Figure 1: Layers in a .125" hole.

Things get dicey when the drilling occurs at a location where internal planes project into the newly drilled hole, especially when the planes are separated by 12, 10, 5, or even 2 thousandths of an inch. And sometimes there are a dozen or more planes stacked all the way through the height of the hole.

See the planes projecting into the hole in Figure 1. In these cases the possibility of smear or the minutest conductive particle connecting the planes is high.

Figure 2: Enlarging a hole.

That's what inspection is for, to catch those potential problems, right? However, inspecting drilled holes using a microscope can be difficult. When the board is thick and large, maneuvering it to obtain a clear field of view covering the 360 degrees of the drilled hole is physically taxing and frustrating.

Proper back-light is crucial and that light requires continual adjustment to keep the viewing area clear. The fact that the board is thick and the plane separation is minuscule, demands a degree of optical magnification which reduces the depth of view, further contributing to the challenge of ensuring a pristine cut surface.

Figure 3: One method of filling and strengthening a repaired hole.

The original drilling and inspection could be done perfectly, yet the epoxy sealing step might introduce or move suspect material to a location where it can cause electrical anomalies.

That can be problematic because optical inspection, as difficult as it already is, may no longer be able to see through the sealing material to the cut planes.

Figure 4: Hole drilled with insulating material separating hole from planes.

Recently we received a board that indicated an anomaly at a drilled hole. The "shorts" were high resistance - in the kilo-ohm to meg-ohm range - but unwanted connections nonetheless. Visual inspection revealed nothing. How could we eliminate the short if we couldn't see it?

The solution was to use the oblique viewing capabilities of our Dage x-ray machine and peer through the board to get a clear view of the planes inside the drilled hole.

Figure 5: Short identified by oblique x-ray.

The result was success on the first attempt. I'm sure the oblique viewing technique is one we'll use again and you may try yourself if you're confronted with a similar situation.