Pads on Ground Plane

Generally, small pads for passive parts are connected ¬†with a single PCB trace of equal size to each pad. That’s the right way to do it.

Top pads solidly connected to copper pour

However, sometimes, circumstances dictate a little different approach. The illustration on the right shows something of a worst-case. This is for a snubber (resistor, capacitor pair) between two power planes.

A couple of things will likely happen. The power plane will act as a heat sink, preventing the solder paste on one side from melting, resulting in a poor connection. Or, the unequal melting could lead to surface tension pulling the part up, causing tombstoning.

Most designers are aware of that, but sometimes, thermals will be deliberately turned off to allow for better current capacity to and from the large power Mosfets (not shown).

Thermal pads on side connected to pour.

If that’s the case, make sure that you can turn the thermals (see figure at bottom right) on or off by the part, rather than just by the plane.

Duane Benson
The rain falls mostly on the ground plane due to static attraction

 

http://blog.screamingcircuits.com/

 

About Duane

Duane is the Web Marketing Manager for Screaming Circuits, an EMS company based in Canby, Oregon. He blogs regularly on matters ranging from circuit board design and assembly to general industry observations.
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