Secrets of Soldering - 2009/09

THIS ARTICLE WILL cover soft
soldering. That’s done with solders that
melt at relatively low temperatures—lower
than 500 degrees Fahrenheit.
Some confusion about solder
terminology arose when lead-free solders
containing approximately 95% tin and 5%
silver became standard in applications such
as installing soldered-together copper-tube
plumbing. That mixture was federally
mandated, to prevent lead contamination of
drinking water.
Some plumbers mistakenly called the
new material “silver solders”; that usage
caught on in other areas in which such
solder is used. However, true silver solders
use a far higher percentage of silver and
require red heat to flow.
Although soft solder bonds easily onto
the surface of suitable metals it’s applied
to, it’s unlike a coat of glue or paint.
Properly applied solder alloys itself with
its underlying “base metal.” Because of
that, it cannot be completely wiped off,
even when reheated, and will never peel
away, because it integrates itself with the
metal underneath.
True, it seems impossible for
intermetallic alloying to occur between
solder and copper, or solder and iron.
Copper melts at 1,984°; iron and steel melt
at roughly 2,800°. Yet “electronic solder,”
with a melting point of only 361°, fuses
readily with both. How could that be?
Under suitable conditions, molecules
can mesh at unexpectedly low
temperatures. A good example is rock salt
added to ice in an old-fashioned ice cream
freezer. Salt itself melts to a liquid at
1,474°. Yet it quickly fuses into ice
(melting point 32°).
I’m making a point of this alloying
action of solder for a good reason: those
“suitable conditions” I mentioned in the
preceding paragraph. Solder’s alloying
action can occur properly only when it’s
applied to clean, unoxidized metal. That’s
the reason soft soldering won’t work on
stainless steel or aluminum.
Both of those metals owe their
corrosion resistance to the fact that a thin,
transparent oxide film automatically forms
on their surfaces. (Aluminum and stainless
steel can be soldered using special solders
and fluxes, but that’s outside the scope of
this article.)
Cleanliness and absence of oxidation is
the key to successful soft soldering. That
applies not only to the metals to be joined,
but also to the soldering iron’s tip. When
in use, that tip gets hotter than 500° and its
surface oxidizes rapidly. That’s why one
of the most important “tools” used in soft
soldering is a cleaning sponge for the iron
tip.
A cellulose sponge that is well
moistened with plain water makes an ideal
wipe-and-clean instrument for solderingiron
tips. The moisture prevents the hot tip
from scorching or melting the sponge as
you rub the iron tip across it immediately
before soldering. But don’t let your sponge
get dripping wet; that would cool the
iron’s tip too much while you were wiping
it.
Now for the items to be soldered. For
Keeping hot soldering-iron tips
bright and clean in use is vital for
producing strong, reliable joints.
That’s why “soldering kits” and iron
holders, such as those that
RadioShack and American Science &
Surplus sell, often come with a sponge
in a small compartment on the base.
Those work well for occasional
electronic soldering tasks. But for jobs
requiring larger, high-wattage irons—
such as landing gear assembly—I’ve
found a much handier “wet sponge
holder.”
As shown, my holder uses a heavy
glass ashtray for its base. That’s
massive enough to resist being
dislodged by wiping the tip of a big
pistol-grip soldering gun on it.
I cut the sponge within from an
inexpensive cellulose kitchen type
that supermarkets sell. The dry
sponge, fresh from its package, is
rigid enough that I can accurately
shape it with a band saw or a scroll
saw.
I made a circular pattern from
cardboard, roughly 10% smaller than
the ashtray cavity, to allow for the
sponge to expand when wet. I used a
felt-tip pen to trace around that pattern
on the sponge, and then I sawed out
the disk and popped it into its glass
base.
That’s all it takes! MA
—Joe Wagner
the strongest and most reliable junctions,
it’s best to clean the areas you want to join
to oxide-free brightness. Wipe them with a
solvent-wetted paper towel, to remove any
oily film, and then scour with fine
sandpaper to remove all surface oxide.
