CA2162941A1

CA2162941A1 - Method and apparatus to contact plating bar without exposing contact copper or rework of the contact pad plating

Info

Publication number: CA2162941A1
Application number: CA 2162941
Authority: CA
Inventors: Thomas Hussey
Original assignee: Hewlett Packard Co
Current assignee: HP Inc
Priority date: 1995-03-07
Filing date: 1995-11-15
Publication date: 1996-09-08
Also published as: JPH08264938A; DE19543894A1

Abstract

A printed wiring board includes a contact pad and a plating bar positioned in spaced apart relation on the surface of the wiring board. An electrically conductive track internal to the printed wiring board has a first end positioned at about the plating bar and a second end positioned at about the contact pad. A first electrically conductive via extends between the plating bar and the first end of the internal track and electrically connects the plating bar and the internal track. A second electrically conductive via extends between the contact pad and the second end of the internal track and electrically connects the internal track to the contact pad.

Description

METHOD AND APPARATUS TO CONTACT PLATING BAR
WITHOUT EXPOSING CONTACT COPPER OR REWORK OF THE
CONTACT PAD PLATING

Background This invention relates to printed wiring boards in general and more specifically to a method of plating contact pads on printed wiring boards.
Printed wiring boards (PWBs), also known as printed circuit boards, are widely used in the electronics industry to connect together the various components of electronic devices. A typical printed wiring board (PWB) may comprise one or more layers of metal circuit paths or conductors that are bonded onto an insulating substrate. The circuit paths or conductors are co~mo~ly thin copper strips and the insulating substrate typically comprises glass-reinforced epoxy, although other materials may be used. Depending on the complexity of the electronic circuit, PWBs may be single sided, double sided, or may comprise multiple layers of circuit paths or conductors, as in a multi-layer PWB.
Complex electronic circuits usually require the use of double sided or multi-layer PWBs, which allow the printed circuit paths to cross over each other without shorting and without the need to add special jumpers. The materials used for producing PWBs as well as PWB fabrication processes are well-known and are documented in numerous technical references such as, for example, Electronic Materials Handbook, Volume 1-Packaging, (ASM International, Materials Park, OH 44073, 1989), pp. 505-630, which is hereby incorporated by reference.
While PWBs may be connected to one another and to various other electrical components hy individual wires or ribbon wire bundles that are soldered directly to the PWB, it is usually more convenient to provide the PWB with a plurality of contact pads positioned adjacent one or more edges of the PWB. The contact pads are designed to align with contacts in a suitable mating connector, thus allowing the PWB to be connected to external circuitry or devices by sim~lv insertinq the PWB into the mating connector. In order to ensure a reliable, low resistance electrical connection, both the contact pads on the PWB and the contacts within the mating connector are usually plated with gold or a hard gold alloy. Gold, being a noble metal, has excellent resistance to corrosion and maintains its low contact resistance indefinitely.
An electroplating process is typically used to coat the contact pads of the PWB with a suitable gold alloy.
However, since the electroplating process requires that an electrical potential be placed between the object to be plated and the electroplate solution, all of the contact pads on the PWB must be electrically connected to the appropriate voltage potential if they are to be plated.
A common method of electrically connecting together the contact pads in preparation for plating is shown in Figure 1. Essentially, a printed wiring board B includes a circuit region C and a waste region W. The circuit region C may include a plurality of contact pads P, as well as a plurality of circuit paths or conductors (not shown) that are required to connect together the various electronic components (also not shown) that will later be mounted on the board B. The waste region W may include a plating tie bar or plating bar PB that is electrically connected to the end E of each contact pad P by a thin conductor strip or track T located on the surface of the board B. A suitable voltage source (not shown) is then connected to the plating bar PB to place each contact pad P at the proper electrical potential required for electroplating. After the plating process is complete, the waste region W o~ board B is removed, usually by routing, to expose a new edge L.
While the foregoing method for plating the contact pads is widely used, it is not without its disadvantages.
For example, one disadvantage is that the underlying copper conductors comprising the contact pads are exposed when the waste region is removed by the router. Over time, the Case No. 1094034-1 exposed copper of the contact pads tends to oxidize, which may compromise the integrity of the overlying gold plate.
In extreme cases, the oxidization of the underlying copper may cause the overlying gold plate to flake away, which, of course, significantly reduces the reliability of the contact pad. Another disadvantage is that the router used to separate the waste region from the circuit region may also cause shorting between adjacent contact pads. More specifically, the copper of the contact pad, being a malleable material, may be drawn along the cutting path by the router bit a sufficient distance so that it contacts the exposed copper or gold plating of the adjacent contact pad. The routing operation may also lift the copper contact pad from the substrate, which, at best will cause contact reliability problems and, at worst may ruin the board.
An alternative method of connecting the contact pads P to the plating tie bar PB is shown in Figure 2. In this alternative method, all of the contact pads P are first connected together by a plurality of lateral tracks T. The contact pads P are then connected to the plating tie bar PB
by a single track S. While this method has been used, it still suffers from all of the same problems as the method shown in Figure 1. In addition, this method also suffers from the disadvantage of requiring a secondary drilling operation, after the plating operation, to cut the lateral tracks T between adjacent contact pads P.
Consequently, there remains a need for a method for connecting the contact pads to the plating bar that is not subject to the disadvantages of current methods. Such a method should result in a reliable contact pad and m;n;m; ze the potential for the gold plate to flake off the pad or otherwise become compromised. The improved method should also be free from problems relating to removal of the waste region, such as potential shorting between contact pads or lifting of the contact pad from the board. Additional Case No. 1094034-1 advantages could be realized if the improved method could be implemented in a conventional PWB fabrication process, without requiring additional materials or process steps, and certainly without the need to resort to secondary drilling operations to break unwanted connections.

