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Soldering is as easy as 1-2-3.
If it is (1) clean enough, (2) fluxed enough and (3) hot enough, it will
solder. Of course there are a few other tricks that help as well.
Clean enough - clean means shiny clean metal, clean means
no contaminants like oil, grease, paint or oxides, and clean means staying
clean through the whole soldering process. The last is where flux comes
in.
Fluxed enough - means fully covered to exclude air using a flux
which can break down any residual oxides that form between the cleaning
process and the conclusion of the soldering process.
Hot enough - means the temperature is high enough to melt the
solder and cause it to flow through and over the joint. It also means the
temperature is not so hot that it causes the metal to oxidise and become
unsolderable.
Lets look at these three in more detail.
Cleaning usually involves mechanical action,
such as filing, sanding or scraping to remove solids from the surface.
But sometimes chemical treatment is more appropriate, for example greasy
surfaces can be cleaned with cleaning solvent, Varsol or lacquer thinner
applied with a brush or rag. Mechanical cleaning would only move the grease
around. Paint can be mechanically removed or dissolved with paint removers
(lacquer thinner often works.) Sometimes heating is the easiest way to
remove contaminants, such as the lacquer often applied to brass to keep
it from tarnishing.
Wire and wiring components often do not need cleaning - they are often
precoated with solder ("tinned") so that when heated, minor contaminants
on the surface of the solder are floated off by the flux. But not all wire
is tinned. Some must be scraped with a knife or sand paper to make it clean
enough to solder. Occasionally, switch lugs are silver plated to enhance
solderability. Silver is very solderable, second only to gold. But silver
tarnish is impossible to solder. Tarnished silver must be cleaned, which
is easy to do with Comet or similar cleanser applied with an old toothbrush.
Some metals are "unsolderable" no matter how much you clean them. This
is usually because they form surface oxides so quickly that you cannot
clean them fast enough. Aluminum is the most common metal in this category.
Under an inert atmosphere, it can be soldered and welded just like any
other metal, however, such techniques require special equipment not generally
available to model railroad hobbyists.
Fluxing can be done with a variety of compounds,
but liquid rosin (rosin dissolved in alcohol) and "plumber's flux" are
the two most useful. Plumber's flux is a greasy concoction containing acid
which is most useful in soldering mechanical assemblies. Canada Metal's
Solder Paste is a good brand. It contains zinc chloride and hydrochloric
acid and comes in 50 gram tins. It can be applied by brush, toothpick or
even finger to the areas to be soldered. When heated, the acid is activated
and cleans off any slight residual surface oxidation while the greasy part,
which may well be stearic acid, a component of animal fat and a flux in
its own right, flows over the surface to prevent further oxidation. After
soldering, any leftover flux can be removed with solvent or hot soapy water
or with Comet and a toothbrush. Plumber's flux is more aggressive than
rosin flux, and is not really suitable for electrical work because it cannot
be cleaned completely from stranded wire. It remains slightly corrosive
and will slowly eat away at the wire until there is no copper left. The
possible exception to this rule would be joining solid, non-stranded track
feeders to rails and soldering rail joiners.
Rosin is the most useful flux for electrical work because it is corrosive
only when hot. When cold, it solidifies and is inert. In practice, this
means it does not have to be cleaned from joints, although if so desired
it can be removed using alcohol (wood alcohol, methyl hydrate, gas line
antifreeze or denatured alcohol all work.) For electrical work, solder
with a rosin core is often used. If used correctly, it works great. If
not, it fails miserably. More on that later. Sometimes electrical work
requires pre-fluxing or extra flux. Then liquid rosin flux is required.
M.G.Chemical's cat. no. 835 No Clean Rosin Flux is a good brand. A 100
ml. bottle will last a long time. Note that this is a rosin dissolved in
alcohol. Unfortunately, it is sold in tall, skinny, tippy bottles. For
ready use, pour some into a used Testor's Plastic Cement bottle - the glass
kind that comes with a brush in the lid. Then set the bottle into the bottle
holder you normally use to keep your plastic cement from tipping over (if
you don't have one now, you surely will after the first time you knock
your flux bottle over.) If your rosin flux needs thinning, use any of the
alcohols listed above. Rosin flux has uses outside of wiring too. It works
on many metals and on some, such as iron, works better than plumbers flux.
Acid core solder has only one use - adding weight to your garbage can.
It is of no use whatever in any kind of soldering, electrical or mechanical.
Heating requirements depend on the amount
of metal to be heated, how well it conducts heat, and the type of solder
used. Tin/lead solder melts at 361º F. for 63% tin "radio solder"
to 460º F. for 60% lead plumbing solder but requires higher temperatures
to flow properly. For electrical work, 600º F. to 700º F. thermostatically
controlled soldering irons in the 25 to 100 watt sizes are ideal but expensive,
especially when several different sizes may be required to cover the whole
range of work. More realistic perhaps would be a 30 to 40 watt non-thermostat
iron with a conical point to solder everything from small wires down to
integrated circuits plus a 150 or so watt soldering gun to handle small
wires up to heavy bus wires and the lighter mechanical soldering jobs.
