Don
I received the same question and I replied as follows:
Martyn
I think there may be four reasons:
1. (probably the most prevalent) the specifiers don't have a clue
2. it is an easy way of ensuring and measuring that the copper thickness
on the board is adequate for the purpose (PCB fab guys are not skimping
on the plating thickness). At the same time, it would help to ensure the
correct Zo
3. maybe for DC reasons with lo-Z power circuitry
4. as you are better aware than I, L/C ratio, defining the Zo, ensures a
clean signal arriving at the load resistance and power is not
particularly significant for, say, high-speed switching. However, if the
load resistance is replaced by a, say, 1/4-wave antenna, giving a
non-reactive load resistance of c. 50 ohms, or a 1/2 wave dipole and c.
72 ohms, any distributed R in the feeder will reduce the power that
could effectively radiate from the antenna.
Personally, I doubt whether any of these would have any significant
effect, in practice, for transmission lines of less than, say, 30 cm,
provided there was no undue reflection anywhere. Interestingly, a high
distributed resistance would probably have a better damping effect in
cases of gross mismatch (e.g. Rload open-circuit), but the values we are
looking at are not significant.
FYI, the Vutrax CAD software (and possibly others) has had R calculation
features for any track or part thereof, for many years. It may be that,
because such a feature exists, users are finding supposed reasons for it
being there and are bugging everyone unnecessarily as a result.
Just some thoughts...
Brian
Don Vischulis wrote:
>
> Techies:
>
> I received the following email today, and it aroused my curiosity. What
> reasons exist for specifying dc track resistance when minimum resistance is
> governed by a combination of copper weight and design width, and track
> length? Is this a case of over zealous specification or is there a purpose
> behind adding another test requirement to a product with extensive
> specifications?
>
> Don Vischulis
>
> Original message:
> You may be able to help me, as I would
> like to know why - in addition to specifying
> impedance - an increasing number of PCB designers
> and PCB fabricators are also specifying DC resistance
> of PCB tracks?
>
> I have had an increasing number of clients ask
> me about series resistance being specified on
> PCB tracks, but no one seems to know why..
>
> If you are able to help with some of the reasons
> why, I would very much appreciate hearing from you.
>
> Kind regards
> Martyn Gaudion
> www.polarinstruments.com
>
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