[SI-LIST] Re: copper thickness of a ref plane

Hi, Jim:

If properly implemented in a multilayer board, 1/2-ounce copper planes are 
fine, particularly for high frequency designs. However, there are some 
caveats. 

Based on skin effect, the shielding (reduction in ground plane current from 
one side to the other) offered by 1/2-ounce copper is ~26 dB (three skin 
depths) at 120 MHz. Higher frequencies are better shielded, lower are not.

As an EMI consideration, low inductance is the main concern/target for 
minimizing development of common mode noise; therefore, use efficient 
paralleling of the reference planes by applying a reasonable grid pattern of 
vias to interconnect the planes. This planning also has beneficial signal 
integrity effects.

If you are delivering 30 or more amperes of Vcc current through the planes, 
the total board power dissipation (and heat extraction) can become a problem. 
Heavier copper planes help get the heat to the board edges and mounting 
points.

In general, using 1/2-ounce copper aids etching accuracy, costs less, and 
tends to tighten controlled impedance tolerances.

There are many more considerations, but that's my input for now.

Mike

Michael L. Conn
Owner/Principal Consultant
Mikon Consulting
(408)727-5697

        *** Serving Your Needs with Technical Excellence ***


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