Soldering irons transfer heat most effectively if they are clean and tinned.
Dull, dead solder can be cleaned off an iron by rubbing it over a dampened
viscose sponge. The iron can then be tinned by fluxing it lightly and adding
solder (or by just touching rosin core solder to it.)
For larger mechanical work requiring more heat, a propane torch is inexpensive
and useful. These torches apply heat quickly, often too quickly, and there
is a limit to how low the flames can be regulated. Heat can be applied
more slowly by using only the tip of the flame, and more slowly still by
"tickling" the work with the tip of the flame, alternately applying it
to the work and moving it away from the work, usually quite quickly.
Along with heating it is appropriate to talk about NOT heating. When
we are soldering on a new piece we often want to avoid unsoldering an old
piece already in place. Often all that is required is to keep the old piece
cool with a bit of dampened tissue or cloth. Depending on the material
we are soldering to and at what distance from the wet tissue, we will probably
have to use more heat for our new soldering job. If the new piece is really
close to the old piece, keeping the old piece cool may not be possible.
Then it is time to bring out the special low melting point solders so that
the new piece can be applied without melting the solder holding the old
piece. Alternately, it may be possible to hold the old piece in place by
tying with wire, clamping or building a jig. Then the new piece can be
applied, and if the solder holding the old piece melts, the old piece will
not move but will simply sit there waiting for the job to cool and the
solder to re-solidify.
Soldering FAQ's
Soldering rail joiners
Why does the solder just make an ugly lump on the side of the rail?
Why does the solder crack after a year or two?
Why do my ties melt when I solder the joints?
About 80% of the questions the Saskatoon Railroad Modellers are asked
about soldering concern soldering rail joiners
Answer - The joints between rails still require track joiners
even if you solder. Solder by itself is not strong enough to join the rails
and resist all the mechanical forces of heating and cooling plus the effects
of humidity changes on the wood frame below the rails. Even if you use
rail joiners, solder that does not penetrate all the way round between
the inside of the joiner and the outside of the base of the rail is still
just solder holding the rail ends together by itself. To get the full strength
of the joint, the solder must penetrate. And if it does so, there is no
need for any solder on the outside of the rail above the joiner. To get
the solder to penetrate, the rails and the joiner must all be clean. The
ends of the rail may be clean enough if they are new, otherwise clean them
with a small wire brush (a rotary wire brush in a Dremel tool is ideal).
This means nickel sliver rail as well as brass, plated iron and solderable
stainless steel rail. Don't bother cleaning aluminum rail - you can't solder
it
anyway. 'Ends of the rails' is meant to include the sides and bottoms of
the rails for at least the length of a rail joiner each way from the joint.
Always use a new joiner - they are cheap and there is no satisfactory way
of cleaning old ones. Assemble the joint, then add a small amount of plumber's
flux along the length of the top edge of the joiner, using a toothpick.
Add a bit of solder to a clean, hot iron. Then lay the tip of the iron
in the groove formed by the rail head, the vertical part of the rail, and
the rail joiner. This means having the soldering iron close to parallel
to the rail. Just bringing a pointed tip straight in will give too small
an area to transfer heat properly. Watch closely. Within seconds the plumber's
flux will melt and run through the joint, closely followed by the molten
solder. Remove the iron and let the joint cool, or force cool it by applying
a damp sponge to the rail head. In HO-scale, it should not take more than
3 seconds to solder the joint, and if applying a damp sponge, the total
time from applying the iron to having the rails cooled again should not
exceed 5 or 6 seconds. This is much to quick for the ties to melt. The
method relies on metal to metal heat transfer being much faster than metal
to air to plastic heat transfer. If the iron is dirty or not tinned properly
or the rail is dirty or not fluxed properly, the heat flow from the iron
to the rail will not be metal to metal, it will be slow, giving the ties
lots of time to heat up and melt. Attempts to heat the rails faster by
using the biggest available soldering gun are usually accompanied by failure
to clean and flux properly, often in the mistaken impression that the flux
from rosin core solder can magically make up for lack of proper preparation.
The result is a predictable melting of ties, misalignment of rails, ugly
gobs of solder on the sides of the joints and ultimately failure of the
joint some years down the road. Ironically, more time, usually much more
time, is wasted correcting such botched jobs than would have been required
to do the job properly in the first place. Best advice here is to practice
on scraps of track until you can do it right every time (probably not more
than half a dozen tries if you follow the procedures laid out here.) While
you are at it, calibrate the amount of solder required, exactly how many
millimetres of your solder are required to fill the joint without leaving
any excess on the sides of the rails. And be sure to take a few of your
trial joints apart, just to make sure the solder really did penetrate all
the way round. Soldering rail joints is a technique you will use hundreds,
perhaps thousands of times over your years as a model railroader. Why not
take the time to learn to do it right, right now.
