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From: neal@weitek.UUCP (Neal Bedard)
Newsgroups: net.arch
Subject: Re: Re: How to fab One-Of-A-Kind systems?
Message-ID: <188@weitek.UUCP>
Date: Thu, 23-May-85 18:23:34 EDT
Article-I.D.: weitek.188
Posted: Thu May 23 18:23:34 1985
Date-Received: Sun, 26-May-85 00:37:42 EDT
References: <202@ganehd.UUCP> <183@geowhiz.UUCP> <1143@sjuvax.UUCP> <259@gcc-bill.ARPA>
Organization: Weitek Corp. Sunnyvale Ca.
Lines: 67

I assume we're talking TTL digital here. I have had experience with both 
Wire-Wrap (tm) and Scotchflex (tm) systems, and I will summarize my
opinions of both here.

Wire-wrap:

	Extremely tedious unless you have the right tools, most
	important of which are:

		1. a *good* wire-wrap gun - that works with the gauge wire
		   you're using. Though expensive (~ $160) my favorite is the
		   NiCd-powered Gardener-Denver unit. 

		2. wire strippers that don't nick the wire.

		3. an unwrapping tool.

	Nets should be split into two levels, alternating between them serially
	so that one does not have to undo more than two wraps to make a fix.
	Wiring should be done serially, as to avoid stubs. Wires should be
	routed so that no tension are placed on them to prevent the pins from
	cutting the insulation.

Scotchflex:

	This is a system marketed by 3M that uses insulation displacement
	to make a gas-tight connection. You insert strips on the wiring side,
	and mating "sockets" on the other. Connecting wires are then punched
	down into the strip's "tangs" on the wiring side. Daisy-chaining bus
	wires is trivially easy, compared to Wire-Wrap. The tangs protrude
	about 1/8" on the wiring side, as opposed to 3/4" for Wire-Wrap pins.
	This allows Scotchflexed boards to be used in normal cardcages without
	sacrificing slots to allow for Wire-Wrap pin spacing.

	While quick and easy, this system is unreliable unless one takes care
	to route the wires at a right angle to the tangs. If this is not
	done, the wires break quite easily. I usually tie bundles of wires
	together to give them additional support.

	The plug sockets aren't the greatest, as they are tin-plated and
	tend to oxidize - no match for the machined contact sockets used in
	Wire-Wrap (3M, are you listening?)

Probably more important is the choice of board. One should choose a board with
the lowest gnd and Vcc impedance possible, as current dumping probably causes
more proto boards to fail than any other cause. Augat panels (with pins already
on them) are good, since they have internal power and ground planes, but are
*very* expensive. I use bare boards from a local company called Twin Industries
that feature gnd and Vcc interspersed between all holes, as well as decoupling
at every IC location. Other vendors also make good bare boards.

Wire-Wrap connections (and, I assume, Scotchflex) ring more than multilayer
boards. The ringing problem shouldn't be an issue if one is careful with stubs
and current dumping.

Someone mentioned Multi-Wire (tm). Multi-Wire isn't quite appropriate for 
one-off systems due to it's cost and not so great turnaround (about 4 weeks
is my experience.) It is tough to  rework, also. Multi-Wire's key advantage is
density over multilayer (but not over Wire-Wrap) since no feedthoughs are
needed in Multi-Wire.


-Neal B.

Wire-Wrap is a trademark of Gardener-Denver
Scotchflex is a trademark of 3M Co.
Multi-Wire is a trademark of Kollmorgen Corp.