Summary of the Invention Apparatus to contact a plating bar without requiring the later exposure of the contact pad copper or rework of the contact pad plating may comprise a printed wiring board having a contact pad and a plating bar positioned in spaced apart relation on the surface of the wiring board. An electrically conductive track internal to the printed wiring board has a first end positioned at about the plating bar and a second end positioned at about the contact pad. A first electrically conductive via extending between the plating bar and the first end of the internal track electrically connects the plating bar and the internal track. A second electrically conductive via extending between the contact pad and the second end of the internal track electrically connects the internal track to the contact pad.
A method of electrically connecting the contact pad and the plating bar may comprise the steps of: Providing an electrically conductive track internal to the printed wiring board; providing a first electrically conductive via between the plating bar and the first end of the internal track; and providing a second electrically conductive via between the contact pad and the second end of the internal track.
A significant advantage associated with the internal track and electrically conductive vias is that they allow the waste portion of the board to be removed after the plating process without exposing the underlying copper of the contact pads. Consequently, the copper comprising the contact pad is not prone to oxidization, with all its Case No. 1094034-1 associated disadvantages. Another advantage is that since the waste portion of the board can be removed without contacting the ends of the contact pads, there is no chance of lifting the contact pad from the board or creating shorts between adjacent contact pads. Still another advantage is that the internal tracks can be provided on the appropriate internal layer at the same time the other conductors are being laid down elsewhere on the internal layer, thus dispensing with the need for additional process steps. Similarly, the electrically conductive vias connecting the internal tracks with the plating bar and contact pads may be provided during the same process steps required to provide other electrically conductive vias between the conductors on the internal layer and the conductors on the external surfaces the board.

Brief Description of the Drawinq - Illustrative and presently preferred embodiments of the invention are shown in the accompanying drawing in which:
Figure 1 is a plan view of a portion of a printed wiring board showing one arrangement for connecting the contact pads to the plating tie bar;
Figure 2 is a plan view of a portion of a printed wiring board showing another arrangement for connecting the contact pads to the plating tie bar;
Figure 3 is a plan view of a portion of a printed wiring board according to the present invention showing the arrangement of the plating tie bar, contact pads, electrically conductive internal tracks, and electrically conductive vias;
Figure 4 is an enlarged perspective view of a portion of the printed wiring board shown in Figure 3 more clearly showing the arrangement of the electrically conductive internal tracks, the contact pads, and the electrically conductive vias;

Case No. 1094034-1 Figure 5 is a cross-sectional view in elevation of the printed wiring board taken along the line 5-5 of Figure 3 showing the arrangement of the plating tie bar, contact pad, electrically conductive internal track, and electrically conductive thru-vias;
Figure 6 is a cross-sectional view in elevation of the printed wiring board shown in Figure 5 after the waste region has been removed; and Figure 7 is a cross-sectional view in elevation of another embodiment of a printed wiring board showing the arrangement of the plating tie bar, contact pad, electrically conductive internal track, and electrically conductive blind vias.

