Flying for Fun 2003/06

58 MODEL AVIATION
D.B. Mathews
F l y i n g f o r F u n
909 N. Maize Rd., Townhouse 734, Wichita KS 67212
IN THE MIDST of telephone
conversations with Larry Sribnic of SR
Batteries and Fred Marks of FMA Direct,
something I’d never really considered
came up: they find it difficult to sell
battery packs or servos that require
soldering on connectors. The simple truth
is that many contemporary modelers don’t
know how to solder!
I have no evidence to support the
theory, but I can’t help but wonder if the
root cause for this lack of an important and
entertaining skill doesn’t tie in with the
inadequate devices sold in most stores as
“soldering irons.” I’ve been joining pieces
of metal with solder for nearly 60 years,
and I can’t get satisfactory soldering
results from those pistol-grip things. How
Required heat sources for soldering are two sizes of irons and a propane torch.
Cleaners needed to create solid solder joints: files, sandpaper, and a knife.
can a novice possibly use one?
Well, that is not exactly fair; if one
needed to repair a hole in a roof gutter or a
water bucket, a pistol unit might be okay.
But for anything requiring precision
positioning and proper heating, those things
make great doorstops.
The advantages of being capable of
adding, changing, and repairing connectors
on radio components are obvious. Many
battery packs are supplied without
connectors, servo connectors often break
wires, or those supplied do not match the
other equipment. Unfortunately we have
never reached that dream of crosscompatible
(or universal) connectors.
Not so many years ago, radio equipment
was only available with the components and
connectors sold separately. You were
expected to wire each servo to a connector
that plugged into another one to be installed
coming out of the receiver. Additionally,
switches were usually unwired, as were the
battery packs. We learned to solder, or else.
Even earlier than that, one bought loose
components and soldered them to a board.
Even today, the art of soldering can save
dollars and add to the reliability of the
components. It’s rather sad to see the
number of servos, battery packs, and other
devices discarded or removed from use
because the owners have no soldering skills.
That lack of skills is unfortunate since
soldering is not a particularly difficult
technique. Once the principles are
understood and the proper materials are
assembled, soldering can be a fun project. In
my opinion, soldering is less difficult than
applying iron-on covering or installing a
control system.
The Heat: For small connections that are to
be made with resin-core solder (this should
be the only type used for electrical
connections), a pencil-type iron is designed
for the job. It is not so huge as to be
impossible to get the tip into the proper
position, nor does its heated point overlap
onto areas one does not want to heat. Their
size and balance is such that they can be
handled like a pencil.
Several manufacturers (such as Weller)
make these 30-watt irons, and they are
usually easy to obtain at RadioShack-type
stores. K&S and X-Acto have 30-watt
pencil-type irons sold through hobby shops.
Although it is possible to create enough
heat with a pencil-type iron to flow solder
onto music-wire joints of roughly 1⁄32 inch in
diameter, anything larger is a real chore.
These irons are best suited to soldering
June 2003 59
connectors and making joints in stranded
Radio Control wire.
On the other end of the heat scale are the
larger units that resemble the pencil-type
configuration but draw approximately 120
watts. These have wider, chisel-shaped tips
and produce enough heat to flow acid-core
solder along the joints of much heavier
material than the 30-watt units. These irons
obviously have much broader footprints and
are capable of heating a wide area of a
music-wire joint to flow the solder into the
soft-wire wrapping and onto the wire itself.
The heat on the tip is no higher than that
produced with the pistol-grip, instant-heat
units, but it is spread over a much wider area
and the heat is more constant; therefore,
they are vastly superior.
One disadvantage, albeit minor, is the
large-tip pencil unit’s much slower heating.
However, this is not a problem if you plan
for it; plug in the 120-watt unit before you
need it. These things do get darn warm and
should be rested on the supplied stand or
some other type of heat sink between steps.
An old glass ashtray or the side of a hammer
will work well.
These 120-watt irons are ideal for
soldering wire-landing-gear segments
together, soldering links onto pushrods, and
soldering washers onto the ends of music
wire for wheel retainers, etc.
Types of Solder: The kind of solder you
use is as important as the iron itself. Resincore
solder is mandatory for soldering
electronic equipment. Never use acid-core
solder on electronic devices! The acid will
corrode and destroy the wiring and the
components. Acid-core solder can be
easily identified by its hollow center core
filled with a jellylike substance.
The stranded wire and the connector terminals have been tinned
and are ready to join. Notice the heat shrink.
This is a three-wire splice: a technique one would use if
changing connector brands that do not have solder lugs.
A general arrangement shows the hammer used as a heat sink, the clothespin, the file
used as a fixture, and the resin-core solder.
