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Ground holes

Ground holes

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By eeNews Europe



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 again – Thank heavens.

This posting drew the following comments from other contributors;

from “Erickk”

About 35 or so years ago, standard PCB practice was NEVER to make solid ground planes. This was in the days before CAD, when we laid boards out with black tape and pads stuck by hand to mylar sheets. You could actually buy peel-and-stick decals pre-printed with mesh ground planes, like an open basket weave. Even on simple double-sided (not even multilayer) boards, PCB manufacturers refused to fabricate solid planes, claiming the boards would warp, and multilayer boards would both warp and delaminate. But of course such planes worked poorly as frequencies increased. Slowly the industry changed, and materials and processes supporting solid planes were developed. I remember one IC data sheet which specified to use a solid plane, and contained the statement: If you board house refuses to fabricate a solid plane, get a new board house!

from “dt_hayden”

I worked many years in design and manufacturing environments and [have] never seen a requirement like this for the reason stated. Not to say there is not some exotic material with unique demands. I am also not a mechanical engineer but nothing is obvious about relieving thermally induced mechanical stress by using the pattern shown. It sounds like a case where an uneducated customer made demands because they could.

from “MWagner_MA”

John, that brings back memories. Early on in my career (lets just say in the 90’s and leave it at that) I remember seeing small bubbles on the solder side (ie ground plane) of a board and thinking that was normal (not knowing any better). Then later on I learned that was due to the vendor not properly baking the board first to drive off moisture before wave soldering. I’m always wary of someone giving me direction without explanation/science to back it up. Thanks for the story!

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