WO2010127496A1

WO2010127496A1 - Back drilling method of through via, circuit board and manufacturing method of circuit board

Info

Publication number: WO2010127496A1
Application number: PCT/CN2009/071680
Authority: WO
Inventors: Victor Taveras
Original assignee: Unimicron Technology (Shenzhen) Corp.
Priority date: 2009-05-08
Filing date: 2009-05-08
Publication date: 2010-11-11
Also published as: CN102007826A; CN102007826B

Abstract

A back-drilling method of through vias in a circuit board and a circuit board made by said method are disclosed. The back-drilling method comprises the steps of: using a drill machine for measuring a localized thickness of the circuit board at a location having a target through via; determining a back drill depth at the location to be back-drilled according to the measured localized thickness and a predetermined percentage; and back-drilling the target through via to the determined depth, thereby forming a back-drilled via extending to a preselected target layer. The circuit board comprises a circuit board body comprising a plurality of spaced conductive layers and a plurality of dielectric layers sandwiched between the conductive layers; and a plurality of through vias drilled through the circuit board body. The through vias are made by said method.

Description

BACK DRILLING METHOD OF THROUGH VIA, CIRCUIT BOARD AND MANUFACTURING METHOD OF CIRCUIT

BOARD

Technical Field

[1] The present invention relates generally to manufacture technologies of circuit boards, and particularly to a back-drilling method of through via to remove selected portions, a circuit board and a manufacturing method of the circuit board. Background Art

[2] In decades, circuit board, especially printed circuit boards (PCBs), are widely used to hold and electrically connect selected electronic components. Printed circuit boards are generally comprised of copper electrical interconnects separated by levels of insulating dielectric materials. Connections between layers are often done using through vias or plated through holes (PTHs). Through vias, i.e., plated through holes, are typically drilled in the vertical direction from top to bottom of the selected layers to which electrical connection is required.

[3] With the advent of wireless digital communication technologies, such as wireless networking, mobile phones, GPS (Global Positioning System), etc., the design and manufacture of high-speed digital circuit boards that handle digital signals within the gigahertz range is in high demand in the electronics industry. In circuit layout design, high-speed digital circuit boards typically use micro-strips, or strip-lines, for transmission of digital signals within the range of high frequencies, typically from 1 GHz to 10 GHz (gigahertz). It is to be noted that throughout this patent specification, the term "high-speed digital signal" refers to a digital signal with a frequency above 1 GHz.

[4] High-speed digital circuits are typically constructed on a multi-layer circuit board composed of multiple circuit layers. In the multi-layer structure, signal transmission lines between different circuit layers are interconnected by through vias. Each through via is plated with a conductor, such as copper, and a pad is formed to connect the through via to a particular one of the conductive layers. Fig. 1 shows a traditional highspeed digital signal transmission structure. As shown in Fig. 1, a multi-layer circuit board 10 includes at least one upper layer 11, at least one lower layer 12 and a plurality of internal layers 13 formed between the on the upper layer 11 and the lower layer 12. Through vias 15 which penetrate through the multi-layer circuit board 10 for interconnecting particular conductive layers, are formed in particular layers using a drilling process. A pad 16 is formed on an upper surface of the upper layer 11 and is connected to an interior conductive layer, for example interior layer 13a, by the through via 15. Accordingly, the through vias 15 may be utilized to conduct a signal from the pad 12 (which is generally connected to an electronic component) to the interior conductive layer 13 a.

[5] In this situation, one drawback to the provision of the aforementioned via 15, however, is that the via 15 only has a small portion 15a (e.g., upper transmission portion) which is actually used for signal transmission. As a result, the via 15 has a lengthy portion 15b on the bottom side which effectively act as an open stub, i.e., an unused portion. The existence of the open stub 15b would then undesirably cause the transmitting high-speed digital signal to be subject to resonance and thus undesirably degrade signal integrity.

