High-speed router with backplane using muli-diameter drilled thru-holes and vias

• US 20090045889A1
• Filed: 08/13/2007
• Published: 02/19/2009
• Est. Priority Date: 08/13/2007
• Status: Active Grant

• Current Assignee: Dell Marketing Corporation (Dell Technologies Inc.)
• Original Assignee: Force10 Networks, Inc. (Dell Technologies Inc.)
• Inventors: Goergen, Joel R., Hunt, Greg
• Granted Patent: US 8,158,892 B2
• Time in Patent Office: Days
• US Class Current: 333/175
• CPC Class Codes: H05K 1/0216 Reduction of cross-talk, noise or electromagnetic interference grounding H05K1/0215
• CPC Class Codes: H05K 1/0237 High frequency adaptations H05K1/0216 takes precedence
• CPC Class Codes: H05K 2201/044 Details of backplane or midplane for mounting orthogonal PCBs
• CPC Class Codes: H05K 2201/09236 Parallel layout
• CPC Class Codes: H05K 2201/09845 Stepped hole, via, edge, bump or conductor
• CPC Class Codes: H05K 2203/0207 Partly drilling through substrate until a controlled depth, e.g. with end-point detection
• CPC Class Codes: H05K 2203/0242 Cutting around hole, e.g. for disconnecting land or Plated Through-Hole [PTH] or for partly removing a PTH
• CPC Class Codes: H05K 2203/175 Configurations of connections suitable for easy deletion, e.g. modifiable circuits or temporary conductors for electroplating; Processes for deleting connections
• CPC Class Codes: H05K 3/0047 Drilling of holes
• CPC Class Codes: H05K 3/429 Plated through-holes specially for multilayer circuits, e.g. having connections to inner circuit layers
• CPC Class Codes: Y10T 29/49165 by forming conductive walled aperture in base

Abstract

A high-speed router backplane is disclosed. The router backplane uses differential signal pairs on multiple signal layers, each sandwiched between a pair of digital ground layers. Thru-holes are used to connect the differential signal pairs to external components. To reduce routing complexity, at least some of the differential signal pairs route through a via pair, somewhere along their path, to a different signal layer. At least some of the thru-holes and vias are drilled to reduce an electrically conductive stub length portion of the hole. The drilled portion of a hole includes a transition from a first profile to a second profile to reduce radio frequency reflections from the end of the drilled hole.

Claims

1. A circuit board comprising:

a plurality of conductive layers separated by insulating layers, at least some of the conductive layers patterned to contain traces; and

a first hole through the circuit board and having a conductive liner in a first longitudinal segment of the hole, the conductive liner electrically connected to a first one of the traces on at least a first one of the conductive layers, the hole having a second longitudinal segment with no conductive liner, adjacent to a first end of the hole, the second longitudinal segment of the hole comprising at least one transition from a first profile adjacent the first end of the hole to a second profile longitudinally separated from the first end of the hole.

2. The circuit board of claim 1, wherein the first hole is a thru-hole providing an electrical connection to a first component mated to the circuit board.

3. The circuit board of claim 2, wherein the first component is mated to the circuit board on a side opposite the first end of the through hole.

4. The circuit board of claim 1, wherein the hole is a via providing an electrical connection between the first one of the traces and a second one of the traces located on a second one of the conductive layers.

5. The circuit board of claim 4, the via having a third longitudinal segment with no conductive liner, adjacent to the second end of the hole, the third longitudinal segment comprising at least one transition from a third profile adjacent the second end of the hole to a fourth profile longitudinally separated from the second end of the hole.

6. The circuit board of claim 5, wherein the first and third profiles are substantially equivalent and the second and fourth profiles are substantially equivalent.

7. The circuit board of claim 1, wherein the length of the second longitudinal segment is substantially equal to the longitudinal depth of the first one of the conductive layers from the first end of the hole, minus a stop clearance.

8. The circuit board of claim 1, wherein the first and second profiles are substantially round, the first profile having a diameter at least ten percent greater than the second hole profile.

9. The circuit board of claim 1, wherein the transition between the first and second profiles occurs approximately midway along the length of the second longitudinal segment.

10. The circuit board of claim 1, wherein the length of the section of the second longitudinal segment having the second profile is related to a primary radio frequency energy component to be suppressed, with the section length substantially different from any integer number of half-wavelengths of the primary radio frequency.

11. The circuit board of claim 1, wherein the length of the section of the second longitudinal segment having the first profile is related to a primary radio frequency energy component to be suppressed, with the section length substantially different from any integer number of half-wavelengths of the primary radio frequency.

12. The circuit board of claim 1, wherein the length of the section of the second longitudinal segment having the second profile is related to a primary radio frequency energy component to be suppressed, with the section length substantially equal to a quarter-wavelength of the primary radio frequency.

13. The circuit board of claim 1, wherein the first hole is a thru-hole providing an electrical connection to a first component mated to the circuit board, the circuit board further comprising a second thru-hole providing an electrical connection to a second component mated to the circuit board, the second thru-hole having a conductive liner in a first longitudinal segment of the second hole and electrically connected to one of the traces on one of the conductive layers, the second thru-hole having a second longitudinal segment with no conductive liner, adjacent to a first end of the second hole, the second longitudinal segment of the second hole comprising at least one transition from the first profile adjacent the first end of the second hole to the second profile longitudinally separated from the first end of the second hole.

14. The circuit board of claim 13, wherein the first and second thru-holes are substantially similar and connected to opposite ends of the first one of the traces.

15. The circuit board of claim 13, wherein the trace connected to the second thru-hole is a second trace on a second one of the conductive layers.

