Patent GB-1450777-A (GB)
Title: Methods of manufacturing printed circuit boards
IPC: B41C1/14
Filed: 1973-09-27
Abstract: 1450777 Printed circuits SIEMENS AG 27 Sept 1973 [29 Sept 1972] 45257/73 Heading H1R A printed circuit board is fabricated and processed by e.g. drilling, spot welding, or piercing at selected points coinciding with the grid points of a standard layout, congruent with the tool pattern of an associated processing device, wherein the points to be processed are defined by images, e.g. optically transferred from a printing plate to the circuit board, and the reference points of the printing plate deviate at random from associated grid points of the standard gridlayout; and wherein the magnitude and direction of the random deviations are determined and the position of the image elements relative to the standard grid layout and/or tool pattern is adjusted from a central position; in such manner that between the reference points of the image elements and the associated grid points of the standard grid layout and/or of the tool pattern there are produced oppositely directed variations in spacing, so that the maximum deviation of any grid point lies within a predetermined limit value. The Figure shows a printed circuit grid layout having rows of grid points R 1  to R 5  along the x co-ordinate, each row containing 5 grid points S 1  to S 5  in the y co-ordinate; the individual intersections R 1  S 1 , R 1  S 2 , to R 1  S 5  being initially equidistant in the theoretical layout. Grid point rows R 2 , R 3 , R 4  form a physical layout having intersection points R 2  S 1  to R 4  S 5  which define graphic elements of a printed circuit, such as soldering tags, conductor paths, or conductor free areas, which are applied to an insulant plate by optical printing and at which holes are to be bored. Mechanical deviations exist between the physical and theoretical grid points in rows R 2  to R 4 ; in R 2  the individual points are progressively and linearly displaced positively (to the right), in R 3  they are progressively and linearly displaced negatively (to the left) while the maximum deviation a (the sum of the individual deviations) lies within a permissible tolerance #2a. In R 4 , grid pointR 4  S 3  already deviates positively by a so that points R 4  S 4  and R 4  S 5  lie outside the permissible tolerance referred to the point R 4  S 1 . Thus if, e.g. soldering tags are located at points R 4  S 4 , R 4  S 5  and grid holes are drilled, e.g. by a numerically controlled machine whose spindles are located accurately with regard to a machine zero and which progressively steps in accordance with the theoretical grid spacing at R 1 , the holes will not register with the tags. In row R 5  comprising grid points R 5  S 1  to R 5  S 5  of a physical layout, the deviations from theoretical layout R 1  are similar to those of row R 4  but the grid points are located so that none is outside the allowable tolerance; the positioning accuracy between the actual and theroetical grid points being measured to assess deviation as predominantly positive or negative; the grid points being also measured on individual layers in relation inter se for multilayer circuits. From the predominance of positive or negative deviations the permissible tolerances are distributed by selection of a suitable reference point. If linear deviation is predominately positive over a row or the entire field, the use of a central reference point R 5  S 3  renders the individual grid points within the permissible tolerances. In drilling printed circuit boards printed or etched with graphic elements, the successive reference points derived from measurement in accordance with deviations of individual grid point rows, e.g. R 4 -R 5  lie along a curve. After drilling in a first co-ordinate direction, e.g. X in accordance with the reference point of a first row of grid points, the drilling machine is displaced in a second co-ordinate direction, e.g. y in accordance with the position of the reference point of the second grid point row on the curve, so as to connect the drilling machine zero point and thus the positioning of the machine itself. Master plates, e.g. of glass on which desired graphic element patterns are, e.g. photographed in a physical grid field deviating from a theoretical grid field may be similarly compensated during photographic transfer of the master plate patterns to the circuit board. In producing multilayer printed circuit boards the positions of the different master plates must be similarly compensated for deviations of physical from theoretical grid layouts in succession to obtain correct registration of corresponding graphic elements in the individual layers. In the case illustrated, the intersection R 5  S 3  is used as a reference point. The deviations, the trend of such deviations, and a comparison thereof may be assessed and analysed by a programmed data processing system.