High density printed circuit board and edge connector assembly
BACKGROUND OF THE INVENTION 
     This invention relates to high density printed circuit boards and their terminals, and associated connector assemblies. 
     Edge type connector assemblies are well known in the electronics field. A typical edge connector and printed board assembly is shown for example in Frank E. Cooney U.S. Pat. No. 3,660,803 titled, &#34;Electrical Connectors&#34;, granted May 2, 1972 and assigned to the assignee of the present invention; and an edge type connector and printed circuit board assembly using two rows of terminals along one edge of the board and matching spring contacts is shown in L. Mayon et al, U.S. Pat. No. 3,160,455, granted Dec. 8, 1964. Additional printed circuit board showings employing two rows of terminals are shown in IBM Technical Disclosure Bulletins entitled, &#34;Connector for Multilayer Circuit Package&#34; by R. C. Miller, Vol. 15, No. 5, October 1972, pages 1614 and 1615; and &#34;High Density Circuits Connector&#34; by M. Jensen et al, Vol. 13, No. 6, November 1970. 
     It may be noted in passing that two of the last three cited references show inner and outer rows of terminals in which surface connections extend between the inner row of terminals to the outer row of terminals. This configuration necessarily increases the spacing between terminals of the inner row or makes for close printed circuit tolerances, or both. 
     Additional patents which are related to the disclosure of the present specification include the following: 
     U.s. pat. No. 3,015,083, entitled &#34;Electrical Connectors&#34;, by Juris; 
     U.s. pat. No. 3,328,749, entitled &#34;Terminal&#34;, by Kukla; 
     U.s. pat. No. 3,530,422, entitled &#34;Connector and Method For Attaching Same to Printed Circuit Board&#34;, by Goodman; 
     U.s. pat. No. 3,253,246, entitled &#34;Printed Circuit Card Connector&#34;, by McConnell et al.; 
     U.s. pat. No. 3,614,706, entitled &#34;Ground Connector&#34;, by Kukla; 
     U.s. pat. No. 3,671,917, entitled &#34;Printer Circuit Board Connector&#34;, by Ammon et al.; 
     U.s. pat. No. 3,673,548, entitled &#34;Printed Circuit Board Connector&#34;, by Mattingly, J., et al.; 
     U.s. pat. No. 3,744,005, entitled &#34;Zero Force Type Connector Block&#34;, by F. C. Sitzler; 
     French Pat. No. 1,475,962, by Labinal. 
     Accordingly, in view of the foregoing background, one object of the present invention is to improve the construction of high density printed circuit boards, and associated edge connector assemblies. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, the multilayer printed circuit boards may be provided with two rows of terminals along one edge of the board. These two rows of terminals, one immediately adjacent the edge and one spaced slightly back from the edge of the printed circuit board serve to make all of the necessary connections to the complex circuit board, and may be provided on both sides of the board. Preferably, the inner layers of the circuit board are connected to the outer row of terminals, and the outer layers of the multilayer board may be connected to the inner row of terminals. 
     With regard to the printed circuit board, the connections may be made by plated-through holes which may extend substantially all the way through the multilayer board, or may extend only from the inner layers in one direction or the other to the outer surface of the board. 
     Another aspect of the invention involves the mating edge board connector which includes a housing for guiding and locating the printed circuit board as it is mounted into place, and two sets of spring type conducting metal connector strips. One of the sets of conductor strips engages the terminals which are closest to the edge of the printed circuit board and another longer set of connector strips engages the set of terminals which is spaced inwardly from the edge of the printed circuit board. By the use of one set of longer connector strips, the required insertion and removal force is reduced. 
     Other objects, features and advantages of the invention will become apparent from a consideration of the following detailed description and from the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a high density edge type connector for use with a mating high density printed circuit board; 
     FIG. 2 is a fragmentary top view of the connector assembly of FIG. 1; 
     FIG. 3 is a schematic cross-sectional view of one edge of a multilayer circuit board which may be used with the connector of FIGS. 1 and 2; 
     FIG. 4 is a fragmentary top view of the edge of the printed circuit board of FIG. 3; and 
     FIG. 5 is a cross-sectional view of an alternative form of multilayer printed circuit board. 
    
    
     DETAILED DESCRIPTION 
     Referring more particularly to the drawings, FIG. 1 shows a novel high density edge type connector constructed in accordance with a preferred embodiment showing one aspect of the present invention. As shown, a connector housing 12 is mounted on a back panel 14. The front end of the housing 12 is preferably provided with tapered or beveled surfaces 16 and 18 to facilitate sliding the high density printed circuit board 20 into the housing and into engagement with the resilient contacts 22, 24, 26 and 28. 