Immediately apply a small amount of
resin-type flux.
(Flux is a pasty compound that, when
heated by the soldering iron, becomes a
thin liquid that flows over the surfaces of
the joint to be soldered and prevents
oxidation. Never use acidic flux for soft
soldering; it can cause corrosion in the
joint. Worse yet, when heated, its vapors
often induce rust on any steel tools in the
vicinity.)
Use this cleaning-and-fluxing
procedure with piano wire; copper, brass,
and steel sheet metal; copper tubing; and
the like. However, most small-gauge wire
used in electric and electronic components
has been “pretinned”; its copper strands
are coated with bright tin at the factory.
Any electrical wire with this silvery
coating needs no further “tinning.”
For the “tinning” method, two of
molten solder’s unusual characteristics
come into play, one of which is its
extremely low viscosity. As will
penetrating oil, molten solder will make its
way swiftly into the smallest crevices, and
it flows most readily toward the hottest
area—even uphill. That’s the reason
behind the long-established advice to place
the iron tip on one end of the area to be
soldered and then apply the solder itself at
the other end. When everything heats
enough throughout the juncture, the solder
melts and rapidly flows into and through
the assembly, toward the iron tip.
However, for tinning, it’s usually best
to apply solder directly onto the iron as I
press it against the surface to be tinned.
That speeds the heating process
considerably. The melted solder provides a
large heat-transfer area between the hot
iron tip and the cooler “base metal.” Also,
the hot tip can be used like a brush, to
smoothly spread the solder over the areas
where “tinning” is needed.
Firmly clamp items to be soldered
before applying heat. I prefer to use
wooden surfaces for clamping the metal
parts. That reduces heat loss from
conductivity.
And during the “tinning” operation,
firm clamping makes an easy job of wiping
off all but a thin surface coat of solder
from the just-tinned areas. (We don’t want
or need blobs of solder on our work; it
adds no strength and looks amateurish.)
Furthermore, after tinning, when the
actual soldering assembly is being done,
firm clamping is even more essential. If
any movement whatever occurs in a solder
joint before its solder has fully solidified, a
weak and unreliable junction will result.
The parts to be connected must be kept
motionless until the solder cools, or it will
become embrittled.
You can see the difference between a
good solder joint and a bad one. If the
solder has a frosty surface appearance, the
intersection is faulty. Its solder is weak
and brittle, rather than strong and ductile.
This can be easily corrected by reheating
the joint until the solder melts and keeping
everything motionless until the solder
cools.
Following are the types of soldering
you might have to do in aeromodeling.
Electrical/Electronic:
• Rosin-core electronic solder seldom
requires additional fluxing.
• Use only low-power—15- to 30-watt—
irons to solder flexible wires, because
excessive heat causes solder to “wick”
along the wire. That spoils its flexibility
and could even cause the wire to fracture if
vibration and/or repeated flexure occurs.
Therefore, whenever you solder
stranded wire, hold the iron tip against the
junction just until the solder flows. Then
quickly remove the iron and blow on the
solder to cool it.
• When assembling Ni-Cd and NiMH
battery packs, it helps greatly to hold the
individual cells together firmly as you
make their soldered interconnections.
For AA-size cells, I use leftover plastic
case halves from old Heathkit and Ace R/C
battery packs. For cells of other sizes, I
make temporary holders from scrap wood.
(I’ve tried tape, but that doesn’t provide as
reliable of an alignment as I would like.)
plastic insulation and possibly causing a short
circuit, I insert a strip of thin sheet aluminum
under each pair of cell tabs before soldering
them. That also acts as a temporary support,
to keep the tabs from separating while being
soldered.
“Structural” (Landing Gear, Wing Struts,
Etc.):
• A solder gun or a RadioShack Mini Butane
Gas-Powered Iron is needed for this kind of
work. The amount of heat required is far
beyond a small “pencil-type” soldering iron’s
capacity. And instead of electronic solder,
use “refrigeration solder” or one of the leadfree
types containing silver for this kind of
application. Maximum mechanical strength is
vital.
