Basic Electronics Soldering for Model Railroaders
This page is an introduction to electrical soldering for Model Railroaders, focused on such basic things as wires and small electronic components. Soldering brass models is a rather different skill, and one I know nothing about, and while many of the same basics apply, you should look for a good tutorial on that elsewhere.
Note: the following is advice based on my own experience and what I’ve read. I’m not an expert at this and don’t pretend to be, but I’ve managed with only a few minor burns over the years, so I’m probably doing something right. Ultimately how you use this advice is up to you; don’t blame me if you manage to hurt yourself.
Soldering isn’t any harder than other craft skills practiced by model railroaders, such a gluing together plastic kits. There is some basic knowledge, a few skills to learn with practice, and the need to choose the right tool for the job. Just as you wouldn’t use super-glue for a part under mechanical stress, you don’t use acid-core solder for electronics. Learn the basics, and then you can work confidently, and gradually work your way up to more complex tasks.
- Just as with power tools, don’t work around pets or children.
- Solder is very, very, hot, and can spatter. Wear safety goggles/glasses at all times.
- Solder can drip. Don’t hold the work or the iron above anything easily damaged, like your lap.
- Soldering irons are hot. Pay attention to where the iron is, and put it on a stand when you can’t.
- Lead is very harmful if ingested. Don’t eat/drink/smoke when soldering and wash your hands afterwards.
- Smoke from burning rosin can damage your lungs, ventilate your work area and don’t lean over the work.
I’ll say it again: lead is dangerous, particularly to children. If you use lead-based solder, be sure to keep it out of reach of children and wash your hands after handling it, and never do anything (like eating, smoking or chewing on a pencil) that might convey lead from your hands to your mouth while working with it.
Beyond that, keep the work area clean so the iron’s cord won’t snag on anything. Use a comfortable seating position that lets you hold the iron to the work without undue stress. And good lighting is essential. I use a shop bench with a pair of 40W fluorescent tubes above it, or a halogen desk lamp when I’m working on a desk or table.
- Soldering iron (see below).
- Rosin-core solder (see below).
- Clips to hold wires together (see below).
- De-soldering braid for the inevitable mistakes.
- A damp sponge (never re-use this with food utensils or pots/plates; see the safety tips).
Non-essential but useful tools:
- A stand to hold the hot iron, preferably with a place for your sponge.
- A modeler’s vice and/or a stand with alligator clips to hold the work.
- A tip-tinning tray (you can just use solder for this)
- RMA rosin flux (or equivalent) with a brush or pen applicator (see below)
A clean, well-lit area with a chair and work surface at the right height is very important, as you’re likely going to spend hours sitting there. You also need to be concerned about keeping the work in place and not dripping blobs of hot solder on fine furniture. Setting the tablecloth on fire is also likely to be frowned upon by domestic authorities (and your insurance agency).
My work area is either a table or workbench, with an anti-static mat on it to protect fine components from static charges. Atop that I place a small board (my preferred size is about 8” x 8”, but I’ll use whatever scrap lumber is handy) which protects the mat (and whatever table is under it) from burning solder. Items not being worked on sit on the mat (or in storage drawers or boxes), and the “work” (whatever I’m working on) sits on the board, or in a vice or other clamp that holds it above the board.
This first question anyone asks after looking at a selection of soldering irons is “which do I need?” The answer is “it depends”. If you’ll always just work on small wires and ordinary-sized electronic components (e.g., putting decoders into trains), a small, light, pencil iron of 20 to 30 Watts is all you need. If you’re soldering feeders to rail, a heavier iron (40 Watts or more) may be more useful, although I used a small pencil iron for this for years with careful use of clamps (and a few melted ties). And if you want to solder to surface mount components (like really small LEDs), then a temperature-controlled iron is very nice to have.