[6] One solution to the foregoing problem is to utilize a back-drilling method using a traditional drill equipment to remove the open stub 15b of the through via 15. Advantages to backdrilling the through via include: less signal attenuation due to improved impedance matching, increased channel bandwidth, reduced EMI/EMC radiation from the stub end, reduced excitation of resonance modes, and reduced via-to-via crosstalk.

[7] Back-drilling necessarily entails a tradeoff between manufacturing costs and electrical performance. Its effectiveness is limited by drilling depth accuracy and the increased cost of multi-depth drilling. As shown in Fig. 2, a conventional backdrilling machine is equipped with a touch sensing system 17. A drill bit 18 is provided at one end of the touch sensing system 17, for drilling from the upper surface of the circuit board 10 to the target layer 13a (See Fig. 1). The upper surface of the circuit board 10 is provided with an aluminum plane. In operation, when the drill bit touches the aluminum entry, a current change will be detected by the sensing system 17, and transferred to CNC as signal. This position becomes the zero reference of the depth to be drilled. The machine then drills down to a predetermined depth equivalent to the depth of the target layer.

[8] Figs. 3A and 3B show representative eye-diagrams of two via structures, one (Fig.

3A) with an intact stub and the other (Fig. 3B) without a stub. Comparing the two figures, it can be shown that via stubs introduce horizontal pedestals in the logic 0 to 1 and logic 1 to 0 transitions. These pedestals close the eye, making it more difficult for the digital receiver to ascertain whether the received signal is truly a logical one or a logical zero. As illustrated in Fig. 3B, "Stubless" vias have clean logic transitions and correspondingly larger open eyes.

[9] In the conventional machine, it can work well and the target layer can be accurately cut by setting one depth when the circuit board is very flat, for example circuit board 30 as shown in Fig. 4A. The drill bit 18 can accurately hit the target layer 13a. However, in effect, the surface of the circuit board 10 is always uneven, for example circuit board 40 having an uneven surface 42, as shown in Fig. 4B. As a result, this uneven profile of the surface 42 brings out uneven or fluctuant depths in different positions. The drill bit 18 will miss or overshoot the target layer 13a in some areas due to thickness variation of the circuit board 40. [10] Therefore, an effective and accuracy drilling method of the through via is required for.

Disclosure of Invention

Technical Problem [11] In accordance with an aspect of the present invention, a back-drilling method of through vias in a circuit board is provided, which reduces the occurrence of missing or overshooting to the target layer during back-drilling operation. [12] In accordance with another aspect of the present invention, a manufacturing method of a circuit board is provided, which reduces the occurrence of missing or overshooting to the target layer during back-drilling operation. [13] In accordance with still another aspect of the present invention, a circuit board is provided and has accurately shot back-drill vias.

Technical Solution

[14] The back-drilling method of through vias in a circuit board comprises steps of:

[15] using a drill machine for measuring a localized thickness of the circuit board at a location having a target through via therethrough; [16] determining a back drill depth at the location to be backdrilled according to the measured localized thickness and a predetermined percentage; and [17] back-drilling the target through via to the determined depth, thereby forming a backdrilled via extending to a preselected target layer. [18] The manufacturing method of a circuit board comprises steps of:

[19] providing a circuit board body including a plurality of spaced conductive layers and a plurality of dielectric layers sandwiched between the conductive layers; [20] drilling a plurality of through vias through the circuit board body at predetermined positions; [21] using a drill machine for measuring a localized thickness of the circuit board at a location with a target through via to be backdrilled; [22] determining a back drill depth at the location to be backdrilled according to the measured localized thickness and a predetemined percentage; and [23] back-drilling the target through via to the determined depth, thereby forming a backdrilled via extending to a preselected target layer. [24] The circuit board comprises a circuit board body and a plurality of through vias drilled through the circuit board body. The circuit board body comprises a plurality of spaced conductive layers and a plurality of dielectric layers sandwiched between the conductive layers. At least some of the through vias have respective backdrilled vias. Each backdrilled via has a depth and extends to a preselected target layer. The depth is determined by a localized thickness of the circuit board at a location which one respective through via is drilled through and a predetermined percentage. Advantageous Effects

[25] Specifically, for an identical circuit board and layers arrangement therein, the back- drilling method described above can achieve a relatively more accuracy hit to the target layer due to the backdrilled depth depending on a predetermined percentage. In other words, the back-drilling method can reduce the drilling error, thereby preventing missing or overshooting the target layer. Accordingly, a circuit board with accurately backdrilled via is obtained and has an excellent performance, e.g., transmission of high-speed digital signal.

