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               TIPS FOR RELIABLE WIREWRAPPING


                        Gary Newman

                     Ampex Corporation
               Redwood City, California 94063



					   ABSTRACT

          The unwary technician  frequently  constructs
     his  first  wirewrapped  circuit with little or no
     instruction, while the  wary  will  first  seek  a
     source  of  information.  The unwary engineer fre-
     quently changes wires on wirewrapped circuits with
     little  or  no  instruction,  while  the wary will
     first seek a source of  information.   The  unfor-
     tunate facts show that in both cases the latter is
     a rare breed.

          By following a few rules,  and  understanding
     why  a  wrapped  wire  makes  a good connection, a
     reliable  circuit  can  be  produced  with  little
     effort.   This paper should serve as a road map to
     both guide the beginner, and improve the style  of
     the more experienced wirewrapper.



June 13, 1980

============================================================


               TIPS FOR RELIABLE WIREWRAPPING


                        Gary Newman

                     Ampex Corporation
               Redwood City, California 94063



1.  Introduction

     When used properly, wire wrapping has been shown to  be
the most reliable connection method|- currently available. It
makes  connections  quickly  and  efficiently,  that  may be
easily changed with a few tools.

     However, the wire-wrapper must be well enough  informed
to  choose the correct wire, tool, and pins to make the con-
nections with, and know how to evaluate the quality  of  the
completed  connection.   These  topics  will  be  dealt with
first, in the section on "The Wrap".

     Once the wire-wrapper is capable of  making  a  correct
connection,  there  are other tips that help make for better
circuits.  The factors that affect the  performance  of  the
connection in the electrical circuit, and the ease of chang-
ing the circuit will be dealt with  in  the  remaining  sec-
tions.

2.  The Wrap

     The connection is made by wrapping a solid wire tightly
around  the corners of a connecting post while applying ten-
sion to the wire.  This process both cuts  a  notch  in  the
wire  and deforms the post at each corner.  At each corner a
cold weld is thus created, causing cross  migration  of  the
metal atoms of the post and the wire|=.  While the weld  does
not  contribute  to  the mechanical integrity of the connec-
tion, it does contribute to its electrical  stability.   The
contact  area  is  only  reliable at the corners, and so the
electrical properties of the connection  are  determined  by
the number of corners of the wrap.


|- Belter, Robert E., "Make ECL wire-wrapped panels  re-
liable with proper routing and tough insulation", Elec-
tronic Design, June 7, 1979, p. 106
|= Davies, D.G., "Micro-Mechanisms of Solderless Wrapped
Joint Formation", The Plessey Company Limited










                           - 2 -


     The tension applied to the wire causes  it  to  stretch
elastically as it is wrapped. This leaves the completed con-
nection with elastic energy stored in the  lengths  of  wire
between corners.  Thus the mechanical properties of the con-
nection are determined by the type of wire  and  under  what
tension  it is wrapped. The wrap may then be considered as a
collection of cold welds (at the corners), all held together
by   individually   stretched   springs  (the  wire  between
corners).

2.1.  The Post

     The connecting post length  determines  the  number  of
wrapped  connections  that  may  be  made  to  it, while its
cross-section determines both the mechanical and  electrical
integrity  of  the wrap.  The cross-section should be either
rectangular or a parallelogram, with radii  at  the  corners
not exceeding 0.003 in.  The corners, being the basis of the
connection, must then be square or `sharp' to achieve a good
connection.   The  effective sharpness of the corners should
be related to the diameter of the wire, and so one should be
wary  of  wiring  too  small  a  post with a particular wire
gauge.  For 30 gauge wire,  the  post  diagonal  should  not
exceed 0.036 inches.

     The posts are usually coated with a metal such  as  tin
or gold.  This coating plays a key role, in forming the con-
nection at the corners, by extruding away from the cold weld
area  into  the  surrounding  connection.   The coating thus
increases the effective contact area of the corner.

2.2.  The Wire

     The wire provides the `spring' in the  connection,  and
so should have good elastic properties.  It should also have
good electrical properties, and bond well with the post  for
a solid cold weld.

     The wire must be solid, and may be metal coated for the
same  reasons that apply to the posts.  Don't wrap with just
any wire that fits into the bit!  Typically, annealed copper
wire is used, but copper alloy wires may also be used, need-
ing fewer wraps for acceptable connection quality.

     The insulation should be tough enough  to  prevent  its
being  cut by the sharp corners of the posts.  Unfortunately
the tougher the insulation, the greater the tendency for the
wire  to `curl' (Curl is a wire's memory of its former shape
on the spool).  This  makes  us  prefer  obtaining  wire  on
large-core spools, and choosing Kynar insulation, instead of
than laminated mylar tape Teflon insulation.  The Kynar is a
compromise  between  the toughness of mylar (with its exces-
sive curl) and the softness of Teflon (with its low  capaci-
tance and excellent thermal properties).









