimes confused with tinning flux, is tinning compound. A tinning compound is a dry powdery blend of solder and flux, usually ammonium chloride or zinc chloride. The usual method is to prepare a slurry or thin paste of tinning compound and water. Paint in on the item to be tinned, and heat the item until the solder melts leaving a tinned film. (Automotive applications may use a slightly different method.) A widely-used brand is Acro-Tin, made by Dutch Boy.

The main drawback to tinning compound, so far as clock work goes, is that it doesn't seem to be available in any quantities smaller than 1 pound, which is more than a clocksmith will need in a lifetime. A pound of Acro-Tin can be purchased on-line for $15-$20. It may be shared with friends.

For hard soldering (brazing, silver-soldering) the basic flux ingredient is borax, with or without additional ingredients such as potash or sal ammoniac. It comes either in dry granular form, or as a slushy paste. The residue it leaves is a hard, glassy substance that needs to be chipped off. It's soluble in water, but takes a lot of rinsing. Some fluxes have additives that reduce this problem.

For an informative article on fluxes, see Soldering flux (Barry L. Ornitz; J. Kimberlin)

For a tutorial on making your own zinc chloride flux, see





This is a method for joining objects using minimum solder and achieving a strong bond. The two surfaces must be smooth and capable of intimate contact. Tin the two surfaces well. position them together. Apply heat until the two tinned surfaces fuse together. Sometimes it is helpful to snip tiny bits of solder put 2 or 3 of them on the bottom piece, cover them with the top piece, and apply heat until the solder flows. But with a good job of tinning, that's not necessary. All of the solder will stay inside the joint, and will be invisible except for a thin line at the edge.



New to me at least. It is called Solder-It Silver Bearing Solder Paste. It is a blend of pulverized solder and flux, and comes in a syringe type dispenser. It is easy to use, and is ideal for sweat-soldering since it is self-tinning.

View attachment 171169

Have both surfaces bright and shiny. Put a tiny amount of Solder-It on one, and lay the other on top. Apply heat with gun, iron or torch. It doesn't take a lot --the stuff is low-temp (430°). It melts, tins the surfaces, and joins them in a tight bond. It's the best of the solder/flux compounds I've seen.


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Current Discussion: Main discussion

  1. Fitzclan

    Fitzclan Registered User

    Jul 20, 2014
    Long Island, New York
    Nice job Bangs. Very informative for someone like me who knows nothing about the subject, but have attempted a few (unsuccessful) soldering ventures anyhow. I now know where I went wrong and will have better prospects of success in the future! Thanks.