[26] This summary is an overview of some of the teachings of the present application and is not intended to be an exclusive or exhaustive description of the present subject matter. Further details about the present subject matter are found in the detailed description and appended claims. Other aspects of the present subject matter will be apparent to persons skilled in the art upon reading and understanding the following detailed description and viewing the drawings that form a part thereof, each of which are not to be taken in a limiting sense. Description of Drawings

[27] Fig. 1 is a cross sectional side view of a conventional circuit board with through vias and a backdrilled via therein .

[28] Fig. 2 is a schematic view of a traditional backdrilling machine during drilling operation. Fig. 3A is an eye diagram showing the characteristic of rise time achieved by the through vias in Fig. 1.

[29] Fig. 3B is an eye diagram showing the characteristic of rise time achieved by the backdrilled vias in Fig. 1.

[30] Fig. 4A is a cross sectional view showing backdrilled vias extending to a target layer of a circuit board having an ideal flat surface using a traditional backdrilling method.

[31] Fig. 4 B is similar to Fig. 4A, but showing backdrilled vias cut in a circuit board having an actual uneven surface.

[32] Fig . 5 i llustrates a flow chart of a back-drilling method of through vias according to an embodiment of the present invention.

[33] Fig. 6 is a structural, cross sectional view showing a drilling operation of the backdrilling method in Fig. 5.

[34] Fig. 7 is a schematic view showing a principle of the back-drilling method as stated in Fig. 5. Best Mode

[35] Referring to Fig. 5, a flow chart of a back-drilling method of through vias is illustrated, in accordance with an embodiment of the present invention. The back- drilling method includes the steps of:

[36] SOl : using a drill machine for measuring a localized thickness of the circuit board at a location having a target through via therethrough;

[37] S02: determining a back drill depth at the location to be backdrilled according to the measured localized thickness and a predetermined percentage; and

[38] S03: back-drilling the target through via to the determined depth, thereby forming a backdrilled via extending to a preselected target layer.

[39] The back-drilling method is achieved by using a drill machine 21, as shown in Fig. 6.

The drill machine 21 includes a touch sensing member 22 and a drill bit 23 provided at an end of the touch sensing member 22. The drill machine 21 is, generally, supported on a working table 24. In step SOl, the measuring of the localized thickness Ti of the circuit board 30 advantageously comprises the steps of: measuring height Ri of the working table 24; measuring height Zi of the start location to be backdrilled; and calculating height difference between the working table 24 and the loaction, i.e., Ti = Zi - Ri, thereby obtaining the localized thickness Ti.

[40] Referring to Fig. 7, a structural model of a cross section of the circuit board 30 is shown to provide a detailed description of principle utilized in the back-drilling method. The circuit board 30 is supported on the working table 24 and has a back surface 31, which the back-drilling is started from, and a front surface 32 opposite to the back surface 31. The back surface 31 of the circuit board 30 is uneven in actual implementation. In the illustrated example, three exemplary through vias 33 are drilled through different locations in the circuit board 30. Then, these through vias 33 are drilled from the back surface 31 to a target layer 35 indicated by an imaginary line, in order to remove unused portions, namely stubs. When reaching to the target layer 35, the back-drilled vias 34 should have different depths, respectively designated as tl, t2, and ti (i represents any location having one through via), due to the uneven back surface 31. Likewise, the locations, which the through vias are drilled through, have different thickness, respectively designated as Tl, T2 and Ti. When calculating the ratio of the depths tl, t2, ti over the thickness Tl, T2, Ti, respectively, it is found that the ratio remains relatively constant. That is, the ratio of the depths of backdrilled vias with respect to the back surface 31 to the target layer 35 over the thickness of respective locations from the back surface 31 to the front surface 32 remains relatively constant K at any given locations. The ratio is as follows:

