Method for forming a back-drilled plated through hole in a printed circuit board and the resulting printed circuit board

A method for forming a back-drilled plated through hole in a printed circuit board and the resulting printed circuit board are described herein. In the preferred embodiment, the printed circuit board includes a via extending through a plurality of stacked layers. The via includes a plated through hole and a back-drilled hole. The plated through hole is located within a predetermined number of the stacked layers and the back-drilled hole is located within the remaining stacked layer(s). The plated through hole without an electrically conductive material located on walls therein has a diameter that is substantially the same size or smaller than the diameter of the back-drilled hole.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to a method for forming a back-drilled plated through hole in a printed circuit board and the resulting printed circuit board.

2. Description of Related Art

Designers and manufacturers of printed circuit boards are constantly trying to develop new and better methods to make printed circuit boards. For instance, the designers and manufacturers have been trying to develop a new and better method to form back-drilled plated through holes in printed circuit boards. A traditional method for forming back-drilled through holes in a printed circuit board is described in detail below with respect toFIGS. 1A-1D.

Referring toFIGS. 1A-1D, there are shown partial cross-sectional side views of a traditional printed circuit board100at different stages during the formation of a back-drilled plated through hole102. The exemplary multi-layered printed circuit board100shown has six stacked material insulator layers (typically laminates and prepregs)104a,104b. . .104fseparating five internal layers105a,105b. . .105eand two external layers108aand108b. Internal layers105a,105band105eare signal layers. And, internal layers105cand105dare plane layers.

In the first step to form the back-drilled plated through hole102in printed circuit board100, a pilot hole106(e.g., first hole106) having a diameter d1is drilled through the printed circuit board100(seeFIG. 1A). In an illustrative example, the thickness of the printed circuit board100is 0.23″ and the diameter d1of the pilot hole106is 0.030-0.032″. Secondly, the walls of the pilot hole106and predetermined areas108aand108bon the surfaces110aand110bof the printed circuit board100are plated with a thin layer of conductive material112(e.g., copper) (seeFIG. 1B). In the illustrative example, the diameter d2of the plated pilot hole104is 0.026″. Thirdly, a drill114having diameter d3is used to back-drill concentrically around and through a predetermined depth of the plated pilot hole106(seeFIG. 1C). In the illustrative example, the drill114has a diameter d3in the range of 0.040-0.045″ and is shown going through stacked layers104d,104eand104f. Fourthly, the drill114is removed from the printed circuit board100which now has a via116formed therein that includes the plated through hole102and a back-drilled hole118(seeFIG. 1D).

The traditional method for forming the back-drilled plated through hole102in the printed circuit board100has a major drawback because of a registration tolerance requirement that requires the back-drilled hole118to have a diameter d3that is much larger than the diameter d1of the pilot hole106. In particular, the registration tolerance requirement demands that the drill114used to make the back-drilled hole118have a diameter d3that is at least 0.010″ larger than a diameter of the drill (not shown) used to make the pilot hole106. This means that the diameter of the back-drilled hole118is a lot larger than the diameter of the plated through hole102which negatively impacts the clearance requirements for signal trace(s)105eand ground plane(s)105din the back-drilled layers and associated material insulator layers104d,104eand104f. Accordingly, there is a need for a new way to form a back-drilled pilot through hole in a printed circuit board that overcomes the aforementioned shortcoming of the traditional back-drilled method. This need and other needs are satisfied by the method and printed circuit board of the present invention.

BRIEF DESCRIPTION OF THE INVENTION

The present invention includes a method for forming a back-drilled plated through hole in a printed circuit board and the resulting printed circuit board. The preferred method for forming a plated through hole in a multi-layered printed circuit board includes the following steps: (1) drilling a first hole (e.g., pilot hole) having a first diameter through the printed circuit board; (2) drilling a second hole having a second diameter concentrically around and through a predetermined depth of the first hole in the printed circuit board; (3) plating the second hole and a remaining portion of the first hole in the printed circuit board; and (4) back-drilling a third hole having a third diameter that is substantially the same size (or may be smaller—depending on the size of the pilot hole) as the second hole concentrically around and through the remaining portion of the plated first hole to remove the plated first hole and leave the plated second hole which forms the plated through hole in the printed circuit board.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring toFIGS. 2-4, there is disclosed a preferred embodiment of a printed circuit board200and a preferred method300for making the printed circuit board200. Although the printed circuit board200is described herein as having only one plated through hole202. It should be appreciated that only one plated through hole202is shown for simplicity of illustration and that the printed circuit board200is likely to have a relatively large number of plated through holes202.

Referring toFIG. 2, there is shown a partial cross-sectional side view of the printed circuit board200in accordance with the present invention. The printed circuit board200is described herein as having six stacked material insulator layers (typically laminates and prepregs)204a,204b. . .204fseparating five internal layers205a,205b. . .205eand two external layers408aand408b. Internal layers205a,205band205eare signal layers. And, internal layers205cand205dare plane layers. It should be appreciated that the number of material insulator layers204a. . .204fand signal or plane layers205a. . .205eshown in the printed circuit board200is selected for simplicity of illustration and that the number of layers is not a limitation on the invention.

