The 'Wiring Pen' - Using the pen

Illustration of underside of 
bd. .gif
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.

Avoiding S/Cs .gif
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.

Right way to feed out wire .gif
Avoid damage to the enamel insulation by feeding the wire at the corect angle.

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.

Pre-solder Checks

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.

[Up sign EWP Guide index]


E-mail:R.Clarke@surrey.ac.uk
1996 April 6th.