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November 2002

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Subject:
From:
David Douthit <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 21 Nov 2002 05:39:02 -0700
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Oliver,

It would be helpful if there was more information about the nature of the "failure".
Cross sections, both latterly and across the "failed" paths, plus environmental conditions
and time in service are needed for a start.

Other manufacturing processes need to be known such as, solder mask type and method,
soldering profiles, fluxes, cleaning methods, was conformal coating used, if so what type, etc...

Operational profiles are also needed.

David A. Douthit
Manager
LoCan LLC

Oliver Yeo wrote:

> 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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