Joe,
Thank you for your response.
1. Soldermask thickness is normal : ~ 0.0007 to 0.001
2. I used a Polar CITS500s to measure and I made sure the probes were in the
right place.
3. I used an airline to check and the machine is good.
4. I serialized the coupons to be sure I measured the same one before and
after soldermask.
5. I flipped the coupon so I didn't measure with the trace touching the
surface.
The only thing that is different is we changed to chemical clean before
soldermask recently. I'm not sure it does anything to the impedance.
Tuan
-----Original Message-----
From: Joe Uhr [mailto:[log in to unmask]]
Sent: Thursday, February 17, 2000 2:23 PM
To: 'TechNet E-Mail Forum.'; 'Tuan Le'
Subject: RE: [TN] Soldermask effect on impedance
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]
<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
[log in to unmask]
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] <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]
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