Shortcuts can lead to wire failure
(no pun intended). These can result from, among other
things, the press of time or contortions from working
in confined workspaces.
For example, when removing insulation
from any wire for connection or termination, nicking
the conductor is all too possible, and it has serious
consequences.
There are enough problems with connections
(that's where nearly all open circuits occur). Why cause
a problem because the connection to the connection is
poorly made?
Stripping The Insulation
In its worst case, a wire-stripper
may actually remove a strand or two along with the insulation,
leaving a shortage of conductors at the termination.
What happens is that, for example,
only five of seven strands have to carry all the current.
This creates a bottleneck, overburdening the unimpaired
conductors and making them prone to failure with possibly
intermittent strands — all of which can produce
extra heat, circuit noise, and/or changes in resistance.
With vibration or even a small amount
of stress, a "mere" nick can develop into
a crack which may break and fail, almost always long
after the connection has been made. While stranded wires
will "bottleneck," as described above, a solid
conductor may open completely! NOTE: There is no such
thing as a "mere" nick.
You cannot "fix" a nicked
or broken conductor: cut it off and start fresh. And
when you do, be sure to use tools which will sever only
the insulation, staying clear of the conductor below.
How do you assure this? After all,
insulation materials are generally a lot softer than
copper, and sometimes you just can't tell by feel that
you have "touched bottom."
Stripping Tools
- Thermal strippers are the kindest to the wire and
will soften most insulation materials. Available in
hand-operated or bench types.
- Motorized hand and bench strippers have a spinning
collet, which receives the wire. Adjustable blades
can be set to a uniform insulation depth and will
slice and then remove the "slug" of insulation
without damage to the conductor. Some of these are
very precision tools. And some are very expensive,
but worth it, for production situations.
- Pliers-like mechanical strippers, with one or a
range of slots for different AWG diameters, are inexpensive,
handy, and perform well — provided the correct
slot is chosen, the wire is well centered in the slot,
and the cycle is smoothly performed. Counterbored
die-type blades help greatly in centering the wire.
- Inexpensive stripping pliers may also have one
or more sharpened notches, often V-shaped —
a poorer choice, requiring considerable care —
and some means of limiting their closure. Experience
is vital — and yours may already have steered
you away from this tool.
- Diagonal cutters are always handy but a poor choice,
relying on just the opposing edges (usually dull and
better at holding than cutting insulation) and considerable
skill. Diagonals grab and stretch the insulation to
the breaking point in order to remove it — kind
of an "all thumbs" approach. This process
also leaves the length of the strip rather unpredictable
due to the stretching. This tool is truly designed
for simply cutting wire, but even so, it is inferior
(for that purpose) to cable cutters which scissor-cut
a nice squared-off end instead of mashing the wire.
- Razor blades. Nice cut, but control can be a problem.
Actually, with skill and care a razor blade can prepare
the insulation for removal with diagonal cutters,
or even by hand. A razor blade is best used to "circumcise"
or score the insulation part-way to define its breaking
point. This can result in a rather precision length
of strip and, in fact, may be necessary in the absence
of more sophisticated tooling. It's not uncommon to
use a razor blade to help in the stripping of coaxial
cables.
- Pocket knives are fine for whittling.
So, given a variety of tools, we recommend
not leaving this delicate task to the inexperienced.
Further, as with any “tools of
the trade,” quality is never a poor investment,
and maintenance is a necessity. A dull anything
is actually a comment on the technician's concern for
quality performance.
Professionalism and aircraft system
reliability demand meticulous attention to detail. Choosing
and using the best available tool for the job, double-checking
everything, and performing careful inspection before
completing the termination will help assure the long-term
quality of installation.
About “Gas-Tight”
Gas-tight means sealed against the
possible penetration of air molecules, as well as any
"tag-along" airborne contaminants. Metal-to-metal
gas-tight connections are those where oxides or other
surface contaminants are absent or removed, if necessary,
by mechanical or chemical means. Such methods are detailed
below.
Hermetic sealing is molecular, impenetrable,
and gas-tight, usually employing insulators, such as
glass or ceramic, which are heated in order to flow
around and seal a metallic conductor. Examples include
ceramic-package semiconductors, light bulbs, mercury-wetted
or dry-reed relays, and the feed-thru connectors incorporated
in PIC quad group connector arrays. There are no plastics
which can effect a true hermetic seal.
