[SI-LIST] : Does solder mask reduce trace impedance ?

John Lin - TAO (LinJohn@digital.com)
Tue, 9 Dec 1997 09:19:42 +0800

Dear all SI experts,

Does solder mask covering PCB reduce the impedance of trace?
If yes, then what will be the amount of impedance changed.

Previously, we have SCSI back plane. We control the stackup to obtain
90 +/- 6 ohms impedance.

After measuring the impedance of real back planes sent back from a
manufacture, we found the impedance is lower than that of our
expectation. It is about 81 ohms. The manufacture analyzed the
backplane by studying the profile of PCB and material used and were
sure that the impedance should be about 89 ohms.

The engineers of PCB manufacture told us the solder mask covering the
PCB will reduce the impedance up to 9 ohms.

I just wonder the solder mask affects the impedance so much for a
PCB board.

JOHNLIN
CAE Engineer of EDA Department
Digital Equipment Corp. Taiwan Branch
Email: Linjohn@mail.dec.com
TEL: 1-886-3-3900000 ext. 2152