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How effective is a power plane

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Peter Johnson

United Kingdom
498 Posts

Posted - 17 Apr 2009 : 17:49:06

A user told me recently that manufacturing practice for multilayer boards is not what I had previously thought. When I toured a plant many years ago, 4-layer boards were made by sandwiching a thin base layer between two equally thin double sided boards to make a 4-layer board with essentially equidistant plane spacing.

He suggested that a thick base layer is now common practice, with sandwiched outer layers effectively on flexible pcb material, so the inner spacing is very significantly more that the spacing between the outer pairs.

Bearing in mind the comments in this article:
<http://www.ultracad.com/articles/todd_h.pdf>
does that then suggest that signal traces should be confined to one side of the board and both planes confined to the other in order to maximise the noise suppression capability of the planes?

Food for thought...

Iain Wilkie

United Kingdom
1015 Posts

Posted - 17 Apr 2009 : 20:28:14

From EMC and SI points of view the tracks must have their reference plane underneath.....its about 8 thou these days ... its this close distance that gives multilayer boards good charactoristics in this respect. Also the further away the plane is, the longer the vias become increasing resistance and inductance (although the inductance could be used positively), also affects impedance balancing when required. Anyway there are many different stack up configurations for different numbered layered boards, all have their pro's and con's, and just like everything else in curuit design there has to be compromises. Cost probably being one of the major ones in most jobs !

Iain

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