The printed circuit board200includes a via201extending through the stacked material insulator layers204a. . .204fand signal or plane layers205a. . .205e. The via201includes the plated through hole202and a back-drilled hole203. The plated through hole202is located within a predetermined number of stacked material insulator layers204a. . .204cand signal layers205aand205b. And, the back-drilled hole203is located within the remaining stacked material insulator layers204d. . .204fand signal or plane layers205c. . .205e. As can be seen, the plated through hole202without an electrically conductive material206plated on the walls therein has a diameter that is substantially the same size (shown) or smaller than the diameter of the back-drilled hole203. Detailed discussions about the advantages of having a back-drilled hole203with a diameter that is substantially the same size or smaller than the diameter of the plated through hole202and about how the printed circuit board200can be made are provided below with respect toFIGS. 3-4.

Referring toFIGS. 3 and 4, there are respectively shown a flowchart of a preferred method300for forming the plated through hole202in the printed circuit board200and various partial cross-sectional side views of the printed circuit board200at different stages in the preferred method300. Beginning at step302, a pilot hole402(e.g. first hole402) having a first diameter d1is drilled through the printed circuit board200(seeFIG. 4A). In an illustrative example, the thickness of the printed circuit board200is 0.23″ and the diameter d1of the pilot hole402is 0.020″. It should be appreciated that the size of the pilot hole402and, in particular, how small one can make the pilot hole402is a function of the plate-ability/aspect ratio of the printed circuit board200.

At step304, a second hole403having a second diameter d2is drilled concentrically around and through a predetermined depth of the pilot hole402(seeFIG. 4B). In the illustrative example, the diameter d2of the second hole403which is shown within stacked material insulator layers204a. . .204cis 0.030-0.031″.

At step306, the walls of the remaining portion of the pilot hole402and the walls of the second hole403are plated with a thin layer of a conductive material406(e.g., copper). In addition, predetermined areas408aand408bon the surfaces410aand410bof the printed circuit board200are plated with a thin layer of the conductive material406(e.g., copper) (seeFIG. 4C). In the preferred embodiment, an electroless plating process and then an elecrolytic process can be used to apply the conductive material406on the walls of the remaining pilot hole402and the second hole403and on the predetermined areas408aand408bof the printed circuit board200. In the illustrative example, the diameter d1′ of the plated pilot hole402is 0.016″ and the diameter d2′ of the plated second hole403is 0.026″.

At step308, a drill412having diameter d3is used to back-drill and form a third hole404concentrically around and through the remaining portion of the plated pilot hole402starting from surface410bof the printed circuit board200(seeFIG. 4D). Then the drill412is removed from the printed circuit board200which now has the via201formed therein that includes the pilot through hole202(e.g., plated second hole403) and the back-drilled hole203(e.g., third hole404) (seeFIGS. 2 and 4E). In the illustrative example, the diameter d3of the drill412is in the range of 0.030-0.035″. It should be noticed that the diameter d3of the drill412is typically at least 0.010″ larger (the exact number is dependent on a fabricator's registration capabilities) than diameter d1of the pilot hole402. This is because of the aforementioned registration tolerance requirement which is applicable to method300like it was to the traditional method for back-drilling the printed circuit board100. It should be appreciated that diameter d2does not have to equal diameter d3as shown but could be smaller or larger, but rarely will be larger, depending on the size of the first hole402(e.g., pilot hole402) and other considerations—such as the plating aspect ratio.

In comparing the traditional printed circuit board100shown inFIG. 1Cto the printed circuit board200shown inFIG. 4D, it is apparent that the diameters of the printed through holes102and202are the same but the diameters of the back-drilled holes118and203are different. The diameter d3of the back-drilled hole118in the traditional printed circuit board100is larger than the diameter d3of the back-drilled hole203in the printed circuit board200. In the illustrative examples, the diameters of the printed through holes102and202are 0.026″ and the diameters of the back-drilled holes118and203are respectively 0.040-0.045″ and 0.030-0.035″. Thus, the printed circuit board200has a back-drilled hole203that is 0.010″ smaller than the back-drilled hole118in the traditional printed circuit board100. This leads to several advantages as follows:There is more room to route traces in the back-drilled layer205e.There is more room so one can use wider traces in the back-drilled layer205e.There can be smaller openings in the back-drilled plane layers205cand205d.The back-drilled plane layers205cand205dcan be more continuous.One can increase the electrical air-gap distance between adjacent plated through holes202on outer layers which reduces the likelihood of having lighting strikes between plated through holes202.
All of these advantages help improve the signal integrity and the electromagnetic interference (EMI) performance of the printed circuit board200.

It should be appreciated that the plated pilot hole402is back-drilled by drill412because it is considered a resonant stub and can cause undesirable reflections at certain frequencies if it was not removed from the printed circuit board200. It should also be appreciated that the plated through hole202extends within the printed circuit board200deep enough so that when a pin (not shown) of a component (not shown), if required, is inserted into the plated through hole202then the pin is electrically interconnected with signal traces205aand205bwithin material insulator layers204aand204b. Moreover, it should also be appreciated that method300can be used to form back-drilled pilot holes202in any multi-layer printed circuit board200. For instance, the printed circuit board200can be a high-speed backplane like Alcatel's Litespan 7201 backplane.