If you’re getting into more complex electronics, there’s value to stepping up to something fancier. I use a Weller WESD51 today for the temperature control, but for years I used a simple 20W/40W switchable Radio Shack iron. Although that one has a number of bad reviews online, it worked fine for me. But for most people, you’re best off starting with a basic iron and deciding later what you need, rather than spending a hundred dollars or more on a fancy model with features you don’t need.
I haven’t tried any of the portable or “instant heat” irons that have come on the market in recent years. I’m a bit dubious about them, in particular the ones that use an electric arc at the tip (flowing electricity and small electronic components is almost certainly a recipe for fried electronics). But for applications like feeder soldering, where you may need to work on a finished section of the layout in a tight space, and you can disconnect any electronics from the track, these might be very useful indeed.
Ultimately, the iron is a tool for applying heat to a point where heat is needed, and not applying it elsewhere. A low-wattage iron can only apply a small amount of heat, then it cools down to the point where it’s not melting the solder (or worse, melting the solder but not heating the object being soldered enough to avoid a “cold solder joint”). A high-wattage iron can apply too much heat if you hold it in place longer than needed. In general, a lower wattage iron is safer for a beginner still learning technique. But higher wattage can be useful if you’re soldering solid-core wire to flex-track rails (you need a lot of heat in a hurry to avoid melting the ties). Some irons have selectable wattage.
Really fancy ones have settable temperatures, which is different from just selecting the wattage. With these, a sensor near the tip reports back to the controller, and more power is fed when needed to keep the iron at a pre-set temperature (or close to it anyway). Unless you’re going to be doing a lot of work with individual circuit components, this is significantly more than you need for typical “wire and decoder” kinds of soldering, and there’s a high cost to such models.
Equally important is the tip. Tips wear out and can be damaged, so an iron with a replaceable tip (make sure you know where to get replacement tips) is a good investment. The tip puts heat onto the work, so a conical point is most commonly used, although for fine work with surface-mount components a “screwdriver tip” or small wedge (really small, about 1-2 mm across) is useful. Cheap irons will use all-copper tips, which wear quickly and aren’t recommended for use with lead-free solder. Better models use copper electroplated with iron.
A grounded iron (one with a three-prong wall plug and a description that it is “ESD safe”) is a good idea if you’re working on sensitive electronics, but not needed for track feeder or simple decoder wiring.
These days, everyone has online reviews. Check out the usual stores (Amazon, Radio Shack) or specialty electronics sites (Digikey under Product Index / Soldering, or Jameco) to find a good price, and read reviews of a few models on several sites that have reviews.
And clean and “tin” the tip frequently to make it last (more on that below). Also, turn off the iron when not in use. Not only is a hot iron dangerous, but if you forget about it for several hours, it could damage itself. I killed a $40 iron that way once.
Solder: How Big, What Kind, and Lead or Lead-free?
After the iron, the solder itself is the most important item. It comes in different sizes, and with and without a rosin core, and now you have the choice of traditional compounds that include lead (Pb) or Lead-free versions. Let’s get the last out of the way first. Solder was originally a lead/tin (Pb and Sn) mixture, now there are more complex formulations available that eliminate the lead. In some places lead-based solder is now illegal to use on consumer items, and becoming harder to find in general.
Lead is a neurotoxin. It can cause harm to brains and other nerves, and affects both adults and children, but children are more at risk as their brains are still developing. If you have kids, particularly kids who want to learn to solder, lead-free is the way to go.
But that comes at a cost. Lead-free solder is a bit harder to work with, as it has a higher melting point. This makes damage to fine components a bit more likely, and requires a bit more skill to use. Many people still like to use lead-based solder and claim the safety issues are manageable. That said, safety regulations may eventually make this a moot point, so learning to use lead-free solder may be inevitable.
If you do use lead, the risk is reportedly limited to ingestion (eating it). At any normal temperature used for soldering lead doesn’t vaporize, so you can’t inhale it. But lead gets on anything that touches it, like your hands, workbench, and tools. So don’t eat, drink or smoke while working, don’t work on the kitchen table, don’t re-use a knife used to push your work around for cutting anything edible, and do wash your hands immediately after soldering.
