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When the cost of copper can impact your design.

If your design has tracking that is resistance sensitive, you need to ensure your track width calculations are based on the minimum finished copper thickness your design can tolerate. We thought we had done this and then we found out the cost of copper had gone up.  We had a batch of flexis made and everything was good. Next batch, oops. When the cost of copper rises, the PCB copper thickness supplied by the PCB material manufacturer can be to the bottom tolerance. Why? To save money. Cynical viewpoint, maybe, what do you think?  If you base your calculations for 1 oz copper on 35 microns or even a little less, allowing for cleaning and etching you will be disappointed with the results.  The finished copper thickness for our manufactured design was sub 30 microns!  We now specify the minimum thickness of copper we require for the manufactured design. If your design has long tracks that are resistance sensitive, talk to your fabricator and enusre you know what the lowest copper thickness you might get.  Once bitten twice shy.

2 Comments

  1. Robert Burbidge 09/19/2016 Reply

    I would always recommend if you have a specific requirement for finished copper thickness and track widths then you should always send a proper drawing along with your Gerber data. This should state finished copper weight you need along with a max / min tolerance you can live with for both thickness and width. Then also state your boards should be manufactured to IPC class 2 as a minimum which tightens up allowable tolerances if none are spec’d in the drawing. This way the manufacture will have to make sure that all boards supplied are within this band width and carry out checks to confirm this. Sound obvious but in my may years on the bare board side of the industry the amount of times I came across similar problems all because no drawing with actual tolerances was included and assumptions were then made.

    • Author
      quantum 12/19/2016 Reply

      Hi Robert
      I missed your reply, sorry. As suggested we do create a drg (we always have), which was created with the PCB manufacturer. Finished copper weight defined and IPC Class 2 stated.
      And we still got caught. So as you say, we now specifiy the minimum we can tolerate on the drg.

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