This is not so say that excellent protection
is impossible without glass-to-metal sealing. It is
the constant goal of designers to defy the pressures
of the environment. Many excellent sealants and techniques
are available to “prevent” [i.e., delay]
leakage of corrosive gases; however, in the strict sense,
they are not hermetic.
Before the impression
is given that there is no practical means of defeating
corrosion, it should be understood that the connection
itself is readily made to be gas-tight. That is, creating
an intermetallic bond is the first step. The second
step is surrounding the exposed metal with enough protection
to keep the environment from causing enough corrosion
to damage the current path.
Gas-Tight Connections
The enemies of electrical continuity
are purely physical. Chemical corrosion is the most
insidious, because it doesn't appear until some time
after the connections are made, tests are performed,
and the installation is pronounced successful.
This is a serious problem, but there
are solutions.
In corrosive atmospheres, considerable
effort is required just to protect the connection against
exposure. This involves seals or enclosures or “goops,”
but underneath it all there must be a gas-tight bond
between the wire and its termination. Only a true hermetic
seal can provide absolute protection of an exposed connection.
Making Sound Connections
It is not difficult to make a gas-tight
connection. Even amateurs do it inadvertently —
yet even professionals can fail unless certain precautions
are taken.
There are many ways to terminate a
wire: soldering; crimping; under the head of a terminal
strip screw; welding … all can be successful in
forming a good, gas-tight connection. While each has
its place, they all require low resistance consistent
with circuit demands. This means the conductors must
be clean at the point of contact — clean enough
to put pure metal in intimate, permanent contact with
pure metal.
To begin with, every conductor deserves
a measure of basic cleanliness. Oils, wax, water, rust,
corrosion, scale, dirt — in short, everything
that can be reasonably removed should be — by
wiping with a solvent or, in some cases, scrubbing or
abrading the surface. After drying, the connection should
be made as soon as possible, before surface corrosion
can take hold.
Some conductors are chemically more
active than others, that is, they will form poorly-conducting
surface oxides which act as a barrier, not always obvious
because they may be, in effect, transparent. In some
cases, however, these oxides are readily broken in the
process of connections made by pressure, or they will
flow into a hot medium such as solder, or evaporate
when welding.
Fluxes
It is common to use oxide-destroying
chemicals — fluxes — before soldering or
welding. Because welding is rare in avionic installation,
we will focus on soldering as the most popular heat-involved
connection process. But keep in mind that safety restrictions
on fueled aircraft forbid soldering without special
precautions.
Acid fluxes. Among the great
fluxes for metal cleaning before soldering is the dreaded
acid flux (several types), which not only dissolves
oxides but etches the metal. However, acid fluxes are
suited only for mechanical (such as jewelry, sealed
containers, copper plumbing, etc.) joinery, never electrical
soldering. Eliminating every trace of flux residue is
impractical, if not impossible, and even a few stray
acid flux molecules will cause corrosion.
Rosin fluxes become chemically
active with heat and dissolve the oxides on tin, silver,
and clean copper reasonably well. They are non-conductive
at room temperature, but it is important to clean residual
flux from the connection because moisture can combine
with it to form a corrosive substance which could affect
the connection over time.
There are many types of rosin fluxes
available, and the solder manufacturers are helpful
in directing you to the best choice for your particular
applications. Suffice to say, however, that high-quality
flux-cored solders incorporate a flux which will perform
well in the great majority of field- or bench-soldering
operations.
Cleanup is another problem —
especially where a solvent can wick into the crevices
of the wire, even up under the insulation, carrying
flux residue with it. Electronic chemical manufacturers
are helpful in selecting appropriate solutions and can
offer advice on maximizing their effectiveness.
Flux-less Soldering
It should be added that no
flux may be needed if the metals to be soldered are
clean, perhaps freshly stripped, and tinned to begin
with. Obviously, this eliminates flux-removal concerns
but such a process calls for careful evaluation and
preparation, not to mention inspection after soldering.
A classic no-flux soldering process is re-flow
soldering, where sufficient clean solder is already
applied to the surfaces to be joined, which are then
placed together. Heating causes the solder to flow and
complete the joint. This process is mandated in some
military and aerospace applications, and it is common
in circuit board manufacturing.
About Fluxes
MIL-F-14256 is the standard for definition
of fluxes used in electronic soldering. Considerations
as to corrosive and/or conductive residues are most
pertinent, and a variety of chemical compositions address
the relative solderability of various metals.
Most prevalent among flux-core solders
is activated rosin (Type RA) — a formulation which
MIL-F-14256 states may cause corrosion under some circumstances.