Frankly, I think washing up after working with chemicals is good advice for any modeling work; just because you don’t know something’s harmful yet, doesn’t mean it isn’t. Over the years many supposedly “safe” substances have turned out to be very harmful indeed. Why take chances you don’t need to?
There are several compounds used for solder, generally described by the percentage of the different elements in them. Traditional lead solder, also called 60/40 solder, is a mix of 60% tin (Sn) and 40% lead (Pb), the amount of rosin isn’t included in the list, but is around 1%. Per wikipedia, 63/37 is preferred for electronics work. Lead-free solders are typically based on a tin-silver-copper compound (Sn-Ag-Cu), often with a fourth element added, which can vary.
The vapor produced by soldering is harmful, but that’s the fumes from the burning rosin (which is basically sap from a tree), which includes the kinds of chemicals you’d get off a cigarette, and worse. Repeated exposure can supposedly cause irreversible asthma, so don’t breathe the fumes.
Solder is sold as a wire on a spool, and comes in three basic forms: Rosin Core, Acid Core, and Plain. Acid-core solder is only used for plumbing and similar things, and it can damage fine details (and needs to be cleaned off after use). Don’t use it. Plain solder is used in some kinds of work where you want to exactly control where the solder goes by applying rosin separately. For all normal uses, you want your solder with a rosin core.
Solder is sold in different diameters. The really thick stuff (1/8” or several mm) is for structural work, not electronics. A heavy-gauge wire (1 mm or a bit larger) is probably the right size for soldering track feeders and large-gauge bus wires. Smaller diameters are useful with electronics. But if you get too small, there won’t be enough rosin in the solder to work properly, and you’ll need to add it up front (using a bottle with a brush or something similar). That’s mostly needed with you get down to surface-mount components and solder that’s a fraction of a millimeter in diameter.
Flux: Rosin and Other Kinds
Up to this point, I’ve used the word “rosin”, because I wanted to be specific. Rosin is one form of what’s more generally called “flux”. Flux has two purposes, to remove oxidation and other impurities from the surfaces being soldered (and to keep it off while working), and as a “wetting” agent, to allow solder to flow more freely. There are many kinds of fluxes, see this Wikipedia page for more detail.
All fluxes are corrosive to some extent, as they’re based on some form of acid. Rosin fluxes are based on pine sap, and come in different grades of activity. The more active ones require the work (area being soldered) to be cleaned after use. Grades R (also known as WW) and RMA are the ones to use without cleaning afterwards, and this is what’s preferred for hobbyist electronics work. RA-grade solder requires cleaning, although the wikipedia page says that some compositions do not. Cleaning, however, could be with a damp lint-free cloth or alcohol wipe or swab dipped in alcohol, whereas more active fluxes would need more careful cleanup.
Cleaning of the soldered joint is needed if an “activated” flux that might have left acid behind was used. This prevents long-term degradation of the joint. The alternative is to use a “no-clean” flux (which typically means “R” grade rosin flux, although as noted above cleaning isn’t strictly necessary for RMA and some forms of RA rosin flux). However, these don’t do as good a job of cleaning or wetting.
In general, RMA flux will give you the best mix of features for hobbyist work: sufficient activity and “wetting” for ease of use, without the need for cleanup.
However, rosin itself is falling out of favor due to its own toxicity issues (more related to groundwater pollution from discarded electronics), and being replaced with other kinds of “flux”. These new fluxes are known generically as “water-soluble” fluxes. Often these are “no-clean” fluxes (equivalent to R-grade rosin flux).
In addition to flux-core solder of various kinds, flux is also available in liquid or paste form, and in pens that can be used to apply it in small quantities to specific locations.
Cleaning and Tinning the Tip
The tip of the soldering iron is the part that conveys heat from the heating element in the iron to the item being soldered. For maximum efficiency, it’s important that this tip have a layer of solder on it, and be free from burnt rosin or other debris. You also don’t want excess solder clumped on it.