     It may be noted that the contacts 22, 24, 26 and 28 are preferably curved so that they make firm engagement with the printed circuit board 20 as it is slid into the housing 12. More specifically, they present slanted surfaces toward the board 20 so that the contacts are readily separated and then they exert significant resilient force onto the board 20 to physically hold it in place, and also to insure adequate contact pressure against the terminal pads on the printed circuit board 20 so that good electrical contact is made and maintained. Following engagement of the contacts with the terminals on the board 20, the contacts 22, 24, 26 and 28 have their curved inwardly-directed portions tangent with the terminals which they engage. The spring contacts 22, 24, 26 and 28 are insulated from one another and mounted in position by the insulating portions 30, 32, and 34 of the housing 12 and by the back panel 14. 
     Incidentally, the greater length of the spring contact members 22, 24 reduces the force required for insertion of the board 20, but the backing for the springs provided by housing 12 insures adequate final contact force. 
     FIG. 2 is a top view of the assembly of FIG. 1. Clearly shown in FIG. 2 is the housing 12, the slanted surfaces 16 and 18, and the springs 22 and 24. 
     The cross-sectional view of FIG. 1 is taken along Section 1--1 of FIG. 2. 
     It is noted in passing that the exact shape of each of the contacts and their mode of mounting and the like can take any known form, as indicated for example in the above-cited U.S. Pat. No. 3,660,803, assigned to the assignee of the present invention. 
     FIG. 3 is a partial cross-sectional view of the multilayer printed circuit board 20 shown in FIG. 1 as being assembled to the connector. FIG. 3, as drawn, has an increased scale in the vertical direction. Thus, the normal printed circuit board has very closely spaced conductive and insulating layers and these are shown to a greatly expanded scale in FIG. 3. More specifically, the four conductive patterns are shown in the multilayer board of FIG. 3 and these include the conductive patterns 42, 44, 46 and 48. These are separated by the insulating printed circuit board layers 50, 52, and 54. In the arrangement shown in FIG. 3 the terminals 56 and 58 are part of the rows on either side of the printed circuit board which are closest to the edge of the board 20. In addition, in accordance with one embodiment of the present invention, terminal 56 is connected by the plated-through hole 60 to the inner layer 44, while terminal 58 is connected by the plated-through hole 62 to the conductive pattern 46. Terminals 64 and 66 are part of the next row of terminals which are spaced inwardly from the outer rows adjacent the edge of the printed circuit board 20 and on opposite sides thereof. 
     In FIG. 4 a fragmentary top view of the board 20 is shown. As shown in this figure, the terminals 56, 56&#39; and 56&#34; form part of the row of terminals along the edge of the printed circuit board 20, while the terminals 64, 64&#39; and 64&#34; form a part of the next row of terminals along the edge but spaced back from the edge for engagement by the contacts such as contact 24 as shown in FIG. 1. 
     FIG. 5 shows an alternative printed circuit board 20&#39; in which plated-through holes 72 and 74 extend substantially all the way through the printed circuit board instead of merely extending from intermediate boards to the surface in accordance with the arrangement shown in FIG. 3. This arrangement shown in FIG. 5 provides staggered terminals, with terminal 76 associated with plated-through hole 72 being closer to the edge of the board than terminals 78 associated with plated-through hole 74. The use of plated-through holes which extend all or substantially all the way through the printed circuit board facilitates production operations, in some cases. It may also be noted of course that, using the arrangement of FIG. 5, the contacts 26 and 28 might have to be shifted somewhat in their engagement points with the terminals on opposite sides of the printed circuit board; alternatively, terminals 76 and 78, for example, could be of sufficient extent that contacts 26, 28, for example, could both be of the same length, despite the slightly different positioning of the terminal pads on opposite sides of the board. 
     With regard to FIG. 5, the inner rows of terminals of board 20&#39; include on one side terminals 80, and on the other side terminal 82. As in the case of the board of FIGS. 3 and 4, one set of terminals may be employed to make connections for the inner layers of the multilayer circuit board, while the other, inner pair of rows of terminals spaced further from the edge of the board 20&#39; may be connected to the circuits on the outermost layers of the multilayer printed circuit board. 
     In the board of FIG. 5, as well as that of FIGS. 2 and 3, simplification of the circuit pattern is achieved by using the outer row of terminals for the inner layers, and the inner row of terminals are preferably coplanar with and are formed concurrently with the conductive pattern on the outer printed circuit board layers. 
     It is to be understood that minor departures from the foregoing specific details may be employed in the implementation of the present invention. Thus, while having a special applicability to multilayer circuit boards, the present invention is not so limited. Also, although the spring contact arrangements as shown in FIGS. 1 and 2 are preferred, other known spring contact engagement arrangements may be employed without departing from the spirit and scope of the invention.