MIL-F-14256 recommends complete removal of RA flux residue,
and states a preference for less activated formulas
Types R (rosin) or RMA (mildly activated rosin).
Solder manufacturers, however, claim
core formulations, meeting military solder specification
QQ-S-571 Type RA, are non-corrosive and non-conducting.
There is long history of satisfactory performance which
lends itself to confidence in this type of flux.
Is there a message here that all is
well with the activated rosin fluxes?
The recommendation is to use solder
and flux according to system manufacturers' recommendations,
or appropriate military designs if called for.
Cleaning residues is always a good
idea — even for Type R fluxes whose residue, while
considered no problem as to corrosiveness or conductivity,
can affect subsequent bonding with conformal coatings,
if used.
And then, while some fluxes are
water-soluble, Types R, RMA and RA require alcohols
or chorinated solvents — the ozone-depleting chemicals
which are said to affect the atmosphere. But that's
a whole 'nother topic.
Crimping
Crimping comprises the majority of
wire terminations in aircraft where quick, easy, and
reliable contact is called for. Crimping may be the
method of choice if other methods compromise safety
in fueled aircraft.
It is generally understood, however,
that a soldered connection is superior where signal
frequencies above 1,000 MHz are involved. This may be
reason enough to consider special accommodations, even
to the point of removing cables to make the connection
or making terminations before installing cables. One
good reason for using pre-made RF cable assemblies.
The barrel of a crimp-type terminal
fits snugly over the wire and is then deformed, or crushed,
using a tool chosen or adjusted to "dent"
or deform the barrel to the proper depth and length.
Depth of this dent is important to assure that the wire
surface(s) and the inside surface of the barrel are
in maximum, intimate (gas-tight) contact. The length
and location of the crimp must be carefully placed so
that only the area surrounding the wire is deformed,
not other parts of a pin or terminal. Both depth and
length contribute to mechanical strength.
One of the benefits of the crimping
process is the breaking-up of surface oxides by the
sheer force of deformation.
To make a gas-tight crimped connection,
it is important to begin with clean wire and properly-sized
terminal or pin. Obviously, a terminal with too large
an internal diameter will not form correctly around
the wire, leaving excessive space to harbor contaminants,
and could even fall off (insufficient deforming) or
crack (excessive deforming). Too small a terminal invites
strand-cutting or some other form of butchery.
 Every
terminal is designed for a specific-size wire (or range
of sizes) and has a recommended tool, die or tool setting
for correct application. See Table 1. Truly consistent
crimps are performed using only cycling-type tools —
those that will not release the terminal until the crimping
operation is complete.
Even the lowly screw terminal (on a
household light switch, for example) is capable of an
excellent gas-tight connection. Assuming things are
clean, the pressure and scuffing of the screw-head on
bare wire penetrate surface oxides of both and make
a good, low-resistance connection. This, of course,
also applies to barrier-strip connections found in many
electronic and power systems.
Low-Loss RF Terminations
Making a good coaxial cable termination
may be "second nature" to those who do it
every day, but some avionic technicians don't have this
luxury. So here are some tips you may find useful.
Almost all PIC coaxial connectors have
the same "cut spec." Basically, this means
that regardless of the cable size or the connector type,
there is uniformity as to where cuts are to be made.
Keeps things simple.
Not so simple is dealing with tape-wrap
low-loss dielectric (the insulation between the conductor
and the shields). This stuff is soft, delicate, sometimes
"stringy" and hard to remove. But this is
the magic ingredient that yields superior electrical
performance.
Tape-wrap Teflon® has a way of
conforming to the conductor — even to the point
of getting buried in the tiny spaces (“interstices”)
between adjacent strands of a stranded conductor. It
may be hard to completely sever when you make the cut,
and surely you don't want to bear down on the blade
just to get it all, only to create nicks in the conductor.
So you'll pull off the slug —
most of it — and then very carefully pick at the
stringy leftovers. This may not be fun & games,
but an important part of making the conductor ready
for the pin.
The advantages of PIC's weatherproofing
on every connector will be realized only if potential
leaks are eliminated. This is accomplished by trimming
shield braid with care, one connector at a time.
If all this seems laborious, it is
not. And we have thorough instructions provided with
every connector. We also have a video — a "how
to" run-through that shows every detail. Click
here to order a copy of this video.
So the process, while vital to
signal or power continuity, is not at all formidable
as long as the proper methods and tools are used. Skill
and experience head the list and can assure long-term
excellent connections.
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