• Strong, dependable soldered joints
involving steel music wire, bicycle spokes,
etc. need each juncture to be assembled in
five stages.
1. Clean and pretin all areas of the wire
that will be soldered. (Electronic-grade
solder is fine for this because of its
strong alloying ability.)
2. Clamp two adjoining sections of wire
together firmly in accurate alignment.
3. “Tack-solder” the wires to each other
with a thin layer of solder.
4. Tightly wrap the joint with copper wire.
5. Neatly fill each wire-wrapped assembly
with more solder.
• For applications such as wire cabane struts
for biplanes, sheet-brass reinforcements
and/or attachment plates might be required. If
so, follow the first three steps and then cut
and preform the required plates.
Firmly clamp the parts together and
solder. Use enough solder to form a visible
fillet at each juncture, but don’t pile on a
blob. As I mentioned, it adds no extra
strength and looks unprofessional.
Fuel System:
• Copper and brass tubing can usually be
soldered with a 30- to 45-watt iron. (I seldom
use brass tubing in my glow fuel systems,
because I’ve found that any alloy containing
zinc will catalyze methanol into acetic acid: a
potent rust inducer.)
• Metal fuel tanks for model engines usually
have too much surface area for a pencil-type
iron to heat adequately and produce a
reliable, leakproof assembly. A solder gun is
needed; however, electronic solder works
nicely in model fuel tanks, and its extra
ductility is a plus.
• The only way to go for disassembly is with
a small butane-fueled “pencil torch.” Use it
like an airbrush, to evenly heat the rim of the
tank end. When the entire edge of the tank is
hot enough, inserting the tip of a small
screwdriver under the tank end’s “lip” will let
you flip the end free without damaging
anything.
(I routinely disassemble nearly all of my
metal fuel tanks before using them, to ensure
that there is no unwanted “crud” inside. [It
happens!] In addition, I might want to
relocate one or more of the fuel tubes inside
the tank—or replace brass tubing with
copper.)
Other Applications:
• Soldered-on washers make excellent
retainers for small pushrods and wheels. I
often use those. Since brass washers in the
sizes I need are almost impossible to find, I
make my own.
Using thin-gauge (.010-.020) brass sheet
from K&S, I drill a series of holes along one
edge to fit the wire I’m using—one hole for
every washer I’ll need and a few spares.
Then I use a hand-operated paper punch (it
looks like a cheap pair of pliers) to make holes
in the finished washers, one by one, from the
perforated brass sheet. Centering the washers
is easy to do visually; concentricity isn’t
needed.
Before soldering each retaining washer
onto its wire, I press a scrap of card stock, torn
from a magazine’s reply card, onto the
pretinned protruding end of the pushrod or
wheel axle. Then I add the washer and solder
it in place.
I tear away the scrap of card stock after
that, since it has done its double duty of
providing clearance space between the washer
and the item it retains and soaking up surplus
flux that might otherwise flow down the axle
and gum up the free-pivoting action.
• “Fairleads” for control-line pushrods and
anchor points for biplane rigging and similar
uses can be made from wire loops or brass
washers. Solder them on edge into deep slots
sawed into the ends of cutoff brass machine
screws. MA
Joe Wagner
[email protected]
Sources:
RadioShack
(800) 843-7422
www.radioshack.com
K&S Engineering
(773) 586-8503
www.ksmetals.com
72 MODEL AVIATION
GRAPHLITE High Performance
Carbon Rods & Strips
67% Carbon Fiber – No Fillers
Optimal Fiber Orientation
Conforms to Curves without Wrinkles
Twice the Stiffness of Aluminum
CST carries over 100 sizes of rods, tubes and strips including DPP and standard.
