Patent GB-1231484-A (GB)
IPC: G01N23/203
Filed: 1968-05-09
Abstract: 1,231,484. Measuring coating thickness. UNIT PROCESS ASSEMBLIES Inc. May 9, 1968 [May 9, 1967; April 1, 1968], No.21925/68. Heading G1A. The thickness of conductive coatings on the side walls of apertures in a printed circuit board or similar workpiece is measured by the beta ray backscatter technique. The apparatus described is designed to facilitate location of a probe comprising a radio-active beta source and a Geiger detector for the backscattered radiation in such a position that measurement of the thickness of side wall coatings is possible. A suitable probe arrangement, Fig. 9, has a source 79 located in holder 13a and a detector tube 14a mounted above it. The probe seats flush against the workpiece W, the radiation thereby being directed at the coating C. Radiation for measurement is scattered towards the detector from the coating through the gap between shield 76, which attenuates scattered radiation 81 generated in the body of the workpiece, and the tapered edge 74a of holder 13a. Two arrangements are described for locating the probe, in a position with respect to the coating to the measured, as indicated in Fig. 9. In one arrangement, Fig. 2, the printed circuit board (not shown) is located on a base-plate 28. Handle 33 of rod 29 is tuned to allow locating pin 22 to be raised by spring pressure 24, carrying its end 27 into the hole of the printed circuit board. The latter is then pushed forward until the end 27 abuts the side of the hole. This precisely locates the hole with respect to the end of the probe 10. An arm 58 is then lowered by adjustment of thumb screw 64 to clamp the board in place, before the probe is lowered on to the top of the board. This is done by turning a knob (52), Fig.3 (not shown), which allows a spring (46) to pull the probe downwards through a bore 35 in block 36. A rod 38 is provided to prevent location of the cylindrical probe within the bore. Alternatively, the probe could be of polygonal cross-section within a polygonal bore. In a second arrangement, Fig. 11, a single pair of coupled control knobs are used to carry out all operations, except for an initial rough location by means of arrangement 160, 163 of the board with respect to the probe assembly. Rotation of the knobs moves a cam-shaft 130 into three different orientations defined by a cam 129 associated with two clamping rods 115. In the first position as shown in Fig. 11, the probe, clamping rods and fine locating pin 145 are retracted. The printed circuit board W is located on the base-plate and using the end of pin 163 as an indication of the measuring location, the board moved until the hole 67a to be investigated is under pin 163. Using the control knob, the cam-shaft 130 is turned to a second orientation, in which a further cam 159 causes arm 147 to be rocked about support 149 and locate pin 145 in the hole 67a. The board is then moved as in the first arrangement to engage the side of the hole against pin 145. Further rotation of the cam-shaft to a third position, causes the pin 145 to be retracted and clamping rods 115 and probe 11a to be lowered on to the board W to allow measurement to take place. Both lowering actions are by means of rocker arms pivoted at 119 and displaced by cams (139 and 129, respectively). Specification 1, 231, 485 is referred to.