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Spindle runout problems can be your hidden quality issues, registration problems, and the cause of excessively high scrap rates in building reliable PCBs.
The simple definition of spindle radial runout is how much wobble a spindle produces at the nose. Axial runout is the measurement of how much play there is perpendicular to the axis of rotation. The reading is represented by total indicated runout (TlR), which means the distance measured between the largest plus direction and the largest minus direction for a total indicated amount. A spindle can be measured cithet dynamically (at speed) or statically. The dynamic measurement is by far the most accurate and requires special non-contact measuring devices.
Today's air-bearing spindles have gotten faster with higher precision than the ball-bearing spindles of the past. Air-bearing PCB spindles can typically be found running in the 120,000 rpm to 180,000 rpm ranges, with some spindles running up at 300,000 rpm. Because the spindle air bearings float on air, no mechanical contact is made with the spinning rotor. Radial and axial runouts can be maintained with very tight precision. Westwind, a manufacturer of high-speed air bearing spindles, claims accuracies of its PCB drilling spindle of less than 10 urn: "Typical synchronous radial runout values: <10 µm (PCB drilling spindle, high speed)," which equates to approximately less than 0.0004 in TIR.
Many of today's CNC PCB drilling machines incorporate laser measurements for checking runout of the spindles at operational rpm. However, this does not always tell the whole story. Many rimes the machine's built-in software is set at a very nominal level of warning for celling an operator that the runout of the spindles is too great a value. The result is the machines may be drilling with less than the desired accuracies for hole locations, and even hole quality can be affected by runout-related problems. This is becoming an even greater issue, as today's circuit board design requirements have increased densities for components, tighter tolerances between the holes and the traces, and the continued direction toward having to use smaller carbide micro drills in the range of 0.004 to 0.008 in (0.10 ro 0.20 mm), with more concentrated holes per square inch of circuit board area. Production panel sizes are also becoming a...