Ground holes

February 03, 2014 // By John Dunn
John Dunn is a design consultant (and contributor to EDN); here, he relates an unexpected “design constraint”.

Sometimes I come across requirements that are really startling.

I was designing a whole set of RF assemblies for this one particular project having frequencies as high as 120 MHz. That isn't really so high by most reckonings, but it does include VHF so certain precautions are, if I may reach for a word, "prudent".

I hold that prudence is especially important with respect to grounding, so I started out with plain copper ground planes like this one.

In each assembly, I had a nice flat, uninterrupted ground plane area in which to lay out my designs as I felt was best and so I did exactly that.

After finishing it all, along came the customer, one of the military services, and they said "No! You can't have the copper foil that way. There have to be thermal stress relief openings in the copper. These boards are going to be wave soldered and if they're made the way you want them made, the copper will separate from the fibreglass layer.

I was told that the ground plane had to be made like this:


This pattern of holes was simply dropped onto my carefully crafted layouts. Plop! Just like that. My ground plane integrity was ruined and there was no use arguing with anyone about it because although I never had any particular mil-spec cited to me as justifying this, certain mortal deities had made up their minds and that was the beginning, the middle and the end of it.

In my mind's ear, I could hear Al Capp's cartoon character Mammy Yokum uttering her classic line of "Ah has spoken!" and there I was like Li'l Abner himself being expected to say "Yas'm.".

Some of my assemblies did malfunction. I had to make compensation with wire jumpers placed across some of the thermal slots. Then with a design tweak here and another one there, I finally got everything working