by Vaughn D. Martin
Rugged and Reliable
It may be hard to believe, but proper wire wrap
construction is more reliable than printed circuits.
Connections are less prone to fail due to vibration or
physical stresses, as well as cold solder joints. The interior
of a wire wrap connection is also air tight, and oxidation
and contamination free — unlike solder that can corrode
over time. Wire wrap connections are firmer and have
lower electrical resistance due to actual cold welding of
the wire to the terminal post at the corners.
Note: In cold welding, you apply pressure to the
work pieces. Plastic deformation occurs. At least one
(but preferably both) of the mating parts must be ductile
(in our case, it’s the wrap post).
Initial pressure at the center of the contact area can
go as high as 100,000 psi. After wrapping, cold flow or
cold welding causes pressure to drop to approximately
30,000 psi, then the metal stabilizes with constant
pressure, completing the plastic deformation phenomenon
Wire wrapping is a technique for
constructing single or small numbers
of simple to moderately complex
electronic assemblies. It is a viable
substitute to limited production
printed circuit board (PCB) runs.
Its most appealing characteristic is
you can quickly and easily change
your prototyping work. Telephone
exchanges, control consoles, radar,
and sonar on submarines all have
successfully used this technique.
Historically, the Apollo guidance
computer was among many early
wire wrap constructed computers.
Before beginning this primer, you are
encouraged to examine the sidebar
glossary of wire wrapping terms.
cycle just mentioned (see Figures 1 and 2).
Wire Wrap Posts
FIGURE 1. By bending the wire around the sharp corner of
the terminal, the oxide layer on both wire and terminal is
crushed or sheared and a clean, oxide free, metal-to-metal
These moderately ductile posts are 0.025 inches
(635 micrometers) square, one inch ( 25. 4 mm) high, and
spaced at 0.1 inch (2.54 mm) intervals. Premium wrap
posts are a hard-drawn beryllium-copper alloy plated with
0.000025 inches ( 25 microinches, 635 nanometers) of
gold to prevent corrosion. Less-expensive hobbyist grade
posts use tin plated bronze.
Wrap Post Materials
Six commonly used
materials for wrap posts are: (1)
the premium beryllium copper;
(2) hobbyist grade phosphor
or tin plated bronze; ( 3) half
hard brass; ( 4) copper nickel;
FIGURE 2. A cross section
through the terminal edge
shows stress distribution
produced in the wire
wrapped with high tension
around a terminal.