On the other hand, it is difficult to
successfully solder sections of metal or
heavier wire using resin-core solder. This
material is usually much thinner and
threadlike and has a solid center. It has a
softer metal blend, melts at lower
temperatures, and is the preferred solder for
electronic connections.
A third type of heat source is the propane
torch. These are sold under many names, but
each uses a disposable tank and a knobregulated
fuel flow. They are for soldering
jobs that require more strength than regular
tin-lead solder provides and more heat than
a 150-watt iron can produce. A small
propane torch generates a large amount of
heat at a temperature high enough to melt
Sta-Brite and silver solder.
Don’t fail to protect surrounding surfaces
with wet towels or some other sort of heat
shield when using an open-flame torch. It is
much too easy to ignite something
important.
Technique: What one is attempting to do is
heat a clean metal object to a temperature
sufficient to melt the solder and flow it
along the exposed metal surfaces, then mate
a second piece to the first by retinning the
solder on the halves.
Key to those descriptions is the word
“clean”; all metals interact with their
surrounding environment to create a thin
layer of contaminated metal on the surface.
This layer is usually the result of
atmospheric gases acting on the metal to
form oxidation. However, dissimilar metals,
moisture, and even the natural oils on our
fingertips can contribute to corrosion. These
corroded elements must be removed from
the metal if there is to be a good flow of
solder.
Several methods can be used to remove
these oxides and corrosion just before
applying heat, ranging from sanding or
scraping the metal to using a chemical
cleaner called Flux. Essentially, the cleaner
the metal, the better the solder will flow, and
60 MODEL AVIATION
the stronger the joint will be. For that
reason, using excessive Flux will leave a
residue that weakens the joint.
Sanding and/or filing the metal are not
only excellent ways to remove
contamination, but the resulting scratches
greatly increase the total surface area
covered in solder.
In the order of things, the most
important step is cleaning the metal,
followed by applying enough heat to melt
the solder. Properly done for maximum
strength, the metal should be heated until
the solder will melt and flow onto its
surface.
A common error is to melt the solder
against the tip of the iron and attempt to
drip/flow it onto the part. Doing this looks
logical and is quicker, but one ends up with
hot solder hitting cool metal; it may stick,
but it is not a strong joint at all.
Heating the part and flowing solder onto
it takes more time, but it is much more
satisfactory. This technique is called
“tinning,” in that the two parts are
precovered with molten solder, then they
are placed together and heat is used to melt
the solder on the halves to flow it together
to produce a joint.
The resultant joint should be cooled,
then the excess Flux, acid, or whatever
cleaned off using denatured alcohol or
lacquer thinner. If this is an electrical joint,
the bare wire should be protected with heat
shrink or other insulation.
Wire Wrap: A few words apply to the
technique used for joining large music-wire
sections to form landing gear, etc. These
joints should use pretinned sections of wire
(doesn’t require great globs, just a coating)
joined by wrapping them with thin copper
wire. The heavy wire is then heated with a
torch, allowing the solder to flow through
the thin wire and into the thick joints. This
thin wire is available in rolls at hobby shops
or in thicker sizes in the craft departments
of some stores.
Something common to all of these thin
copper wires, and significant, is a coating of
shellac, varnish, etc. applied to their
exteriors to make them look nice and shiny
on the roll. This coating must be removed;
pull sections of the copper wire through
folded sandpaper until all of the coating is
removed.
As an alternative, Sta-Brite is supplied
with a squeeze bottle that can be used to
apply a drop or two of muratic acid to the
parts. (It should be thoroughly washed with
water after soldering.)
What is so insidious about this wire
wrapping is that the heat of the iron or torch
will burn enough of the coating off to allow
a flow of solder between some areas of the
wire while not adhering to the rest. A
wrapped and soldered joint can look fine
but only be partially soldered.
The Torch: After adjusting a torch to give
a steady point without fluctuations, the
flame will have three different-colored
parts. The clear inside portion of the flame
is the hottest, the blue portion is the next
hottest, and the red outer part is
considerably cooler and full of oxides.
If one can hold the center of the flame
over the object, fine, but practicality usually
involves the blue portion. Studiously avoid
attempting to heat with the red part of the
flame.
Fixtures: When joining parts by soldering
them, it is of great help if a fixture is made
to hold the parts close together during the
procedure. This is mostly because soldering
is already a two-handed operation, making
holding parts anatomically difficult.
I’ve included a photo of a simple unit
fabricated from clothespins. It could just as
easily be alligator clips on a length of solder
or a chunk of Styrofoam. When assembling
landing gear, it is often handy to
temporarily install the wire parts to the
fuselage or wing, then wrap and solder. Be
careful to avoid igniting the model.
I’d suspect that many who have read this
can solder far better than I can and found
the text boring. However, don’t forget the
modelers to whom this is all a black art.
And as I pointed out, there must be a large
group who needs some help with the simple
techniques of soldering. MA