Pat:

IPC-D-275, Design Standard for Rigid Boards and Rigid Board Assemblies, 
includes formulas for thermal relief. The relationship between the hole 
size, land and web area is critical. Typically, divide 60% of the minimum 
land area diameter by the number of webs desired to obtain the width of 
each web. If the land diameter is greater than the minimum value 
calculated, the percentage difference between the land diameters must be 
subtracted from the total web width calculation. Total cumulative copper 
web for all layers in any plated through hole should not exceed 4.06mm 
for 1 oz. copper of 2.03 mm for 2 oz. copper.

The total of the thermal relief cross-sectional area divided by the
number of planes connected to the plated through hole shall not violate 
the current carrying capacity requirements for a given hole.

Mike Buetow
IPC Technical Staff
7380 N. Lincoln Avenue
Lincolnwood, IL 60646
P: 708-677-2850, ext 335
F: 708-677-9570
[log in to unmask]

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On Mon, 31 Jul 1995, Pat McGuine wrote:

> Designing thermals on power and ground planes:
> 
> I've heard that rule-of-thumb is an ID of 0.015" over the hole size, and an
> OD of 0.020" over the ID.
> 
> Questions:
> 1.  Does IPC have a spec for thermal relief?
> 2.  Do the rules change as the hole size changes?
> 
> Thanks in advance.
> 
> 
> -Pat-
> 
> -------------
> Patrick McGuine
> Nicolet Instrument
> [log in to unmask]
> (608) 276-6334
> 
>