WO2003077619A1

WO2003077619A1 - Process for drilling circuit board panels

Info

Publication number: WO2003077619A1
Application number: PCT/US2003/006649
Authority: WO
Inventors: Aric Joseph Olson
Original assignee: Kennametal Inc.
Priority date: 2002-03-06
Filing date: 2003-03-05
Publication date: 2003-09-18

Abstract

Drill stacks and methods of drilling holes in drill stacks. A preferred method of drilling one or more holes in a stack of panels (230, 240, 250) wherein each panel of the stack of panels includes at least one conductive layer (233, 235, 243, 245, 253, 255) and at least one dielectric layer (232, 236, 242, 246, 252, 256), includes: (a) providing a stack of panels (230, 240, 250), a laminate spacer (220), and an entry layer (210); (b) providing a laminate spacer; providing an entry layer (210); forming a drill stack by sandwiching the laminate spacer (220) between the entry material and the stack of panels (230, 240, 250); and (c) drilling a hole (201) that passes through the entry layer (210) and laminate spacer (220) and at least partially through the stack of panels. A preferred drill stack (230, 240, 250) includes a stack of non-woven aramid skinned panels and at least one epoxy resin impregnated woven glass and metal laminate sandwiched between an entry layer and a backer board (260). Another preferred drill stack includes at least three aramid skinned, mechanically drilled panels (230, 240, 250) comprising at least one through hole (201) passing through all of the at least three panels (230, 240, 250) wherein there is substantially no upheaval of the surface copper surrounding the at least one hole.

Description

PROCESS FOR DRILLING CIRCUIT BOARD PANELS

Field of the Invention

[0001] The field of the invention is circuit board formation.

BACKGROUND OF THE INVENTION [0002] During multilayer panel fabrication, it is sometimes desirable to drill holes in one or more partially completed multilayer panels each of which may comprise of a partially cured (b-stage) non- woven aramid (THERMOUNT®) skin. An "aramid skinned panel", as the term is generally used herein, is a panel having a non-woven aramid b-stage layer positioned between a surface copper layer and the next layer in time panel, generally another copper layer. Such panels are generally double sided in that they comprise aramid b-stage and copper layers on opposite sides of the panel with at least one cured internal layer separating the aramid layers. Such panels will often have several alternating copper and cured dielectric inner layers. [0003] To increase throughput, it is generally desirable to form one or more of the aramid 15 skinned panels into a stack of panels and to drill holes through the entire stack rather than through individual panels. Figure 1 helps to illustrate a prior art method by showing a drill stack. The term "drill stack", as the term is generally used herein, refers to a stack of panels sandwiched between entry and exit material layers, hi figure 1, the drill stack 100 comprises a drilled hole 101, an entry material layer 110, non-woven aramid skinned panels 130 and 140, and exit material layer 160. The use of entry and exit material layers is known and it is not uncommon to use a layer of aluminum as entry material layer 110 and a phenolic and paper laminate backer board as exit material layer 160. Drilling hole 101 may involve first drilling through layer 110, then through layers 130 and 140, and finally drilling at least partially into layer 160.

[0004] Unfortunately, it is generally not feasible using known methods to mechanically drill holes in stacks of non-woven aramid skinned panels that are greater than two panels high. This may be due, at least in part, to the random fiber characteristics of non- woven aramid materials. Using such methods to drill holes in non-woven aramid skinned panel stacks greater than two panels high may often result in fuzzy drilled hole-walls, and unacceptable drill bit breakage levels. Another problem often encountered using such methods is deformation of the panels near the entry point of the drill, mounding. As discussed below, Figure 3 provides an example of such "mounding". [0005] It is thus desirable that new methods and devices be developed which facilitate the drilling of holes in non-woven aramid skinned panels stacks having a height greater than two panels.

SUMMARY OF THE INVENTION

[0006] The present invention is directed to drill stacks and methods of drilling holes in drill stacks. In particular, a preferred method of drilling one or more holes in a stack of panels wherein each panel of the stack of panels includes at least one conductive layer and at least one dielectric layer, comprises (a) providing a stack of panels, a laminate spacer, and an entry layer; (b) providing a laminate spacer; providing an entry layer; forming a drill stack by sandwiching the laminate spacer between the entry material and the stack of panels; and (c) drilling a hole that passes through the entry layer and laminate spacer and at least partially through the stack of panels.

[0007] A preferred drill stack comprises a stack of non-woven aramid skinned panels and a spacer comprising at least one epoxy resin impregnated woven glass and metal laminate layers sandwiched between an entry layer and a backer board. The thickness of the laminate spacer can range from between .007 (.0178 cm) and .062 (.15748 cm) inches in thickness.

