Operation Hole Shorts

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X-Ray Finds Short Route to Failure

Figure 1: Layers in a .125" hole.

Drilling boards in the post-production environment is tricky work. Certainly if the board is populated with components and other hardware the setup alone can be time consuming. Additionally, datum points (the spots that ensure drilling precision) are not always easy to calibrate because these points are often designed for electronic versus mechanical location.

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.

Figure 2: Enlarging a hole.

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. 

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

There are many reasons a board may be drilled in post-production. Figures 2, 3 and 4 provide one view of how the drilling might be accomplished.  In this case it is to repair a damaged non-plated hole. 

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..

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

Now, some of you may be thinking, "Why don't you just meter the board at locations that provide feedback as to whether suspect planes may be shorting?"  Great idea, shows you know your business.  Unfortunately, believe it or not, that information is not always available to the people doing the drilling, so the drillers are dependent on using sharp cutters, stable drilling platforms, complete inspection and clean environments to accomplish their tasks. Things can go wrong.  Not often, thankfully. 

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. 

Figure 5: Short identified by oblique x-ray.

 What an idea! The Dage was able to obliquely (look top down all the way an angle of 70 degrees from the perpendicular) view the hole.  Additionally, it was able to rotate around the 360 degrees of the hole circumference without the laborious twisting and turning required with optical inspection.  See Figure 5. Using this method we were able to get a view of a ghostly fibrous piece or two that were likely causing our "shorts" and therein have a place to focus our repair attention rather than blindly drilling and hoping to clear the problem.  The result was success on the first shot.  That won't always be the case, I'm sure, but the oblique viewing technique is one we'll use again and you may try yourself if you ever find yourself in a similar situation.

Several members of the Circuit Technology Center team contributed to this feature story.