Plated mounting "holes"/slots

[hpmaxim]

I'm trying to make a PCB which mounts to a metal chassis with 4 holes in the corner of the PCB.  Screws would be used to hold the board down and I'd like to ground the PCB to the chassis through the mounting screws.  The bottom of the PCB has a ground plane.  The catch here is that my "holes" are actually slots.  I'm making a 3.2mm (1/8") long cut with a 3.2mm mill bit.  This gives a rectangular 3.2x3.2mm hole with semi-circle caps on the ends.  This seems to be fine, but when I attempted to draw the same line on the pad layer with a wider diameter to "plate" it, I get a "Layer Abuse" error in DRC.

Is there a proper way to do this or will what I did work fine?  I tried to put an "oval" shaped via there, but its hard to get the size I want.  Also, this would put a drill hole and a mill over the same area.  I'm not clear what the consequences (if any) are for this, or if its even possible to properly do this by using a via.

[rob77]

and what's wrong with plain round holes for mounting screws ? but anyways... you would need a manufacturer which can do plating of milled slots (they usually do only drilling during the process and milling is possible only during the final phase)

[hpmaxim]

There is a through-hole connector which mounts into the chassis as a result, the exact location of the holes on the board is critical, I'm planning to use #4 screws which have a 2.8mm diameter, with a 6mm diameter head.  I wanted to get at least +/- 1.6mm tolerance which means I'd have to expand the hole diameter to 6mm, the size of the head.  It is possible I could use a washer, which seems to have an 8mm OD, and a 3.2mm ID. 

Not really the ideal solution but if you are claiming that the board house is going to balk at plating the slot no matter how I do it in Eagle, then it may be my only alternative.

It's actually worse than I originally thought.  Eagle only has 5.5mm holes, and they won't fit on the board unless I enlarge it.

[hpmaxim]

Is there a way to grab everything and realign it, since the bottom of my board is now at (0,-.6604mm) instead of (0,0)?  When I do a group and move I have to be so far zoomed out that I can't possibly zero the location.

[uncle_bob]

Hi

A lot of PC design guides will tell you *not* to plate a mounting hole. The reason is that as you tighten things down, you locally distort the board That can "rip" a plane connected to the hole with un-predictable results. Even if the planes do not rip, the hole can easily crack or separate from the plane(s). That makes it a bad idea to depend on a hole like that for a through connection.

Yes we could spend a lot of time debating how much risk there is. The cracked hole part is common enough that I've seen it happen.

Bob

[hpmaxim]

Ripping a plane concerns me, cracking the metal around the hole doesn't (at least not in a direct sense).  I have four mounting holes, and I'm not strictly speaking relying on it.  I have multiple ground connections coming into the PCB through the connector.  I'm not sure of any other reasonable way to guarantee that the chassis is grounded other than connecting it to the PCB.  The thing that concerns me most actually if I can't use a slot is that the screw head (or washer) could wind up shorting my VDD plane (top of the board) to the ground. 

[uncle_bob]

Hi

Well no matter how you do it, you must have a ground around the area (on top and bottom) that the metal hardware can contact. That includes tolerances for slop and a bit more "just in case".

If you don't *need* the slots for grounding, just leave them un-plated. Put in a keep out on the inner plane in the vicinity of the plane and that takes care of it. Same basic approach, keep it back far enough and then some. It will be a manual process on any package I have ever seen (draw in a polygon sort of thing).

Bob

[hpmaxim]

So after looking into this more, what I'm inclined to do is go back to my original plan (milled slot), except in lieu of the through plating on the slot I'll drop multiple vias between the top an bottom metal along the perimeter of the slot.

I don't really have that much experience with Eagle, so I'll ask some dumb questions:

1) I can't use pads or vias within the pad, so it looks like I can only use holes.  Are holes automatically plated through holes if they are surround by both top and bottom metal?
2) It seems like I can't assign both top and bottom metal to the same pin name.  Is there a way to do this or this just a limitation?
3) What layers besides top and bottom metal do I need (to allow plating of the metal itself and to keep the top (VDD) plane away from it?