A damp (not soaked, just damp to touch) kitchen sponge is the easiest way of cleaning the tip. Just buy a bunch of these in a light color, and throw them away when damaged too much. You can also buy special iron-stands with a space to keep the sponge, and these are handy (you can always cut a normal sponge to fit rather than buying expensive replacement sponges). While working, use brief passes across the sponge to clean debris and excess solder off the tip (let the iron heat back up for a few seconds after this). When debris builds up on the sponge, you can usually wipe it off with a finger when holding the sponge above a trash can. Toss the sponge and get a new one when you can’t clean it (or if the surface has gotten burned because you let it day out while using it).
In industrial applications, use of specialty sulfur-free sponges and de-ionized water are recommended, but both are unnecessary for hobbyist soldering. It’s all about tip wear and how often you need to replace it, and contamination of solder joints with foreign substances and the effect that has on reliability. None of that is likely to matter much in small-scale model railroad applications; I use tap water, although I do buy the special sponges, but that’s mostly a matter of convenience, as I can get them at the local electronics supply store.
Note: as mentioned previously, keep solder away from food. Don’t wet the sponge in the kitchen sink. Fill a small cup with water and pour on the sponge outdoors or over a bucket if you don’t have somewhere else to wet it.
To “tin” the tip, get the iron hot, touch the end of the solder to the tip to melt a small blob onto the tip, roll the iron around and re-touch the solder as necessary to coat the whole tip, then wipe off any excess on the sponge. Expect hot solder to drip while you’re working. A correctly tinned tip should be shiny silver all over the point. Re-tin whenever simple cleaning on the sponge isn’t enough, which means often.
Surface Preparation: Cleaning the Object to be Soldered
Solder works by bonding to metal. And to do that, the metal (both pieces) needs to be clean of dirt and even oxidation. The “flux” in the solder (typically rosin) does part of this, but it can’t do it all. You need to start with a clean surface, and in some cases you may need to add flux directly to the surface.
With insulated wire, when you strip the insulation you expose wire that hasn’t had much opportunity to oxidize. That’s generally sufficient. To make things go quickly you can “tin” the wire, in much the same way as you tinned the iron’s tip, but you need to have clean wire first. If the wire was previously stripped and you can’t cut off the exposed end and strip more, fine emery cloth or a file could be used to clean it, or a commercial “contact cleaner”, although many of those leave a residue behind that could interfere with solder.
For track, not only do you need to remove oxidation, but if the rail has been painted or treated with a “weathering” compound, you need to remove that from the part where you’re working. Soldering to the flat underside of the rail, which you can clean with a file and emery cloth until it’s shiny, is easiest. Soldering to the outside of the rail is harder to do in this regard, because it’s harder to clean it. A glass-fiber pen, which can be found in auto-supply stores where paint for repairing chips is sold, can be used to clean small, hard-to-reach areas. But be careful, as these shed small glass fibers, which can irritate your skin (wear gloves, and if possible wipe the part with an alcohol wipe or swab or similar afterwards, to remove any fibers and loose dust).
When you solder with rosin-core solder, a small amount of rosin is released as the solder melts, and cleans the area where you are applying solder of remaining oxidation and other impurities. But rosin boils quickly at soldering temperatures, and if it boils off it can’t do its job. This is one reason you apply solder to the metal surfaces being soldered, and not to the iron (the other is that the metal needs to be hot too, to form a good joint).
Flux can be applied in advance, using a brush or other applicator. This can be useful as a way to clean hard-to-reach places. Because flux also serves as a wetting agent, which allows solder to flow easily, this can be used to more precisely control where the solder goes (in this case, solid solder could be used rather than rosin-core solder). As noted above, the kind of flux used matters, as you don’t want to have a corrosive one on an electronic circuit or wire joint (unless you clean it afterwards, and that’s hard to do well for the more active fluxes). Even with RMA flux, you want to use the minimum necessary, not slather it on the parts.
