Tuan,
 
3 things off the top of my head:
 
1. Soldermask thickness greater than normal? ~2-4 mils?
2. If you are using a Tek scope to measure impedance, make sure the signal and ground pins are not reversed when inserting into the holes of the coupon. This usually causes a 4-6 ohm drop in impedance even though the waveform looks fine.
3. Scope / cable not calibrated to nominal settings. Eg. 50 ohm cable measuring 45 ohms vs. 50 ohms.
 
It is assumed you measured the exact same coupons before soldermask and after soldermask application. Also,
make sure you are not lying the coupons flat on a conductive surface while you are measuring.
 
Joe
 

-----Original Message-----
From: Tuan Le [mailto:[log in to unmask]]
Sent: Thursday, February 17, 2000 3:16 PM
To: [log in to unmask]
Subject: Re: [TN] Soldermask effect on impedance

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
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 -----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]