 

[uncle_bob]

So after looking into this more, what I'm inclined to do is go back to my original plan (milled slot), except in lieu of the through plating on the slot I'll drop multiple vias between the top an bottom metal along the perimeter of the slot.

I don't really have that much experience with Eagle, so I'll ask some dumb questions:

1) I can't use pads or vias within the pad, so it looks like I can only use holes.  Are holes automatically plated through holes if they are surround by both top and bottom metal?
2) It seems like I can't assign both top and bottom metal to the same pin name.  Is there a way to do this or this just a limitation?
3) What layers besides top and bottom metal do I need (to allow plating of the metal itself and to keep the top (VDD) plane away from it?

Hi

I don't use Eagle so I'll only answer in a general way:

You can't put vias inside parts in general. On the stuff I use, there is a way to turn on that feature. Putting the via's outside the slot / hole area is a better choice (less chance of being damaged).

On the stuff I use, you define a plane to fill the open areas with. I routinely fill both top and bottom with ground. With a pad, I would simply connect them to the same net.

If you are going to plate through the hole (and likely get an extra charge for it) you will need top and bottom metal. The other stuff needs to be kept away by "enough". That may be 0.01" it could be more depending on a lot of things. 0.05" is pretty safe.

Bob

[hpmaxim]

I think you misunderstood what I said...  Let's pretend I'm drilling a hole instead of milling a slot.  I'm drilling a 3.2mm diameter hole, I'm putting a 6.4mm diameter circle of top and bottom metal at the same center point, then I'm putting small holes around the perimeter of a 4.8mm circle.  I want the small holes plated, just like a via would be. 

[uncle_bob]

I think you misunderstood what I said...  Let's pretend I'm drilling a hole instead of milling a slot.  I'm drilling a 3.2mm diameter hole, I'm putting a 6.4mm diameter circle of top and bottom metal at the same center point, then I'm putting small holes around the perimeter of a 4.8mm circle.  I want the small holes plated, just like a via would be.

Hi

With most pcb houses, you send them two files. One is the drill file for the plated holes, the other is the drill file for the un-plated holes. They work out which one to use when (if the files are properly labeled). In your case the holes in the 4.8 mm circle would be described in the plated hole drill file. The layout could have solid metal on top and bottom where those holes go. It would not have cutouts for the holes since they are drilled through the metal and then plated.

Did I miss the point again? ... hmmm ... maybe ...

Bob

[hpmaxim]

Nope, I think you got it... 

Although it doesn't make sense based on what I've seen.  Eagle seems to have two layers: 44 (Drill), and 45 (Holes).  According to something I read, 44 is for vias (plated through holes), and 45 is for mounting holes (i.e. non-plated).  The default Excellon CAM job seems to dump both into a ".drd" file, so I can't tell that there is any sort of differentiation between plated through holes and non-plated.

I wish there was some way to confirm what I actually have.  I tried to use EagleUp and when I look at the results, the milling hole isn't showing up, the holes aren't through-plated, and the bottom side doesn't seem to be plated.  But another milling hole on the board and vias seem to have it.  Now its possible, the lack of through-hole plating is because mine are on Hole layer instead of Drill layer (no idea how to change that), but that may not actually make a difference at the board level.  This is very frustrating.

[rob77]

just simply use vias for the "plated holes" you can choose the size and drill diameter for vias. so basically just do a ground fill top and bottom and vias around the mount hole. just use vias for vias and holes for holes and  that's it. they're even named "via" and "hole" in eagle.

[hpmaxim]

I made my own package (3,2-PAD-SLOT) as opposed to (3,2-PAD).  You cannot instantiate vias within a package, only holes.  So, not an option without it getting annoying and having to instantiate the vias manually from within the board which is not a good way to do it.

[uncle_bob]

Hi

As much as I recognize that all packages have problems (the stuff I use at work most certainly does ... I could go on for hours) Eagle does seem to be a hassle to work with.

Bob