Thank you for your response, Mike.  However, I guess I wasn't clear on my
question: I didn't expect the impedance to drop that much after soldermask.
Usually I see 2 to 3 ohms the most.

Tuan

-----Original Message-----
From: Beckman, Michael W [mailto:[log in to unmask]]
Sent: Thursday, February 17, 2000 11:54 AM
To: 'TechNet E-Mail Forum.'; 'Tuan Le'
Subject: RE: [TN] Soldermask effect on impedance


Hi Tuan,
Just real briefly;  Zo is = to L/C (inductance over capacitance) and
capacitance is proportional to Er (dielectric constant), so C= Er A/d (area)
over d (distance/separation).
It is important to understand that solder resist is a dielectric and acts as
insulation to the surface traces, thus reducing the circulating currents.
I hope this helps.



Mike Beckman


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Packaging Engineer
Assembly Technology Division
CH5-157                                 Ofc. (602) 554-4232
5000 W. Chandler Blvd.            Fax (602) 554-7615
Chandler, Arizona 85226          Pgr. (602) 340-3983




 -----Original Message-----
From: Tuan Le [mailto:[log in to unmask]]
Sent: Thursday, February 17, 2000 12:13 PM
To: [log in to unmask]
Subject: [TN] 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]