[41] tl/Tl « t2/T2 « ti/Ti « K; [42] Wherein ti is depth of the target layer with respect to the back surface 31 at a given location i; Ti is overall thickness at the location i; and K is the ratio of ti/Ti at percentage (of ti/Ti x 100%).

[43] Fig. 7 shows an actual surface of the circuit board to be back-drilled through. This actual surface of the circuit board has an uneven or fluctuant profile, which is represented by thickness in the range of, for example, from about 240 to 280 mils at various locations. Due to the uneven or fluctuant profile of the surface, it is almost not possible to accurately hit the target layer of the circuit board by setting one depth as used in a conventional back-drilling method. However, the back-drilling can accurately hit the target layer by setting a percentage, as used in the present invention. For example, it is supposed that layers L1-L9 are cut through and the stubs of L1-L9 have various depths tl, t2, ti, with respect to the actual surface, for example, in Fig. 7. The actual percentages at any locations remain approximately a constant 23%. Accordingly, when a back-drilling is performed through layers L1-L9, the percentage could be preset as, e.g., 23%. As shown in Table 1, a biggest percentage error of back- drilling extending through layers L1-L9 is less than 0.9%. This error can substantially be acceptable to demand of precisions in the back-drilling method.

[44] Likewise, for example, when layers L1-L26 is drilled through, the actual percentages at any locations remain approximately a constant 67%, for example, 66.4%, 67.4%, and 66.9% as shown in Table 2. Accordingly, when using percentage 67% as a back- drilling parameter, the actual back-drilling operation can reach the target layer, with a percentage error less than 1.6%. Accordingly, the information as shown in Tables 1 and 2 validates the effect of percentage used in back-drilling operation.

[45] In step S02, the percentage depends on the ratio of layer depth of the target layer to total thickness of the circuit board (i.e., board thickness in this location). Specifically, the percentage is verified to be in accordance with design by using a cross-sectional slice method. The slice method comprises the steps of: selecting a plurality of slices at a plurality of locations to be backdrilled; measuring total thickness of the circuit board at the plurality of locations; measuring layer depths of the target layer at the plurality of locations; and obtaining the percentage based on layer depth of the target layer and the total thickness of the circuit board at the plurality of locations. In one embodiment, the percentage is usefully the average ratio of layer depth of the target layer and the total thickness of the circuit board at the plurality of locations (of ti/Ti x 100%).

[46] Tables 1 and 2 illustrate target depth information and target depth percentage information when back-drilling two kinds of vias across layers L1-L9 and layers L1-L26 in an example of circuit board, respectively. In the two tables, the number of slices is fifteen, i.e., cut at fifteen different locations of the circuit board. Then the depths of layers L1-L9 and Layers L1-L26 at fifteen locations are respectively measured and record in the Table 1. The total thickness of the circuit board at fifteen locations is measured and record in the Table 1. Each percentage of layers L1-L9 and Layers L1-L26 at each location is obtained according to the layer depth of the target layer and the total thickness of the circuit board at the respective location, i.e., by calculating the ratio of the layer depth to the respective total thickness, as shown in Table 2.