Visit our website at
www.cstsales.com
Order Desk
CST–The

THIS ARTICLE WILL cover soft
soldering. That’s done with solders that
melt at relatively low temperatures—lower
than 500 degrees Fahrenheit.
Some confusion about solder
terminology arose when lead-free solders
containing approximately 95% tin and 5%
silver became standard in applications such
as installing soldered-together copper-tube
plumbing. That mixture was federally
mandated, to prevent lead contamination of
drinking water.
Some plumbers mistakenly called the
new material “silver solders”; that usage
caught on in other areas in which such
solder is used. However, true silver solders
use a far higher percentage of silver and
require red heat to flow.
Although soft solder bonds easily onto
the surface of suitable metals it’s applied
to, it’s unlike a coat of glue or paint.
Properly applied solder alloys itself with
its underlying “base metal.” Because of
that, it cannot be completely wiped off,
even when reheated, and will never peel
away, because it integrates itself with the
metal underneath.
True, it seems impossible for
intermetallic alloying to occur between
solder and copper, or solder and iron.
Copper melts at 1,984°; iron and steel melt
at roughly 2,800°. Yet “electronic solder,”
with a melting point of only 361°, fuses
readily with both. How could that be?
Under suitable conditions, molecules
can mesh at unexpectedly low
temperatures. A good example is rock salt
added to ice in an old-fashioned ice cream
freezer. Salt itself melts to a liquid at
1,474°. Yet it quickly fuses into ice
(melting point 32°).
I’m making a point of this alloying
action of solder for a good reason: those
“suitable conditions” I mentioned in the
preceding paragraph. Solder’s alloying
action can occur properly only when it’s
applied to clean, unoxidized metal. That’s
the reason soft soldering won’t work on
stainless steel or aluminum.
Both of those metals owe their
corrosion resistance to the fact that a thin,
transparent oxide film automatically forms
on their surfaces. (Aluminum and stainless
steel can be soldered using special solders
and fluxes, but that’s outside the scope of
this article.)
Cleanliness and absence of oxidation is
the key to successful soft soldering. That
applies not only to the metals to be joined,
but also to the soldering iron’s tip. When
in use, that tip gets hotter than 500° and its
surface oxidizes rapidly. That’s why one
of the most important “tools” used in soft
soldering is a cleaning sponge for the iron
tip.
A cellulose sponge that is well
moistened with plain water makes an ideal
wipe-and-clean instrument for solderingiron
tips. The moisture prevents the hot tip
from scorching or melting the sponge as
you rub the iron tip across it immediately
before soldering. But don’t let your sponge
get dripping wet; that would cool the
iron’s tip too much while you were wiping
it.
Now for the items to be soldered. For
Keeping hot soldering-iron tips
bright and clean in use is vital for
producing strong, reliable joints.
That’s why “soldering kits” and iron
holders, such as those that
RadioShack and American Science &
Surplus sell, often come with a sponge
in a small compartment on the base.
Those work well for occasional
electronic soldering tasks. But for jobs
requiring larger, high-wattage irons—
such as landing gear assembly—I’ve
found a much handier “wet sponge
holder.”
As shown, my holder uses a heavy
glass ashtray for its base. That’s
massive enough to resist being
dislodged by wiping the tip of a big
pistol-grip soldering gun on it.
I cut the sponge within from an
inexpensive cellulose kitchen type
that supermarkets sell. The dry
sponge, fresh from its package, is
rigid enough that I can accurately
shape it with a band saw or a scroll
saw.
I made a circular pattern from
cardboard, roughly 10% smaller than
the ashtray cavity, to allow for the
sponge to expand when wet. I used a
felt-tip pen to trace around that pattern
on the sponge, and then I sawed out
the disk and popped it into its glass
base.
That’s all it takes! MA
—Joe Wagner
the strongest and most reliable junctions,
it’s best to clean the areas you want to join
to oxide-free brightness. Wipe them with a
solvent-wetted paper towel, to remove any
oily film, and then scour with fine
sandpaper to remove all surface oxide.
