[SI-LIST] Re: controlled impedance requirements
- From: Larry Miller <ldmiller@xxxxxxx>
- To: si-list <si-list@xxxxxxxxxxxxx>
- Date: Mon, 17 Jan 2005 11:24:43 -0800
On Jan 17, 2005, at 1:37 AM, kobik74 wrote:
>
> Hi gurus
>
> How do you specify to the PCB manufacturer the controlled impedance
> requirements
>
> Except the layer stack up, layer thickness (oz), board thickness and
> text that define the controlled impedance for signals in each layer
> do you also define the PCB material type (Er, Silkscreen, Solder
> =85), inner dielectric thickness or you trust the manufacturer to
> reach the controlled impedance?
You have to be careful not to over-specify. Usually you supply a
"proposed" stack-up and let the vendor come back with a design that
accommodates the materials to be used. For example, Er is very
sensitive to glass/resin ratio in thin layers. You don't know what that
is.
Info you supply is:
Layer stack-up
Desired board thickness (not more than 8X-10X the diameter of your
smallest vias)
Design trace width for each layer and the impedance you expect that to
give
Cosmetic items, such as silkscreen, solder mask, solder thieving
If you have to specify everything exactly, you probably have the wrong
PCB vendor.
>
> How much tolerance do you permit?
+/-10% is usually economical and satisfactory; per H Johnson, the
signal overshoot is no worse than 1/2 the mismatch.
Larry Miller
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