The 'Wiring Pen' - Using the pen
Showing a connection being added to the underside of the circuit board.
Using the wiring pen
Mount a reel of enamel wire between the lugs at
the top of the pen and check that it is held with sufficient firmness to
generate tension in the wire as it is unwound but not so much that there is
a danger of damage to the insulation. If tension is incorrect, the lugs
may be adjusted by GENTLY warming with a hot air blower and forming them
inwards or outwards. Alternatively, generate the tension by wrapping an
elastic band around the lugs on the pen. Thread the wire down the centre
of the pen, leaving about 2mm protruding from the tip.
Minimise the risk of short circuits by taking the correct approach at
the pins .
Insert the end of the wire down between the leg of the IC holder and the
side of the hole. Holding the pen vertically, wrap the wire around the leg
two or three times to anchor it in place. A little thought should be given
whether to wrap clock-wise or anti-clockwise. This will affect
the proximity of the wire to adjacent pins as it crosses over to the
channel and the likelihood of short circuits. Any shorts that do occur are
likely to be at this point. Do not push more wire down the hole than
neccessary; the bare end may contact an adjacent pin.
Avoid damage to the enamel insulation by feeding the wire at the corect
The route taken to reach the destination pin is largely unimportant but
avoid concentrations in one channel or crossing. Make no more right angle
turns than neccessary; it is these that become the limiting factor in the
number of wires that can be accomodated. In general, select routes which
avoid the centre of the board since this area is liable to become congested
first. When the destination pin is reached secure the wire with two or
three turns around it.
When a signal line has multiple destinations place a marker (such as a
matchstick) next to each IC which is to be connected. It should then be
easy to glance at the way the markers are distributed in order to plan the
best route to take to join all points. As each is reached remove the
marker. This helps to ensure that no pins are missed.
If it is suspected that a wire has been wrongly routed, the only reliable
method of confirming its destination may be to remove it altogether. This
is the chief drawback of the EWP technique. Use a scalpel to reach down
between the channel and the IC holder leg to cut the wire. With a pair of
tweezers push the wire back into the channel until it becomes possible to
separate it from its neighbours. It may be helpful to use a pin to tease
out the end.
Safety During Soldering
When satisfied that all connections are correct begin soldering. The idea
is to use an iron hot enough to melt off the synthetic enamel insulation.
When this happens the fumes emitted contain a small quantity of toluene
di-isocyanate gas which is toxic and irritant. Use adequate
ventilation. The joint will have been made correctly after the insulation
is seen to 'bubble' for a second or two. If this does not happen then the
iron is not at the right temperature. Provided the soldering temperature
is adequate, 'dry' joints are very rare.
EWP Guide index]
1996 April 6th.