[47] Further, two groups of average depths, maximum depths, and minimum depths of

L1-L9 and L1-L26 at fifteen locations are calculated and shown in Table 1. An average board thickness, a maximum board thickness, and a minimum board thickness of the circuit board at fifteen locations are calculated and shown in Table 2. Two groups of average percentages, maximum percentages, and minimum percentages of L1-L9 and L1-L26 at fifteen locations are calculated and shown in Table 1. It is shown in Tables 1 and 2 that the percentages of L1-L9 and L1-L26 may advantageously use 22.9% and 66.8%, namely the average percentages, respectively. When expressed as absolute depth (in mil), the target layer varies up to a maximum of 10.8mil from one location to another for the circuit board. However, when expressed as a percentage (of ti/Ti x 100%), the variation becomes about 1.6% of 268.5 mil, or 4.3 mil maximum. Therefore, the back-drilling method according to the embodiment of the present invention can achieve a relatively more accuracy hit to the target layer. In other words, the back-drilling method according to the embodiment of the present invention can reduce the drilling error, thereby preventing missing or overshooting the target layer and improving performance of the circuit board.

[48] Table 1 Target depth information (mil) of an example of circuit board

[Table 1] [Table ]

[49] Table 2 Target depth information (%) of an example of circuit board

[Table 2] [Table ]

[50] Tables 3 and 4 illustrate six groups of silces at fifteen locations, respectively cut across L1-L4, L1-L7, Ll-LlO, L1-L13, L1-L16, L1-L18 in another example of circuit board (total layers 26). In accordance with the same comparison manner as used for Tables 1 and 2, it is can be found that, when expressed as absolute depth (in mil), the target layer varies up to a maximum of 13.7 mil from one location to another for the circuit board. However, when expressed as a percentage (of ti/Ti x 100%), the variation becomes about 1.3% of 262.8 or 3.4mil maximum. Thus, the occupied percentage of the back-drilling depth error in layer thickness of the target layer is less than 2%. For an identical circuit board and layers arrangement therein, the back- drilling method of the present invention can significantly reduce the drilling/hit error resulting from the uneven surface of the circuit board.

[51] Table 3 Target depth information (mil) of another example of circuit board [Table 3] [Table ]

[52] Table 4 Target depth information (%) of another example of circuit board [Table 4] [Table ]

[53] After verifying that the theoretical back drill depth matches the actual back drill depth at the location to be backdrilled according to above method, the drill bit is driven to move down to the target layer, thereby removing the unused portions, i.e., stubs. As a result, a backdrilled via is formed on the back portion (relative to useful transmission front portion) of the through via, thereby substantially achieving step S03. The backdrilled via extends to the target layer and has a depth as calculated in step S02.

[54] The back-drilling method above described is typically used to manufacture a circuit board. Specifically, before performing the back-drilling process (e.g., before Step 01), a circuit board body is provided, including a plurality of spaced conductive layers and a plurality of dielectric layers sandwiched between the conductive layers. Then a plurality of through vias is drilled through the circuit board body at predetermined positions, for example, using a convention drilling method. The circuit board could be used for a specific function, e.g., transmitting high-speed digital signal. For improving transmission performance of high-speed digital signal, the plurality of through vias is typically subject to the back-drilling process including steps S01-S03.

[55] The circuit board manufactured by above process includes a circuit board body and a plurality of through vias drilled through the circuit board body. The circuit board body comprises a plurality of spaced conductive layers and a plurality of dielectric layers sandwiched between the conductive layers. At least some of the through vias have respective backdrilled vias. Each backdrilled via has a depth and extends to a preselected target layer. The depth is determined by a localized thickness of the circuit board at a location which one respective through via is drilled through and a predetermined percentage. Specifically, the depth is equal to the multiple of the localized thickness and the predetermined percentage. It is to be understood that the circuit board advantageously includes each of the features described and formed in above-mentioned method.

[56] It is to be realized that, for an identical circuit board and layers arrangement therein, the back-drilling method described above can achieve a relatively more accuracy hit to the target layer due to the backdrilled depth depending on a predetermined percentage. In other words, the back-drilling method can reduce the drilling error, thereby preventing missing or overshooting the target layer. Accordingly, a circuit board with accurately backdrilled via is obtained and has an excellent performance, e.g., transmission of high-speed digital signal.