                           - 3 -


     The wire must be  correctly  stripped  to  provide  for
seven  wraps on the post (for 30 AWG).  A slight nick in the
wire at the stripping point would cause  the  connection  to
fail  either  during  the strain of wrapping, or from future
vibration, so a non-nicking wire stripper is a necessity.  A
separate  stripping  tool  must  be  obtained  for each wire
gauge, as each one is precision machined for a specific wire
diameter.  Use  of the wrong tool will cause unreliable con-
nections.

2.3.  The Bit

     The bit rotates in a sleeve and wraps the wire onto the
post.   It  is  mated with the wire-wrapping tool that turns
the bit at between 3000 and  5000  rpm.   The  correct  bit-
sleeve  combination  must  be chosen for the job to be done.
This is frequently where  wire-wrapping  fails.   The  wire-
wrapper  uses  the  wrong  bit for the posts and wires to be
wrapped.

     Read the instructions for choosing  the  bit  for  your
post  size, and wire gauge.  A bit that provides a `modified
wrap' should be used. It wraps one extra wrap of  the  insu-
lated  part  of the wire around the post to keep the corners
of the connection from unwelding.  For specifying this  type
of bit, the insulation thickness must also be known.

     The bit has a lifetime  limited  to  somewhere  between
50,000 and 250,000 wraps, depending on use.  There may be no
visible signs of the wear, so a  strip-off  test  should  be
used periodically|-.  The force necessary to slide (with  the
correct  tool)  the  wrapped  connection off the post should
exceed 3 pounds for 30 gauge wire.

     Because the drive train  in  a  wire-wrap  gun  expands
slightly  under power, the collet nut (that holds the bit in
place) should be tightened by hand with the  motor  running.
This will give longer tool life, and maximum wrapping speed.

2.4.  The Connection

     After sliding a piece of stripped wire  into  the  off-
center hole in the bit, the center hole of the bit is placed
over the post.  Applying the weight of the tool only, to the
connection,  the wrap should be applied, letting the tool do
the work.  Bad wraps are frequently made by pushing, pulling
or twisting the tool.




|- Various tests for measuring quality of wirewraps  are
described in MIL-STD-1130 and EIA-STD-RS280.










                           - 4 -


3.  Making A Net

     A net|= should  be  wired  by  connecting  one  post  to
another  with  as little slack as possible in the connecting
wires.  A common mistake in connecting a net is to create  a
daisy chain.

     If a net contains posts 1 through 4, then a daisy chain
is  made  by  connecting post 1 to post 2, post 2 to post 3,
then post 3 to post 4.  If a future change requires  post  1
to  be  removed  from  the net, all wires in the daisy chain
must be removed (see section 5).  However,  by  using  level
wrapping, this can be avoided.

     The term level wrapping means that all  wires  will  be
level  with the surface of the card, or both wraps of a wire
will be applied to the same level of the posts.  So  in  the
above  daisy  chain, the wire from post 1 to post 2 is level
because both wraps are on the bottom of the post.  The  wire
from  post 2 to post 3 is not level because the wrap on post
3 is on the bottom level, while the wrap on post 2 is on the
second  level.   A  level wrap for the above net would be to
connect posts 1 to 2, and posts 3 to 4 on the bottom  level,
and  then  posts  2  to  3  on the second level.  The wiring
change here would only require removal of two wires.

     The desirable feature of level  wrapping  is  that  any
wiring change requires removal of at most three wires.

4.  Unwrapping

     When removal of connections is necessary,  they  should
be unwrapped with an unwrapping tool.  The tools for unwrap-
ping have a hole in their center for the post, and a  spiral
track  around  the  outside  for  the wire to slide in as it
unwraps.  At the tip of the spiral, there is a  sharp  blade
that  lifts  the tail of the wire off the wrap.  Once again,
let the tool do the work, do not bear down with the  unwrap-
ping tool as the blade will be damaged.

5.  Changing A Wire

     Making wiring changes is simply a matter of  completely
removing the old wire, and replacing it with the rewire.  If
any other wraps are affected, the involved  wire  should  be
completely  replaced.   The  end of a wire from an unwrapped
connection should never be straightened out  and  rewrapped.
It  may  seem easier at the time to cut a wire, or rewrap an
already used piece of wire, but the  difficulty  of  finding
the  errors induced surely outweighs the short term benefits

|= The electrical node created by wiring posts  together
is called a network or `net'.










                           - 5 -


of these practices.  The practice of cutting a wire close to
the  post  can lead to almost invisible and sometimes inter-
mittent problems.

     The wrapping process causes irreversible damage to  the
corners  of the posts, and every rewrapping on a post furth-
ers this damage.  The only sure test of damage to a post  is
the  strip-off test.  Replacement of posts is a good feature
to look for in choosing the wirewrap card.

