John Urry @ SLG wrote:-
>I am looking for help on determining recommended dimensions for thermal   
>vias and thermal via legs (spokes).  I understand there is a trade-off   
>between making the vias large enough with small enough legs to allow the   
>board to be soldered without the use of a blast furnace versus making the   
>vias smaller with larger legs to allow for the required current capacity   
>of the connection.  Any suggestions would be most appreciated.

My advice would be to keep the cross-sectional area as large as possible
by using a two spoke thermal to the ground plane which you can design to
be as wide as you need. This would give you a wider trace than using
four spokes and avoid the possibility of break out toward a single
connection. If you tear drop the connections to the pad, it will
increase cross-sectional area of contact to the hole without increasing
the heat sink.

Someone else asked why put thermals on non-soldered vias? If the finish
is HASL then the holes will clear better if they are not connected
directly to a ground plane. It is no problem to add thermals and they
also help to determine good drilling registration over the whole panel.
Besides that, if a hole goes straight into a ground plane these days, I
always query the design in case there has been a data error.

Regards
-- 
Paul Gould
Teknacron Circuits Ltd
[log in to unmask]
Isle of Wight,UK

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