Holding the Work: Clamps and other Tools
Since solder is molten metal, you can’t hold the part being worked on with your hand (this should be obvious, but you’ll likely learn this the hard way a time or two before it sinks in). Further, since it takes time to cool, the two objects being soldered can’t be allowed to move until it has cooled sufficiently to form a bond, which takes several seconds. Finally, since the points being soldered are often surrounded by other parts that could be damaged by heat, you need some way to keep the heat from spreading. In general, a few small clamps will do all of this.
Oddly, one of the best tools for the job is a surgical hemostat. This is a small clamp made entirely of metal that locks in the closed position. These are chrome-plated, which keeps solder from sticking to them. They come with smooth or serrated jaws, and with straight or angled tips. I prefer the angled-jaw version with serrated tips for track feeder wiring, using two of these (one at each end of the length of wire touching the rail). This both holds the two securely together, and also serves as a heat sink, so adjacent plastic ties are much less likely to melt. You used to be able to get these at Radio Shack, but you’ll probably need to order online from an electronics specialty store now, as they seem to have dropped off the Shack’s online catalog.
Hemostats are, unfortunately, a bit large and clunky for fine work. You can get special soldering clips made of aluminum (which has the benefit that solder doesn’t stick to it very well), and these work fairly well, although the springs are too strong for really fine items, and can bend them. Chrome-plated clips used by hairdressers also work as heat sinks, and have gentler springs. You can often pick up a bag of several dozen for only a few dollars (they work well for other model-clamping tasks also).
With surface-mount components (or similarly small items), you need to clamp the objects rather than the point being soldered, which isn’t ideal. I’m still working on the right way to do this when you have something like a surface-mount LED and a wire, but no “surface”. I’m pretty sure the answer involves growing a third arm (alligator clips can also work).
If you have an iron with an adjustable-temperature tip, your first question, like mine, will likely be “what’s the right temperature?”. Again, the answer is “it depends”. Solder melts at different temperatures depending on its composition, but you want the tip temperature to be sufficiently above that so that even as it cools in use, it’s still making the objects being soldered hot enough to melt solder.
Lead solder melts at 183°C (361°F) or below. Lead-free solder has different melting points depending on the compound, from 187°C (369°F) for “Sn77.2 In20 Ag2.8”, a common form used in electronics, to temperatures of 230°C (446°F) for more specialized forms, and even as high as 250°C (482°F). But you want to be hotter than that, as the tip will cool quickly as heat flows into the item being heated, and the two need to equalize at a temperature above the melting point.
The recommended temperature depends on what you’re using for solder, and on other constraints. I’ve seen a recommended range of 316°C to 371°C (600°F - 700°F) for lead solder, and 371°C to 426°C (700°F - 800°F) for silver-solder or lead-free solder. However, I’ve also seen 400°C given as an upper limit.
And several recommendations have been to start low (around 260°C/500°F) and raise the temperature until you get “the desired result”, which isn’t the most helpful tip I’ve ever seen.
One very specific tip (which I can’t find now) said to use 288°C (550°F) for lead solder, and to raise it about 28°C (50°F) for lead-free. The latter seems excessive to me, since the difference for typical electronics solders between lead and lead-free is around 4°C.
I initially tried using 288°C (550°F) when I purchased an adjustable-temperature iron, using lead-based solder. This didn’t work as well as I hoped, and I switched to a temperature of 343°C (650°F), which has worked better, for both through-hole semiconductor components and wires up to 16 gauge.
Aaron Cake’s How to Solder - An excellent overview, with a fair number of photos
Circuitrework.com’s Soldering Basics - Fairly terse
Instructables How to Solder guide - an ad-heavy guide, but with lots of photo’s and detail
Sparkfun Electronics’ SMD How To - a guide for how to solder surface-mount components, excellent!
Tips and other References:
Cooper’s Better Soldering - a short manual focused on tip care and forming a proper joint on through-hole PCB components.