One method to create non-plated holes is to plug them (usually with little rubber corks) before plating. This is an old technique though, and I don’t think many manufacturers use it any more.
Hi gregben,
I’ll add a few thoughts to what you’ve written about manufacturing processes.
First of all, a lot of routers can drill too. All of them should be able to, since they’re basically just drilling machines with spindles designed to be be dragged sideways through panels.
As for unplated holes, if the PCB shop uses the procedure I’m familiar with, then tenting will result in a non plated hole. That is, if you don’t put pads on a hole, and there’s no other copper around the hole, the result will be that the hole will be unplated.
There are three ways to produce an unplated hole.
One is post drilling, one is plugging (this is extremely tedious and error prone) and the third way is tenting.
I’ll try to explain how tenting works. The manufacturing procedure is first to drill the panel, then to use electroless copper plating, or direct metalisation, or some other way to get a very thin layer (a few microns) of copper in to the hole barrels.
Next dry film photoresist is applied to the panels, and the film with the track image is lined up with the drill holes and the photoresist is exposed to UV light through the film. Then the panel goes through a developer which strips off all the unexposed dry film. The result is that the panel now has plastic film stuck to it everywhere where there’ll be NO copper on the final board. (This means that holes with no copper pads will be ‘tented’ by the film top and bottom.)
The next stage is electroplating copper. The factory where I work plates close to 1 ounce thickness of copper, so the holes will get about 1oz copper thickness and the top and bottom copper will be close to 1.5 oz since we start with 0.5 oz material.
After copper plating, tin/lead is electroplated as etch resist.
Now the dry film is stripped from the panel. The panel now has plated copper covered with a tin/lead layer whereever there’ll be copper on the final panel. The remainder of the board has thinner bare copper shown, not plated with tin/lead.
The next step is etching, which removes all the copper which does not have the tin/lead etch resist coating. This includes the few microns of copper that was inside those tented holes.
So… Provided that the manufacturer follows a procedure similar to this one, tenting holes by removing top and bottom copper pads will result in unplated holes.
I have two notes to add to this. First of all, if there’s any copper at all touching, or very close to the hole (on the design) including tracks, pads, or fills, then there’ll likely be some plating in the hole. This will also be the result if the tenting over the hole breaks or is washed off. This is more likely to happen with large holes, perhaps 4mm and larger.
The other point is that manufacturers increase the diameter of the holes by about .004" to allow for copper plating and solder coating. So if you choose the tenting method for acheiving unplated holes, your unplated holes are likely to come out slightly larger than you specified.
The advantage of using the tenting method is that you can do it yourself and not even bother to tell the manufacturer that you want unplated holes. Once again, of course, this assumes that they use the manufacturing process I’ve described above.
Another advantage is that the unplated holes, since they are drilled at the same stage as your component holes, will align with them better.
On the other had, the advantage of post drilling unplated holes is that they’ll align better with the routed edges of the board, if they’re done on a driller/router in one step.
By the way, the term ‘tenting’ is also used in relation to vias. If there’s no solder mask pad on a via then it will end up ‘tented’ with solder mask ink.
Steve.
