pcb trace cross section: mushroom

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buenos

Joined: 24 Oct 2005
Posts: 657
Helped: 15
Location: UK

23 Apr 2009 20:14 pcb track cross section

hi

why is the musroom shaped cross section being created on PCB traces?

i heard at few places that the plated layer traces after etching have a mushroom shape cross section.

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mikal

Joined: 05 Jun 2006
Posts: 28
Helped: 2
Location: Austin, TX

01 May 2009 7:38 cross section pcb

They might be referring to the 'etch back' of the copper during the etching process. While in some processes, the anti-etch is coating the very top, the sides however, are not coated, and as the layer gets etched, the chemicals 'dig in' from underneath the anti-etch coating. This plays with numbers when you're trying to calculate trace impedance, but overall, it's not really an issue.
Usually PCB fab houses, when the customer calls for very tight trace/space, this becomes an issue and they'll want to use half-ounce copper, because sometimes the etch-back will actually eat through the entire trace, from the 'inside out'.

Mike

www.pcb-experts.com