[0008] Another preferred drill stack comprises at least three aramid skinned, mechanically drilled panels comprising at least one through hole passing through all of the at least three panels wherein there is substantially no mounding of the surface copper surrounding the at least one hole.

[0009] Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention, along with the accompanying drawings in which like numerals represent like components. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Fig. 1 is a cutaway view of a prior art drill stack. [0011] Fig. 2 is a cutaway view of a drill stack embodying the present invention. [0012] Fig. 3 is a cutaway view of a multilayer circuit board having a through hole drilled 5 without the use of a spacer.

[0013] Fig. 4 is a cutaway view of a multilayer circuit board drilled with the use of a spacer.

DESCRIPTION OF THE INVENTION

[0014] Referring first to figure 2, a preferred drill stack 200 comprises a drilled hole 201, an entry material layer 210, laminate spacer 220, aramid skinned panels 230, 240, and 250, and exit material layer 260. The stack of figure 2 differs from the stack of figure 1 primarily because it includes laminate spacer 220. Stack 200 may be formed by sandwiching laminate spacer 220 between entry material layer 210 and the stack of panels 230-250, and by sandwiching laminate spacer 220 and the stack of panels 230-250 between entry material layer 210 and exit material layer 260. Hole 201 may be formed by causing a drill bit to pass through layer 210, then through layer 220, cut into the top surface of the stack of panels, then through panel layers 230-250, and finally into layer 260. [0015] Use of laminate spacer 220 may be particularly advantageous when panels 230-250 each comprise a pair of conductive layers (231 , 237, 241 , 247, 251 , and 257) sandwiching pair of b-stage non- woven aramid reinforced (such as DUPONT® THERMOUNT®) layers (232, 236, 242, 246, 252, and 256), with the b-stage non- woven aramid reinforced layers (232, 236, 242, 246, 252, and 256) sandwiching a multi-layer core that includes at least two conductive layers (233, 235, 243, 245, 253, and 255) and at least one dielectric layer (234, 244, and 254). However, it is contemplated that use of laminate spacer 220 may also be advantageous when used in conjunction with other types of panels as well. Laminate spacer 220 is preferably an FR4 laminate material comprising a cured layer 222 of woven glass impregnated with epoxy resin sandwiched between two copper layers 221 and 223. The thickness of the laminate spacer 220 can range from between .007 inches (.0178 cm) and .062 inches (.15748 cm) in thickness. With thick laminate spacers 220 it has been discovered that the drilled hole in the panels improves, mounding on the top surface of the first panel 230, and fuzzy holes can be reduced or nearly eliminated. However to reduce costs it is desirable to use a thin laminate spacer 220. If the laminate spacer is less than .007 (.0178 cm) inches unsatisfactory mounding occurs on the top surface of the first panel 230. It is believed that such tin laminates are not stiff enough to resist mounding as the hole in the top panel is being drilled. Mounding it has been discovered begins to occur if the thickness of the spacer 220 is less than .012 inches (.03048 cm). Whenever a spacer 220 having a thickness of approximately .013 inches (.03302 cm) or greater is used mounding in the first panel is nearly or completely eliminated. Wherein in one embodiment the laminate 220 is between about .015 (.03810 cm) and about .020 inches (.5080 cm).

[0016] The required thickness of the spacer layer 220 is dependent upon the material composition of the spacer. For spacers having compositions other than DUPONT® THERMOUNT® sandwiched between two conductive layers as described above, the thickness of the spacer layer 220 needed to prevent mounding will vary.

[0017] Entry material layer 210 will typically be chosen at least in part based on the size of holes to be drilled, the composition of panels 230, 240, and 250, and the total number of panels to be drilled. Although the use of a sheet of aluminum may often be advantageously used as entry material layer 210, it is contemplated that the laminate spacer 220 may be advantageously used with other types of entry materials including aluminum/paper composites, phenolic, phenolic/paper composites and other well-known compositions in the industry. Any and all material that is easy to drill and functions as a bushing to assist in guiding and holding the drill in proper alignment during cutting would be suitable as an entry material layer 210.

[0018] Similarly, exit material layer 260 will typically be chosen at least in part based on the size of holes to be drilled, the composition of panels 230, 240, and 250, and the total number of panels to be drilled. Although the use of a backer board comprising a sheet of phenolic material sandwiched between paper layers may often be advantageously used, it is contemplated that the laminate spacer 220 may be advantageously used with other types of back boards/exit material layers as well. A harder material such as phenolic is better at reducing undesirable burring around the drilled exit hole on the bottom panel 260 than aluminum. Aluminum is a softer material and does not provide as much support against the bottom panel 250 so that burring is more likely to occur when the drill exits the bottom surface of panel 250. Aluminum offers the advantage that it provides some support and is softer to drill resulting in a lower drill wear rate and less drill breakage than a phenolic exit material layer.