16. The circuit board of claim 15, wherein the length of the second longitudinal segment of the first thru-hole is substantially equal to the longitudinal depth of the first one of the conductive layers from the first end of the first hole minus a stop clearance, and the length of the second longitudinal segment of the second through hole is substantially equal to the longitudinal depth of the second one of the conductive layers from the first end of the second hole minus the stop clearance.

17. The circuit board of claim 16, wherein for the second longitudinal segments of the first and second holes, the lengths of the segments having the second hole profile are each substantially equal to half the respective lengths of the second longitudinal segments.

18. The circuit board of claim 16, wherein for the second longitudinal segments of the first and second holes, the lengths of the sections of the second longitudinal segments of the first and second holes having the second hole profile are substantially equal.

19. The circuit board of claim 15, further comprising a via providing an electrical connection between the first one of the traces and the second one of the traces.

20. The circuit board of claim 19, the via having a conductive liner in a first longitudinal segment of the via and electrically connected to the first and second traces, the via having a second longitudinal segment with no conductive liner, adjacent to a first end of the via, the second longitudinal segment of the via comprising at least one transition from a third profile adjacent the first end of the via to a fourth profile longitudinally separated from the first end of the via.

21. The circuit board of claim 20, the via having a third longitudinal segment with no conductive liner, adjacent to the second end of the via, the third longitudinal segment comprising at least one transition from the third profile adjacent the second end of the hole to the fourth profile longitudinally separated from the second end of the hole.

22. The circuit board of claim 20, the conductive liner in the first longitudinal segment of the via passing through a nonfunctional conductive pad on at least one of the conductive layers.

23. A circuit board comprising:

a plurality of conductive trace layers and a plurality of conductive ground plane layers separated by insulating layers, the conductive layers arranged such that the conductive trace layers are each positioned between a pair of ground plane layers, the conductive trace layers each comprising a plurality of differential signaling trace pairs; and

a plurality of hole pairs positioned at the termination points of the differential trace pairs, at least a first subset of the holes having a partial conductive liner electrically contacting the differential trace pair termination point associated with those holes, the holes in the first subset each comprising an end section with no conductive liner, the end section comprising at least one transition from a first profile to a second profile, wherein the hole pairs included in the first subset consist of hole pairs that are substantially identical to each other.

24. The circuit board of claim 23, wherein the first subset of the holes comprises signaling thru-holes mated to components on a top side of the circuit board and coupled to conductive trace layers positioned in the top half of the circuit board.

25. The circuit board of claim 24, at least a second subset of the holes comprising signaling thru-holes mated to components on a top side of the circuit board and coupled to conductive trace layers positioned in the bottom half of the circuit board, the second subset of the holes having a complete conductive liner.

26. The circuit board of claim 25, the second subset of the holes passing through and electrically connected to nonfunctional conductive pads on selected ones of the conductive ground plane layers.

27. The circuit board of claim 23, wherein a second subset of the first subset of the holes comprises via pairs, each via in each such pair having a partial conductive liner electrically contacting two differential trace pair termination points on two different conductive trace layers.

28. The circuit board of claim 27, wherein when the two different conductive trace layers exist in the top half of the circuit board, the end section of the via with no conductive liner is formed in the bottom end of the circuit board, and when the two different conductive trace layers exist in the bottom half of the circuit board, the end section of the via with no conductive liner is formed in the top end of the circuit board.

29. The circuit board of claim 27, wherein the via pairs have an end section with no conductive liner that is of a different length than the end section of the other holes in the first subset.

30. The circuit board of claim 23, wherein the transition from the first profile to a second profile occurs approximately midway along the end section in each hole of the first subset.

31. A router comprising the circuit board of claim 23.

32. A method of fabricating a circuit board comprising a plurality of differential signaling trace pairs formed on conductive trace layers within the circuit board, the method comprising:

assembling a plurality of conductive trace layers comprising differential signaling trace pairs and insulating layers into a panel, each differential signaling trace comprising terminating pads;

forming a hole through the circuit board at the location of each differential signaling trace terminating pad so as to pass through the terminating pad;

plating the holes to form a conductive liner in each hole connected to the trace terminating pad;

for at least some of the plated holes, removing the conductive liner from a longitudinal segment adjacent at least one end of the plated hole such that the longitudinal segment comprises at least one transition from a first profile adjacent the end of the hole to a second profile longitudinally separated from the end of the hole.

33. The method of claim 32, further comprising placing the at least one transition approximately halfway along the longitudinal segment of the hole.

34. The method of claim 33, wherein removing the conductive liner from one of the plated holes comprises drilling the hole a distance equivalent to the length of the longitudinal segment with a first bit having a diameter larger than the outer diameter of the conductive liner, and drilling the hole to the depth of the transition with a second bit having a diameter at least 10 percent greater than the diameter of the first bit.

35. The method of claim 34, wherein drilling with the first bit precedes drilling with the second bit.

36. The method of claim 33, wherein for holes that will be used to inject or receive signals into or out of the circuit board from a top side of the circuit board, removing the conductive liner from the longitudinal segment adjacent the bottom end of the hole when the hole connects to a trace on a conductive trace layer within the top half of the circuit board.

37. The method of claim 36, wherein for holes that will be used to inject or receive signals into or out of the circuit board that have a longitudinal segment adjacent the bottom end removed, removing the longitudinal segment to form a similar longitudinal segment profile for each such hole.

38. The method of claim 36, wherein at least some of the holes are via holes that pass through terminating pads on two different conductive trace layers, the method comprising removing the conductive liner from the longitudinal segment adjacent the bottom end of a via hole when the via hole connects to traces on two conductive trace layers within the top half of the circuit board, and removing the conductive liner from the longitudinal segment adjacent the top end of the via hole when the via hole connects to traces on two conductive trace layers within the bottom half of the circuit board.