[57] The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

Claims

[1] A back-drilling method of through vias in a circuit board, comprising steps of: using a drill machine for measuring a localized thickness of the circuit board at a location having a target through via therethrough; automatically determining a back drill depth at the location to be backdrilled according to the measured localized thickness and a predetermined percentage; and back-drilling the target through via to the determined depth, thereby forming a backdrilled via extending to a preselected target layer.

[2] The back-drilling method of claim 1, wherein the percentage depends on the ratio of layer depth of the target layer to total thickness of the circuit board.

[3] The back-drilling method of claim 1, wherein the percentage is substantially equal to the ratio of layer depth of the target layer to total thickness of the circuit board.

[4] The back-drilling method of claim 1, wherein the percentage is verified by using a cross-sectional slice method, the slice method comprising the steps of: selecting a plurality of slices at a plurality of locations to be backdrilled; measuring total thickness of the circuit board at the plurality of locations; measuring layer depth of the target layer at the plurality of locations; and obtaining the percentage based on the measured layer depth of the target layer and the total thickness of the measured circuit board at the plurality of locations.

[5] The back-drilling method of claim 1, wherein the measuring of the localized thickness of the circuit board comprising the steps of: measuring height of a working table for supporting the circuit board thereon; measuring height of the start location to be backdrilled; and calculating height difference between the working table and the location, thereby obtaining the localized thickness.

[6] The back-drilling method of claim 1, wherein the occupied percentage of the back-drilling depth error in layer thickness of the target layer is less than 2%.

[7] A manufacturing method of a circuit board, comprising steps of: providing a circuit board body including a plurality of spaced conductive layers and a plurality of dielectric layers sandwiched between the conductive layers; drilling a plurality of through vias through the circuit board body at predetermined positions; using a drill machine for measuring a localized thickness of the circuit board at a location with a target through via to be backdrilled; determining a back drill depth at the location to be backdrilled according to the measured localized thickness and a predetermined percentage; and back-drilling the target through via to the determined depth, thereby forming a backdrilled via extending to a preselected target layer.

[8] The manufacturing method of claim 7, wherein the percentage depends on the ratio of layer depth of the target layer to total thickness of the circuit board.

[9] The manufacturing method of claim 7, wherein the percentage is substantially equal to the ratio of layer depth of the target layer to total thickness of the circuit board.

[10] The manufacturing method of claim 7, wherein the percentage is verified by a cross-sectional slice method, the slice method comprising the steps of: selecting a plurality of slices at a plurality of locations to be backdrilled; measuring total thickness of the circuit board at the plurality of locations; measuring layer depth of the target layer at the plurality of locations; and obtaining the percentage based on the measured layer depth of the target layer and the measured total thickness of the circuit board at the plurality of locations.

[11] The manufacturing method of claim 7, wherein the measuring of the localized thickness of the circuit board comprising the steps of: measuring height of a working table for supporting the circuit board thereon; measuring height of the start location to be backdrilled; and calculating height difference between the working table and the location, thereby obtaining the localized thickness.

[12] The manufacturing method of claim 7, wherein the occupied percentage of the back-drilling depth error in layer thickness of the target layer is less than 2%.

[13] A circuit board, comprising: a circuit board body comprising a plurality of spaced conductive layers and a plurality of dielectric layers sandwiched between the conductive layers; and a plurality of through vias drilled through the circuit board body, at least some of the through vias having respective backdrilled vias, each backdrilled via having a depth and extending to a preselected target layer, the depth being determined by a localized thickness of the circuit board at a location which one respective through via is drilled through and a predetermined percentage.

[14] The circuit board of claim 13, wherein the circuit board has a high-speed digital signal transmission structure.

[15] The circuit board of claim 13, wherein the percentage depends on the ratio of layer depth of the target layer to total thickness of the circuit board.

[16] The circuit board of claim 13, wherein the percentage is substantially equal to the ratio of layer depth of the target layer to total thickness of the circuit board.

[17] The circuit board of claim 13, wherein the occupied percentage of the back- drilling depth error in layer thickness of the target layer is less than 2%.