Patent Application
20070278001
An understanding of the present teachings can be gained from the following detailed description, taken in conjunction with the accompanying drawings of which:
In the present invention, the characteristic impedance of the through hole via is optimized by creating sufficient capacitance between ground layers and adjusting the via drilled hole diameter and clearance to surrounding metallization of the ground planes. This new high frequency through hole via has a via hole characteristic impedance that now matches the characteristic impedance of the traces on the layers which the via will connect together. This will permit a high frequency signal to pass through this entire signal path with minimal reflections on the signal waveform caused by impedance mismatches. The via hole will act essentially as a coaxial transmission line.
To optimize the characteristic impedance of the through hole via 270, the via hole 270 is treated as a coaxial transmission line. The through hole via 270 may be plated, which acts as a center conductor. The through hole via 270 may be plated with copper as a base material. Additionally, the copper plating may or may not be followed by nickel and then finally gold. The printed circuit board material acts as a dielectric and the metallization in the ground planes 3-7 (240-248) surrounding the plated through hole acts as the shield of the coax transmission line.
By adjusting the separation of the ground planes, sufficient capacitance between the planes can be created to make a high frequency short between the ground planes 3-7 (240-248). Basically, the ground layers 3-7 (240-248) act like a solid copper wall running parallel to the plated 272 through hole 270. Then, the anti-pad 230, or clearance in the ground planes 3-7 (240-248) surrounding the plated through hole 270, will be selected to create the desired characteristic impedance for a given plated through hole diameter.
The anti-pad may be created by selectively etching copper from solid planes of cold rolled or sputtered copper of thickness ranging from 0.1 mil to 0.3 mils.
The characteristic impedance can be adjusted from about 25 ohms up to about 75 ohms for applications where the printed circuit board thickness does not exceed about 0.125 inches. Adjusting the impedance may be adjusted by varying the ratio of the anti-pad to the drilled via hole diameter. As the ratio decreases, the impedance decreases. The ratio increases, the impedance increases.
It will be readily apparent that the above fabrication method may be accomplished in different orders than that given. Also, many different materials may be used without deviating from the heart of the invention.