Immediately apply a small amount of
resin-type flux.
(Flux is a pasty compound that, when
heated by the soldering iron, becomes a
thin liquid that flows over the surfaces of
the joint to be soldered and prevents
oxidation. Never use acidic flux for soft
soldering; it can cause corrosion in the
joint. Worse yet, when heated, its vapors
often induce rust on any steel tools in the
vicinity.)
Use this cleaning-and-fluxing
procedure with piano wire; copper, brass,
and steel sheet metal; copper tubing; and
the like. However, most small-gauge wire
used in electric and electronic components
has been “pretinned”; its copper strands
are coated with bright tin at the factory.
Any electrical wire with this silvery
coating needs no further “tinning.”
For the “tinning” method, two of
molten solder’s unusual characteristics
come into play, one of which is its
extremely low viscosity. As will
penetrating oil, molten solder will make its
way swiftly into the smallest crevices, and
it flows most readily toward the hottest
area—even uphill. That’s the reason
behind the long-established advice to place
the iron tip on one end of the area to be
soldered and then apply the solder itself at
the other end. When everything heats
enough throughout the juncture, the solder
melts and rapidly flows into and through
the assembly, toward the iron tip.
However, for tinning, it’s usually best
to apply solder directly onto the iron as I
press it against the surface to be tinned.
That speeds the heating process
considerably. The melted solder provides a
large heat-transfer area between the hot
iron tip and the cooler “base metal.” Also,
the hot tip can be used like a brush, to
smoothly spread the solder over the areas
where “tinning” is needed.
Firmly clamp items to be soldered
before applying heat. I prefer to use
wooden surfaces for clamping the metal
parts. That reduces heat loss from
conductivity.
And during the “tinning” operation,
firm clamping makes an easy job of wiping
off all but a thin surface coat of solder
from the just-tinned areas. (We don’t want
or need blobs of solder on our work; it
adds no strength and looks amateurish.)
Furthermore, after tinning, when the
actual soldering assembly is being done,
firm clamping is even more essential. If
any movement whatever occurs in a solder
joint before its solder has fully solidified, a
weak and unreliable junction will result.
The parts to be connected must be kept
motionless until the solder cools, or it will
become embrittled.
You can see the difference between a
good solder joint and a bad one. If the
solder has a frosty surface appearance, the
intersection is faulty. Its solder is weak
and brittle, rather than strong and ductile.
This can be easily corrected by reheating
the joint until the solder melts and keeping
everything motionless until the solder
cools.
Following are the types of soldering
you might have to do in aeromodeling.
Electrical/Electronic:
• Rosin-core electronic solder seldom
requires additional fluxing.
• Use only low-power—15- to 30-watt—
irons to solder flexible wires, because
excessive heat causes solder to “wick”
along the wire. That spoils its flexibility
and could even cause the wire to fracture if
vibration and/or repeated flexure occurs.
Therefore, whenever you solder
stranded wire, hold the iron tip against the
junction just until the solder flows. Then
quickly remove the iron and blow on the
solder to cool it.
• When assembling Ni-Cd and NiMH
battery packs, it helps greatly to hold the
individual cells together firmly as you
make their soldered interconnections.
For AA-size cells, I use leftover plastic
case halves from old Heathkit and Ace R/C
battery packs. For cells of other sizes, I
make temporary holders from scrap wood.
(I’ve tried tape, but that doesn’t provide as
reliable of an alignment as I would like.)
plastic insulation and possibly causing a short
circuit, I insert a strip of thin sheet aluminum
under each pair of cell tabs before soldering
them. That also acts as a temporary support,
to keep the tabs from separating while being
soldered.
“Structural” (Landing Gear, Wing Struts,
Etc.):
• A solder gun or a RadioShack Mini Butane
Gas-Powered Iron is needed for this kind of
work. The amount of heat required is far
beyond a small “pencil-type” soldering iron’s
capacity. And instead of electronic solder,
use “refrigeration solder” or one of the leadfree
types containing silver for this kind of
application. Maximum mechanical strength is
vital.
