ECO - Missing Ground Connections

Figure 1: Line of plated through holes with no connections.

This latest problem is a board fabricated like the picture you see in Figure 1.

It may not clear in the photo, as connections can be buried, but the featured plated through holes (A through V, Row 1 through PS) are supposed to be connected to ground in order to properly perform their function. They're not connected to each other, or to ground. 

It would seem easy enough to find a ground point and connect the pins once the board is populated, or simply buss a wire along the row of pads. But, the customer nixed both of those ideas.

What's required is a connection at the bare bord level, one that is rock solid and does not interfere with the barrel of the plated through holes. And the whole row has to be connected to ground on layer 3.

The dimension from the pad edge to the inside of the plated through holes is only .010". That's very small and would make it difficult to solder a wire or circuit to the pad without getting solder into the hole.

So here's the solution. We mill a groove down the row of pads. Next, under high magnification, we excavate under the each of the PTH pads in order to provide a connection point for a new circuit to the PTH without getting solder into the hole. See Figure 2.

That's a nifty piece of work and requires a patient, steady hand. Additionally, at the end of this row of plated through holes we will use a precision milling system to mill down to layer three in order to expose the ground plane for connection.  The connecting buss will be soldered to the ground plane and run out to the row of plated through holes. See Figure 3.

A tinned copper circuit is laid in the groove and carefully soldered to the underside of the barrel and pad of each plated through hole. Once that is complete the circuit is connected to ground.

Figure 4: Circuit run out to ground and masked.

That portion of the rework is visually inspected and electrically tested. Next the reworked area is protected by covering it with high strength thermosetting epoxy.


Figures 4 shows an in-process view of the ground connection and Figure 5 is of the line of plated through holes bussed together.

Figure 5: Complete row bussed.

So there it is, good stuff, done right and done quick. One more fire quenched!

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

Web Page: http://www.circuitmedic.com/features/548.shtml