Re: gEDA-user: PCB layout for dc/dc-switching converter

[Steven Michalske <smichalske@xxxxxxxxx>]

On May 29, 2009, at 6:10 AM, Stefan Salewski wrote:

> I think I can make the copper areas with PCB polygons. But the pads of
> the components will not touch the copper polygons by default. I wonder
> if I should connect the pads to polygons by traces, or if I should
> reduce the clearance of the pads for full contact. The later may make
> hand soldering difficult.

I use these two options.

1.  draw minimal polygons near around the pads of your part and then  
hit the s key.
	This turns off the clearing in the polygon,  i.e. lines cannot clear  
these polygons, this is why you make these polygons minimal.

2. connect the pad with many lines forming a solid block,  one of the  
main goals with the small switchers is that you want to give heat a  
conductive path out of the part.
	This is why the shutdown signal is a polygon, not a thin trace in the  
recommended drawing.


My opinion, the polygons deserve chamfered corners,  knock off the  
sharp 90 degree edges and you'll reduce any EMI sources,  sharp  
corners radiate.  I doubt that you'll have any issues in this design,  
but someday you may.   You can get rid of all edges by drawing lines  
that are the outlines of the polygons and filling it with a poly, so  
that you get the rounded edges of the line corners.  This is also  
important for high voltage supplies.

This brings up a point about polys, pads, and thermals.

I know that we cant apply a thermal to a pad on a poly because we  
can't truly decide how it should layout.  We can thou turn off the  
pads clearing of the polygon.

Yes that pin will take more heat to solder,  but in reflow that  
doesn't matter.  With hand soldering, make sure you have a good high  
power or high thermal capacity iron.


Thanks for the PDF


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