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Glossary of Terms




Thermal Relief Pad - AKA Thermal Ties, Wagon Wheels, Thermal Webs



Thermal connections to a plane are described in MIL-STD-275E paragraph 5.2.5 Thermal Relief in Conductor Planes and referenced in Figure 14 'Ground Plane Lands' in the Appendix. IPC has since replaced MIL-STD-275 with IPC-2221 and IPC-2222. There in section 9.1.3 of IPC-2221 you find the same information as was published in MIL-STD-275 with a reference to IPC-2222 the 'Sectional standard' for 'more detailed information'...


Regarding the IPC-2222 9.1.2 Thermal Relief in Conductor Planes you will find the relationship between the hole size, land and web area is has a very specific relationship which they choose to term as 'critical'.


Typically, to calculate the width of the thermal tie connections you need to divide the minimum land area diameter by 60% and then divide that number by the number of spokes desired to obtain the width of each thermal connection trace.




    Maximum hole size = 1.0 [0.040]
               Annular ring = 2 x 0.13 [0.005]  = 0.26 [0.010]
 Fabrication allowance = 0.25 [0.010]
     Minimum land size = 1.0 [0.040] + 0.26 [0.010] + 0.25 [0.010] = 1.51 [0.060]dia.


Total thermal width = 60% of land size
                            = 0.6 x 1.51 [0.060]
                            = 0.91 [0.036


2-web width = one-half of total thermal width
                  = 0.5 x 0.91 [0.036]
                  = 0.45 [0.018]
3-web width = one-third of total thermal width
                  = 0.33 x 0.91 [0.036]
                  = 0.30 [0.012]
4-web width = one-fourth total thermal width
                  = 0.25 x 0.91 [0.036]
                  = 0.23 [0.009]

The total cumulative copper in the spokes for all layers in any plated thru hole must not exceed 160 mils or 4.0mm for 1 oz. copper... 80 mils or 2.0mm for 2oz copper.

Also you need to make sure that you do not restrict the current carrying capacity of the plated thru hole structure by removing too much copper.

If the land size used in the design is greater than the minimum value calculated above, then the percentage difference between the land diameters must be subtracted from the total web width calculation.  This balances the thermal distribution in the pad compensating for the greater copper volume in a larger than minimum pad.

The next consideration is the clearance area between the land and the copper plane. Section  9.1.3 Clearance Areas in Planes, points to an illustration that clearly states that the clearance area should not be less than 10 mils or 0.25mm and/or not less than the electrical spacing requirement based upon the voltage being applied to the conductors.






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