[0019] In some instances were mounding is the only potential problem in drilling stacks of panels it may be advantageous to not include either entry material layer 210 and/or exit material layer 260 as part of the drill stack and only incorporate a spacer 220. Such instances may occur and are dependent on the size, shape and speed of the drill; and the thickness and composition of the panels.

[0020] It should be noted that, using the described drill stack and method, it is possible to drill a stack of at least thee aramid skinned, mechanically drilled panels without mounding, without fuzzy drilled hole-walls, burring and without unacceptable drill bit breakage levels. Figure 4 provides an example of a section of the topmost panel of such a stack after drilling a hole 320 using the described method. The panel comprises said hole 320 having a central longitudinal axis A — A, a top surface 340 formed by the exterior surface of an outer conductive layer 330, a plurality of aramid layers 300 sandwiching at least one conductive layer 331 there between. Figure 3 illustrates a top section of a panel that had a hole drilled therein, without a spacer; the panel is otherwise similarly constructed as the panel shown in Figure 4. The panel shown in Figure 3 is magnified approximately 2X that of the panel illustrated in Figure 4. As can be seen by comparison with figure 3, the perimeter 325 of the hole 320 in figure 4 is less distorted than perimeter 325P of the hole 320P of figure 3 in that there is no mounding/upheaval of the surface copper 330 surrounding the 25 hole 320. The perimeter 325 of the end of the hole shown in figure 4 is substantially planar and level with the surrounding top surface 340 of the panel. In contrast, the perimeter 325P of the end of the hole 320P shown in figure 3 is substantially higher than the surface 340P. Moreover, the hole 320 of figure 4 does not have aramid fibers protruding into the hole wall 321 as would a prior-art "fuzzy" hole. [0021] Thus, specific embodiments and applications of circuit board drilling methods and devices have been disclosed. It should be apparent, however, to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive subject matter, therefore, is not to be restricted except in the spirit of the appended claims. Moreover, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible manner consistent with the context.

Claims

CLAIMSWhat is claimed is:

1. A method of drilling at least one panel, the at least one panel having a top surface comprising the steps of: placing a spacer adjacent said top surface, placing an entry material adjacent said spacer, and drilling through the entry material, through the spacer, and into the at least one panel.

2. The method of claim 1, wherein said spacer is a laminate material.

3. The method of claim 2, wherein said laminate spacer includes a cured layer of woven glass impregnated with epoxy resin.

4. The method of claim 3, wherein said cured layer is sandwiched between two copper layers.

5. The method of claim 4, wherein said entry material is a composition selected from the group comprising; aluminum, phenolic, phenolic/paper composites and paper composites.

6. The method of claim 5, wherein said entry material is aluminum.

7. The method of claim 1 , wherein said at least one panel is a stack of at least two panels.

8. The method of claim 1, wherein said laminate spacer has a thickness greater than 0.3302 centimeters.

9. The method of claim 1 , wherein said laminate spacer has a thickness between 0.3810 centimeters and 0.15748 centimeters.

10. The method of claim 4, wherein said laminate spacer has a thickness between 0.3810 centimeters and 0.15748 centimeters.

11. The method of claim 1 wherein said at least one panel is aramid.

12. The method of claim 4 wherein said at least one panel is aramid.

13. The method of claim 1 wherein said at least one panel is a printed circuit board.

14. A method of drilling at least one panel, the at least one panel having a top surface comprising the steps of: placing said at least one panel on an exit material layer, placing a spacer adjacent said top surface, placing an entry material adjacent said spacer, and drilling through the entry material, through the spacer, and into the at least one panel.

15. The method of claim 14 wherein said laminate spacer includes a cured layer of woven glass impregnated with epoxy resin and said cured layer is sandwiched between two copper layers.

16. The method of claim 15 wherein said exit material layer is a composition selected from the group comprising; aluminum, phenolic and phenolic/paper composites.

17. The method of claim 14 wherein said at least one panel is aramid.

18. A spacer for use in drilling printed circuit panels comprising: a cured layer of woven glass impregnated with epoxy resin and said cured layer is sandwiched between two copper layers.

19. The spacer of claim 18 wherein said laminate spacer has a thickness greater than 0.3810 centimeters.

20. The spacer of claim 19 wherein said laminate spacer has a thickness less than 0.15748 centimeters.