Targeting Through Holes in Printed Wiring Boards (PWB) – Use of Reference Pins

When using the PC-MET and PWB-MET targeted grinding fixtures from Buehler, it’s necessary to set the total grinding distance from a reference pin on which the sample is mounted to the feature of interest.  In the standard equipment, this distance is set to 0.150” as shown in the diagram.

The perfect target is reached by grinding to diamond stops in the fixture. Setting the diamond stops accurately is of paramount importance in accurate targeting.  A dial gage is used to do this.  Setting the Dial gage top, the correct distance is important to target grinding to the specific plane prior to polishing to the center of target hole. When the Dial Gage is set to zero with reference pin of known value, the target plane is set at the center of holes.

Targeting Through Holes in Printed Wiring Boards (PWB) – Use of Reference Pins
Targeting Through Holes in Printed Wiring Boards (PWB) – Use of Reference Pins

A standard distance of 0.150” requires a reference pin of 0.1972” diameter to set the Dial Gage zero. For other distances, different reference pins may be used to set up the Dial Gauge.  This can be calculated as follows:

Distance from Reference holes to Target holes + 0.047” (half the value of 0.094” diameter index pin).

A note on use: It is advisable to set the diamond stops to specific values, typically 7 mils for the ‘Long Stop’ (first grinding step) and 3 mils for the ‘Short Stop (second grinding step). This ensures that the grinding surface plane remains above the target plane and will not pass beyond the center.

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