Dear All,

Recently a customer added track resistance requirements to their boards.  We
have made boards for them for a number of years and they had a new R & D
Manager start this year.  Apparently, their products have been failing
prematurely in the field and the new R & D Manager has since stipulated that
particular tracks have to meet certain resistance.

eg.     Track width - 1.27mm (50 tho)
        Track Length - 68.9mm
        Copper Thickness - 105um
        Resistivity of copper 0.0000000177 ohm.metre
        Required resistance - less than 9 milli-ohms

The customer is using a four wire resistance measurement method to determine
the resistance.  A known current power supply runs through the board at two
points and the voltage is measured along the track.  Using R=V/I to
determine the resistance.

As a PCB manufacturer we manufacture to plating thickness and the
verification of the resistance is an after process check.   We made some
boards in excess of 105um (plating up from 70um base copper) but they still
did not meet the resistance requirement even when we plate up to 141um.

We know that resistivity of copper is directly proportional to the ambient
temperature of copper and that plated copper have different density than
rolled copper.   But from their mathematical analysis of our copper
thickness, the difference is as much as 35um.  Our microsection has shown
the boards to be 120um and their calculation says that it is only 75um!

The customer is a manufacturer of inverters and transformers.  So my
questions are:

a)      Can the board be re-designed better not to rely so heavily on the
plating?
b)      Would using 105um or 141um base copper achieve this resistance
requirements?
c)      Would redesigning the board to have thicker tracks improve the situation?
d)      Do you think pre-mature failure is due to the 'under' plating or design?

I know this email and the questions are long winded, but I would really
appreciate it if anyone with similar experience can offer suggestions that
can help us as well as the customer achieve their goals.

Thank you.

Oliver Yeo
Quality Manager
IMP Printed Circuits Pty Ltd
Tel : (08) 8262 1444
Fax : (08) 8262 6004
www.imppc.com.au

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