It is well known in some types of electrical apparatus, such as radio apparatus, to provide a shielding structure for the printed circuit board, such as shown in FIGS. 1 and 2. FIG. 1 is a cross sectional view of the shielding structure and FIG. 2 is a cross sectional view of the structure taken along line A--A of FIG. 1.
As shown in the figures, printed circuit board 1 comprises a dielectric substrate 11 which serves as a support for the printed wiring, including circuit trace 12 and ground trace 13. Ground trace 13 is that portion of the printed wiring which is adapted to be connected to a source of ground potential and circuit trace 12 is the remainder of the printed wiring on printed circuit board 1 and is usually operated at potentials other than ground potential. Circuit trace 12 is only printed on the bottom surface of dielectric substrate 11 and ground trace 13 is printed on both the top and bottom surfaces of the dielectric substrate. The top surface of printed circuit board 1 is coated with a material that is resistant to solder, except for a part of the land of circuit trace 12 and a part of ground trace 13 which is in contact with metallic cases 2a and 2b. The method of construction of printed circuit board 1 and the materials from which it is made are well known and will not be discussed here.
Circuitry blocks 3 comprise a plurality of electrical elements, such as transistors, amplifiers, etc., and are mounted on the top surface of printed circuit board 1 with the input and output terminals in circuitry blocks 3 being connected to the land of circuit trace 12 with solder via through holes (not shown). Circuitry blocks 3 are also isolated from one another by metallic cases 2a and 2b.
Narrow grooves 2c are formed in a portion of metallic case 2b proximate circuit trace 12 to prevent contact between metallic case 2b and circuit trace 12, as best shown in FIG. 2. The portions of metallic cases 2a and 2b adjacent ground trace 13 are held in contact with ground trace 13 by securing elements, e.g., screws, (not shown). Contacting material 4, formed of an elastic material inserted into a cylindrical wire netting, is inserted into grooves formed in metallic cases 2a and 2b and is sandwiched between cases 2a and 2b and ground trace 13 to ensure firm contact between ground trace 13 and metallic cases 2a and 2b. The effect of this arrangement is that each of circuitry blocks 3 is covered by metallic cases 2a and 2b and is shielded against interfering electric waves, such as high frequency radio waves, from another circuitry block 3.
As explained above, in the prior art shielding apparatus, it is necessary to form narrow grooves 2c in metallic case 2b to prevent contact between metallic case 2b and circuit trace 12. Because electric waves may be transmitted from one circuitry block to another circuitry block via narrow grooves 2c, interference between circuitry blocks may occur, particularly in the case of high frequency radio waves. In addition, the manufacturing process is also complicated because of the need to suitably adjust the length of contacting material 4, to account for narrow grooves 2c, so that contacting material 4 can be positioned within the area between metallic case 2b and ground trace 13. It is also necessary in manufacturing metallic case 2b to accurately position narrow grooves 2c to prevent contact between metallic case 2b and circuit trace 12.