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X-Ray Finds Short Route to Failure
Things get dicey when the drilling occurs at a location where internal planes project into the newly drilled cylinder, especially when the planes are separated by 12, 10, 5, or even 2 thousandths of an inch. Holy Moly! And sometimes there are a million planes stacked all the way through the height of the cylinder. OK, maybe not a million, but 20 or so is not a stretch (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.
That's what inspection is for, to catch those nasties, right? However, inspecting drilled holes can be difficult. When the board is thick and large in size, maneuvering the substrate to obtain a clear field of view covering the 360 degrees of cut 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.
On occasion, I'm sorry to say, things get worse. The environment the drilling is performed in is not always as clean as it could be which means the original drilling and inspection could be perfect and an epoxy sealing step might introduce or move suspect material to a location where it can cause electrical anomalies Now 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..
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. We were at a loss at to where to start. How could we eliminate the short if we couldn't see it? Then someone came up with the bright idea of using our version of oblique viewing x-ray machine, the Dage XD7500, to "burn" through the epoxy material masking and get a clear view of the planes inside the drilled hole.
Several members of the Circuit Technology Center team contributed to this feature story. |
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CircuitMedic