Re: Wire wrap

From: K Williams (krw_at_att.biz)
Date: 06/28/04 

Date: Sun, 27 Jun 2004 21:50:13 -0700

John Fields wrote:

> On Sun, 27 Jun 2004 20:53:22 GMT, Joerg
> <notthisjoergsch@removethispacbell.net> wrote:
>
>
>>Just why is it then that several agencies do not endorse it for
>>printed circuit boards? Maybe you can enlighten us here.
>
> ---
> OK.
>
> For several reasons, one of which is that ww boards are more
> voluminous and weigh more than a PCB supporting the same component
> population and function, another being that sockets, which are
> often used on ww boards, are more or less verboten in a high-rel
> environment because of galvanic corrosion problems and the
> likelihood of chips falling out in a high-vibration environment.

Have "you" used the machined Augat style pins? These things hold
like hell, though are very expensive. They make wonderful
prototyping boards for DIP stuff. Of course they're far less
useful today.
> ---
>
>
>>All the WW boards I
>>have seen (not the back planes) were essentially printed circuit
>>boards with a ground and power plane.
>
> ---
> Not really. They were wire-wrap boards with ground and power
> planes. Big difference.

Sure. I never understood people trying to prototype a high-speed
widget daisy-chaining power and ground on a Wire-wrap board,
because it was "only" a prototype. One must be even more cautious,
though it is possible. Power distribution is the biggest trap I've
seen engineers fall into. Why should a prototype be any lesser of
a problem?

> ---
>
>>The socket corner pins were soldered to
>>the GND and power planes respectively and decoupling caps were on
>>the boards as well. To see an example of what agencies prescribe
>>scroll to "3.2.2.2 Wire Wrap" in this document:
>>
>>http://www.faa.gov/and/and300/and360/coldfusion/library/specifications/FAA-E-2100g.pdf
>
> ---
> They said "Wire wrap shall not be used on printed circuit boards."
> , which is a whole different thing from "Wire wrap boards shall
> not be used."

Dunno. I've seen both used on the same "board". Maybe we have a
difference of opinion of what constitutes a "board". IBM
Mainframes used both on the "mother-boards". The cards (plug into
boards) were all printed, with soldered over-flows, mainly because
there was no space for WW pins. IBM mainframes aren't generally
known to be unreliable.
> ---
>
>>The PDP-11 was made by DEC, not by my colleagues. What I was
>>talking about is contamination.
>
> ---
> What you're talking about is something you obviously have very
> little experience with but which, for the purpose of saving face,
> you have to keep on prattling about.

Indeed.
>
>>There is another issue that occasionally comes up with WW and this
>>one has nothing to do with the gas-tight connection and all. It is
>>the fact that you end up with lots of dense and tall posts that
>>the wire is wrapped around. That invites the accumulation of
>>contaminants and I
>>remember a case (in the UK) where that was considered a factor in
>>a very dangerous situation in the air. Luckily the crew got the
>>plane somewhat stabilized and back to the ground. I believe the
>>yaw damper control is what had failed and the investigation
>>concentrated on some of the wire wrapped connectors.
>
> ---
> And do you remember the outcome of the investigation, or does the
> outcome of the investigation not matter as long as you can use the
> investigation itself to conveniently imply guilt?

By association...

>>With a circuit board a coating takes care of this but I have not
>>seen coating on a WW assembly.
>
> ---
> there are mechanical contrivances available which are designed to
> keep the posts from touching each other, as is compliant foam.

Coating PCBs isn't always a win either. Often one is better off
leaving things alone. That said, I can't imagine 2-level posts
finding their way over to meet each other, unless there is severe
trauma (that would likely kill a modern PCB). 

-- 
  Keith

Relevant Pages

  • Re: placing addiional caps across existing caps to reduce noise
    ... power planes have much capacitive effect at these frequencies (the ... "planes" being polygons, with other signals on the same layer, and thus ... It is not just that the caps work at different frequencies, ... they work with the power planes. ...
    (comp.arch.fpga)
  • Re: Houserule positive and negative energy planes away?
    ... > be true infinite planes just like them. ... planes is forbidden by the Tuatha De Dannan, the pantheon of gods ... > come about from using the power of Sunaru. ... > evil and wrong impulses. ...
    (rec.games.frp.dnd)
  • Re: Board layout for FPGA
    ... analyze a complex power distribution system (PDS). ... makes sense to me that the pin has more capacitance than the ... between the two FPGA planes. ...
    (comp.arch.fpga)
  • Re: Board layout for FPGA
    ... analyze a complex power distribution system (PDS). ... resistance and back to the FPGA ground. ... between the two FPGA planes. ...
    (comp.arch.fpga)
  • Re: 69 Mustang instrument lights
    ... If you're getting power to the blue-red wire, you can rule out the light ... It sounds like the ground connection or the ... printed circuit is broken.. ...
    (rec.autos.makers.ford.mustang)