6.  The Electrical Characteristics

     The electrical characteristics of wire-wrapped  connec-
tions  can be controlled to some extent by the layout of the
wires, but more control still can be obtained by the  proper
choice  of  the  wire,  and card.  The layout for high-speed
logic (Schottky-TTL and ECL) is  much  more  stringent  than
that  for  low-speed  logic,  so  they  will  be  dealt with
separately.

6.1.  Low Speed Logic

     Low speed logic can be troublesome where a  many  wires
all  change  state  at the same time.  The effect is serious
where the wires are grouped closely together in a bundle, as
the radiated fields can be large and induce spurious signals
in neighboring wires.  To alleviate this, the layout  should
avoid  parallel  runs  of  wires,  and  bundles of any kind.
Wires are sometimes randomized during the making of a wirel-
ist to avoid parallel runs.

     The wire-wrap card may have enough ground plane to sup-
port the bandwidth of a single low-speed logic signal chang-
ing in the local area but not  enough  to  support  many  of
these  signals  changing  at the same time.  This problem is
frequently seen when a large synchronous counter is changing
from  all ones to all zeros.  The solution here is to spread
out the ground plane loading over a larger area  by  distri-
buting the counter chips around the card.

     Use of more ground plane than you think you  need  will
save you from many of the noise related debug headaches with
low speed logic.

6.2.  High Speed Logic

     For Schottky-TTL, or  ECL  logic,  special  precautions
should  be  taken  in  both  the selection of the cards, the
wires, and the layout.  The  wire  insulation  used  may  be
Kynar,  but  Miline  and  Teflon  insulation  may be used to
reduce the distributed capacitance by half.

     Give serious consideration to cards that are labeled as
Schottky  or  ECL  wire-wrap cards as some of these have met









                           - 6 -


with great  success  even  with  ECL-100,000  series  logic.
These  cards  may  have dedicated power and ground pins that
will give greatly reduced supply and ground impedance at the
cost  of  somewhat  restricted layout.  When choosing cards,
remember that wirewrapping to your bypass capacitors is  not
effective.   Be  wary  of the manufacturers claims.  Get the
name of satisfied customers who are building  similar  tech-
nology circuits and talk to them!

     The layout for high-speed logic should group functional
blocks  of  logic  together  to  minimize wire lengths.  The
wires should be randomized by either straight point-to-point
wiring, or using a programmed routing of wires.

     To obtain uniform impedance  along  the  wires,  use  Z
level wiring|- and specify two level wire-wrap  pins  on  the
cards.   The  reduction  in  post  length  will  reduce  the
impedance discontinuity caused  by  transmission  line  stub
effects.

     For the typical wirewrap card, wire  length  should  be
limited to 6 inches for an unterminated signal|=.  This  will
keep signal reflections to less than 12% undershoot, that is
safely within the range of the input  clamp  diodes  of  TTL
logic.

7.  Debugging wire-wrap

     Finding problems with wire-wrapped cards can be agoniz-
ing.   By  following good construction practices, these will
be reduced, but the following list should  help  in  finding
the more elusive problems.

1.   Two posts are touching together. Do not touch the  pins
     in handling.

2.   Small curls of wire debris are caught in the rats  nest
     of  wire,  causing a short.  When removing wires, shav-
     ings or curls of wire might break off  the  wire  being
     removed.  With the board standing on end, rap the board
     edge on a solid surface to shake out loose wire  (don't
     get  carried  away here).  Do not lay a board with pins
     down on an unclean surface, the board will act  like  a
     magnet and collect nuts, bolts, and loose wires!

3.   A nick in the insulation has shorted.  Be careful  with
     soldering irons on wire-wrapped cards.

|- Z level wiring specifies that a wire must  have  both
ends wrapped at the same level on the posts.
|=  Malek  and  Schwartz, "IC Packaging Panels for High-
speed Logic  Applications",  Electronic  Packaging  and
Production, April, 1976










                           - 7 -


4.   The insulation has separated on a wire and shorted.  Do
     not  overstrip and push back the extra insulation after
     it has been cut.  Use pre-stripped wire when possible.

5    There is a pigtail shorting to an  adjacent  post.   Do
     not remove the wrapping tool prematurely, let it finish
     the wrap.

6.   A wire has been stripped back too far and  is  shorting
     to  an  adjacent post.  Insert the wire completely into
     the bit. Strip correctly and choose long enough  pieces
     of prestripped wire.

7.   The printed circuit power or ground plane  is  shorting
     to  a  pin.   Check  the  card before wiring for solder
     flecks and hair line shorts  in  the  printed  circuit.
     Broken  traces and missing solder should also be looked
     for at that time.

8.   A wire has fatigued and is making intermittent  contact
     to  the post.  Use the right bit and wire-post combina-
     tions.

9.   The sharp corner of a post has cut through the  insula-
     tion  of  a  wire.   Leave  a little slack where a wire
     makes a bend around a post.   Use  wire  with  a  tough
     insulation.





See also:


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