How do you solder wires together?
Answer - First slip a piece of shrink tubing over one of the
wires. Then strip a half inch or so of insulation off both wires, twist
the bare ends together, and solder. If the wires are tinned (shiny silver
in colour) then rosin core solder is all that is required. If the wires
are untinned (copper coloured), then add a little liquid rosin to the joint
before soldering. Bring the soldering iron and the solder to the same point
on the joint at the same time. This will melt a little solder onto the
wire and onto the iron to help heat transfer. Immediately move the solder
a little distance from the iron, and when it again starts to melt, feed
a little into the joint. This will assure the joint was solder melting
hot at least that far away from the iron. When the joint is filled with
solder, remove the iron and let the joint cool. Then slide the shrink tubing
over the joint (you did remember to put on the shrink tubing, didn't you?)
and shrink it with some heat (a disposable cigarette lighter works well
here, particularly if you just tickle the tubing with the tip of the flame.)
Taping a soldered joint is about fourth choice, right after the first three
choices:
shrink tubing, shrink tubing or shrink tubing.
How can I solder two wires together without twisting
them?
Answer - You will want to do this when installing decoders, or
anywhere else that you want a joint that can be easily taken apart. The
problem is this - one hand for wire A, one hand for wire B, one hand for
the soldering iron and one hand for the solder, and nobody around to help.
The solution is to strip and tin both wires (even if they were factory
tinned) and sweat them together. To do this, arrange one wire so that it
does not move (not necessarily sitting on a surface, it can just as well
be hanging in the air.) Flux both wires with liquid rosin. Hold the second
wire a little bit away from the bare end so you don't burn your fingers.
Align the tinned part of the second wire with the tinned part of the first
wire. Holding the soldering iron in your other hand, touch the tip of the
iron against the tinned wires, heating them up enough for the solder to
re-flow. This only takes a couple of seconds. The added flux will assure
a smooth, clean joint. Remove the iron and keep the joint from moving until
the solder sets. Slide the shrink tubing over the joint and shrink it in
place. (Oops - this time it was me who forgot the shrink tubing. Oh well,
a touch of the iron and this joint is apart. More flux and a touch of the
iron and it is together again. As many times are you want.) A final note
on this type of joint - if you have to take one apart and it has shrink
tubing on it, you can hack away with a knife trying to strip off the tubing
without damaging the wires, or you can burn through the center of the shrink
tubing with your soldering iron, melt the solder and gently pull the joint
apart. The ends of the shrink tubing can be easily slid off the wires if
this is done immediately while they are still warm.
What is sweat soldering and why would I use that technique?
Answer - Sweat soldering is pretinning both surfaces, adding
flux, then reheating both pieces together until the solder remelts and
flows together. This is a very useful technique when mechanically soldering
larger pieces. It assures you that both pieces were adequately cleaned
and that the solder actually reached them and covered them (you can see
it - no having to guess.) It is also useful when you run out of hands,
for example, soldering a detail on a locomotive boiler. Conventional wisdom
says one hand to hold the detail part, one hand to hold the iron, one hand
to apply the solder. Perhaps conventional wisdom has more hands than you
and I. For two handed mortals, it is possible to tin the detail by fluxing
it and applying solder where needed using one hand for the iron and the
other hand for the solder. Same with the spot on the boiler where the detail
part is to go. Then it is simply a matter of re-fluxing both parts, holding
the detail part against the boiler using pliers or tweezers in one hand
and applying the hot iron using the other hand.
I can't get solder to "take" to the metal, even though
it has been soldered before.
Answer - The metal may be a marginally solderable stainless steel.
Some stainless steels oxidize at soldering temperatures, so no matter how
clean you get them, by the time you heat them and try to add rosin core
solder, it just won't take. The solution is to clean the metal then coat
it with flux. Pick up some solder on a good hot iron and plunk it down
in the middle of the flux. This way you can get the metal up to soldering
temperature and get it tinned before enough oxygen diffuses through the
layer of flux to disrupt the process. If this does not work, try a bit
of silver-bearing solder which contains about 2% or so of silver. Sometimes
it will work when nothing else will.
What the heck is a viscose sponge?
Answer - Viscose comes from cellulose and is used for making
artificial sea sponges. The cleaning section at your local grocery store
likely has sponges that have holes of various sizes, just like a natural
sponge. These are viscose sponges, as opposed to foam rubber or plastic
foam which have holes all the same size. Cut a viscose sponge to fit an
old sardine tin then dampen it with water and you have a great tip cleaner
to wipe your hot soldering iron over.
More soldering questions? - We probably
have the answers. E-mail us and ask.
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