Is there any problem spelling out "ALL GROUND/POWER PLANE CONNECTIONS SHOULD
HAVE THERMAL RELIEF " on my fab drawings.


Should you not be specifying the thermal relief shape and size, Ken?


(No thermal relief is needed on press fit parts).

I tried to get a discussion on this topic going a few months ago. The PCI
organization actually specify Press Fit because of this fact.


 Vias to
>power and ground planes do not usually need thermal relief and it makes for
>a much more continuous plane which give better electrical characteristics

Why this can't be achieved using Pins through the board is only a matter of
process and equipment, ie getting sufficient heat into the planes to allow
solder to wet in the wave. (Quite why the design boys feel it neccessary to
specify the production process I'll never know).


>I've also had dense designs which would have had large gaps in the power
>and ground planes if I'd used thermal relief on the hundreds of vias that
>connected to these planes.
>
Apparently the contact factor is better if there are numerous pins making
the contact to the plane, making it possible to live with thermal relief
from an inductance point of view.


>Problems with lack of thermal reliefs may occur if a very short and wide
>(>.025 in) trace is used to connect a SMT part to a via to a gnd/pwr plane.
>
>Ray Klein
>
We never specify thermals on SMT and have never had problems. If you are
using forced convection ovens then it is easy enough to get the planes up to
temperature.

I suppose this is where specs fall down. They are independant of process and
equipment. If you are using old IR ovens then the ability to get heat into
planes isn't great.

Wave still uses IR preheating, if equipment manufacturers move to convective
preheat then maybee this thermal relief aspect (and the need for press fit
connectors on high speed digital), would disappear.


Regards

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