• Strong, dependable soldered joints
involving steel music wire, bicycle spokes,
etc. need each juncture to be assembled in
five stages.
1. Clean and pretin all areas of the wire
that will be soldered. (Electronic-grade
solder is fine for this because of its
strong alloying ability.)
2. Clamp two adjoining sections of wire
together firmly in accurate alignment.
3. “Tack-solder” the wires to each other
with a thin layer of solder.
4. Tightly wrap the joint with copper wire.
5. Neatly fill each wire-wrapped assembly
with more solder.
• For applications such as wire cabane struts
for biplanes, sheet-brass reinforcements
and/or attachment plates might be required. If
so, follow the first three steps and then cut
and preform the required plates.
Firmly clamp the parts together and
solder. Use enough solder to form a visible
fillet at each juncture, but don’t pile on a
blob. As I mentioned, it adds no extra
strength and looks unprofessional.
Fuel System:
• Copper and brass tubing can usually be
soldered with a 30- to 45-watt iron. (I seldom
use brass tubing in my glow fuel systems,
because I’ve found that any alloy containing
zinc will catalyze methanol into acetic acid: a
potent rust inducer.)
• Metal fuel tanks for model engines usually
have too much surface area for a pencil-type
iron to heat adequately and produce a
reliable, leakproof assembly. A solder gun is
needed; however, electronic solder works
nicely in model fuel tanks, and its extra
ductility is a plus.
• The only way to go for disassembly is with
a small butane-fueled “pencil torch.” Use it
like an airbrush, to evenly heat the rim of the
tank end. When the entire edge of the tank is
hot enough, inserting the tip of a small
screwdriver under the tank end’s “lip” will let
you flip the end free without damaging
anything.
(I routinely disassemble nearly all of my
metal fuel tanks before using them, to ensure
that there is no unwanted “crud” inside. [It
happens!] In addition, I might want to
relocate one or more of the fuel tubes inside
the tank—or replace brass tubing with
copper.)
Other Applications:
• Soldered-on washers make excellent
retainers for small pushrods and wheels. I
often use those. Since brass washers in the
sizes I need are almost impossible to find, I
make my own.
Using thin-gauge (.010-.020) brass sheet
from K&S, I drill a series of holes along one
edge to fit the wire I’m using—one hole for
every washer I’ll need and a few spares.
Then I use a hand-operated paper punch (it
looks like a cheap pair of pliers) to make holes
in the finished washers, one by one, from the
perforated brass sheet. Centering the washers
is easy to do visually; concentricity isn’t
needed.
Before soldering each retaining washer
onto its wire, I press a scrap of card stock, torn
from a magazine’s reply card, onto the
pretinned protruding end of the pushrod or
wheel axle. Then I add the washer and solder
it in place.
I tear away the scrap of card stock after
that, since it has done its double duty of
providing clearance space between the washer
and the item it retains and soaking up surplus
flux that might otherwise flow down the axle
and gum up the free-pivoting action.
• “Fairleads” for control-line pushrods and
anchor points for biplane rigging and similar
uses can be made from wire loops or brass
washers. Solder them on edge into deep slots
sawed into the ends of cutoff brass machine
screws. MA
Joe Wagner
[email protected]
Sources:
RadioShack
(800) 843-7422
www.radioshack.com
K&S Engineering
(773) 586-8503
www.ksmetals.com
72 MODEL AVIATION
GRAPHLITE High Performance
Carbon Rods & Strips
67% Carbon Fiber – No Fillers
Optimal Fiber Orientation
Conforms to Curves without Wrinkles
Twice the Stiffness of Aluminum
CST carries over 100 sizes of rods, tubes and strips including DPP and standard.
Visit our website at
www.cstsales.com
Order Desk
CST–The