Tuan,
this is to be expected. You are adding a dielectric layer on top of your
outerlayer conductors. The impedance calculator program I got from Polar
Instruments can take this into account. You need to know the thickness over
the traces and the dielectric constant of the cured mask.

Timothy Reeves
ECD Circuit Board Division
13626 South Freeman Road
Mulino, OR 97042
[log in to unmask]
(503) 829-9108 (800) 228-8198  FAX (503) 829-5482


> ----------
> From:         Tuan Le
> Sent:         Thursday, February 17, 2000 11:12
> Subject:      Soldermask effect on impedance
>
> Hi everyone,
>
> I have done some tests recently by measuring impedance (on traces from the
> coupons) before and after soldermask.  I found that there are quite a
> large drop in impedance after soldermask: 4 to 6 ohms for single ended and
> 8 to 10 ohms for differential impedance.  Has anyone experienced this?
> Could anyone offer me an explanation of what happened?  We use the same
> Taiyo product for a long while.
>
> Your help is greatly appreciated.
>
> Tuan Le
> [log in to unmask]
>
>

##############################################################
TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
##############################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in
the body:
To subscribe:   SUBSCRIBE TECHNET <your full name>
To unsubscribe:   SIGNOFF TECHNET
##############################################################
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information.
If you need assistance - contact Gayatri Sardeshpande at [log in to unmask] or
847-509-9700 ext.5365
##############################################################