Detailed Description of the Invention Apparatus 10 to contact a plating bar 18 without requiring the later exposure of the contact pad copper or rework of the contact pad plating is best seen in Figures 3 and 4 and may comprise a printed wiring board or PWB 12 having a circuit region 32 and a waste region 14. The circuit region 32 of printed wiring board 12 includes a plurality of contact pads 22 as well as a plurality of circuit paths or conductors (not shown) required to connect together the various electronic components (also not shown) that will later be mounted on the PWB 12 and soldered to the conductors to complete the electronic circuit. The waste region 14 of PWB 12 comprises a plating tie bar or plating bar 18 that will later serve as the connection point for the voltage source (not shown) required to accomplish electroplating of the contact pads 22. As will be described in greater detail below, the waste region 14 later will be separated from the circuit region 32 along a line 16, which then becomes the new edge of the printed wiring board or PWB 12.
In order to electroplate the contact pads 22 with a suitable material, such as gold or gold alloy, each contact Case No. 1094034-1 2~6294~

pad 22 must be connected to the plating bar 18, which in turn must be connected to a suitable electroplating voltage source. In one preferred embodiment, each contact pad 22 is electrically connected to the plating bar 18 by an electric conductor or track 34 that is internal to the PWB
12. However, since the plating bar 18 and the contact pads 22 are located on the external surface 36 of PWB 12, but the tracks 34 are internal to the PWB 12, a pair of electrically conductive vias 40 are used to connect each internal track 34 to the plating bar 18 and to the contact pad 22.
Once the all of the contact pads 22 have been connected to the plating bar 18 by the internal tracks 34 and electrically conductive vias 40, the portion of the PWB
12 containing the contact pads 22 then may be submerged in a suitable electroplating solution (not shown) containing, for example, gold or a gold alloy. A suitable electroplating voltage source (also not shown) may then be connected to the plating tie bar 18, thus placing the appropriate voltage potential between the contact pads 22 and the electroplating solution. After the plating operation is complete, the PWB 12 may be removed from the electroplating solution and the waste portion 14 of the PWB
12 removed, for example, by routing along line 16. Line 16 then becomes the new edge of the PWB 12. However, while the new edge 16 is located near the ends 20 of the contact pads 22, it does not contact the ends 20, thus leaving the ends 20 of the pads 22 covered with a protective layer of gold plate 42, as best seen in Figure 6.
A significant advantage of the present invention iS
that it prevents the ends 20 of the contact pads 22 from being exposed after the waste portion 14 of the PWB 12 is removed. Consequently, the copper comprising the contact pad 22 is not prone to oxidization, with all its associated disadvantages. Another advantage is that since the ends 20 of the contact pads 22 are not contacted by the router bit ~ N~ ln94n~4-l 2~62941 (not shown) during the routing operation, there is no chance that the bit will drag along the edges 20 of the contact pads 22, thus eliminating the possibility of lifting the contact pad 22 from the board or creating shorts between adjacent contact pads 22.
Still other advantages are associated with this printed wiring board configuration. For example, in most cases the printed wiring board 12 will comprise a multi-layer printed wiring board with at least one internal layer, although PWB 12 may comprise many internal layers.
In that case, the internal tracks 34 can be provided on the appropriate internal layer at the same time the other conductors are being laid down elsewhere on the internal layer. Similarly, the electrically conductive vias 40 connecting the internal tracks 34 with the plating bar 18 and contact pads 22 may be provided during the same process steps required to provide other electrically conductive vias between the conductors on the internal layer and the conductors on the external surfaces 36 of the PWB 12.
Consequently, the provision of the internal tracks 34 and electrically conductive vias 40 may be accomplished without the need for additional process steps. The present invention also dispenses with the need for secondary drilling operations to break any unwanted contacts between the contact pads.
Referring now to Figures 3-6, the apparatus 10 to contact a plating bar 18 without requiring the later exposure of the contact copper or rework of the contact pad plating will be described in detail. As mentioned above, the printed wiring board (PWB) 12 may comprise a multi-layer printed wiring board comprising one or more internal layers 50, 52 of circuit paths or conductors, as best seen in Figure 4. Such multi-layer printed wiring boards are manufactured by first manufacturing a plurality of single sided wiring boards (not shown) having the appropriate routing of electrical conductors. Methods for fabricating ~ No. 1094034-1 2~ 6~94 1 single sided circuit boards are well-known in the art (see for example Electronic Materials Handbook, Vol. 1-Packaging, pp. 505-630). The various single side boaras are then stacked, one on top of another, and pressed together with a suitable bonding agent (not shown), thus forming a single, unitary board with a plurality of layers 50, 52 (Figure 4) of internal circuit paths or conductors, each of which corresponds to one of the individual wiring boards (not shown). Methods for stacking and bonding of single sided circuit boards to provide a multi-layer board are also well-known in the art. The multi-layer printed wiring boards 12 shown in Figures 3-7 comprise such multiple layers of individual boards that have already been stacked and bonded together, and are referred to herein in the singular.
Referring now to Figure 3, printed wiring board 12 also comprises a circuit portion 32 and a waste portion 14.
The circuit region 32 of printed wiring board 12 includes a plurality of contact pads 22, as well as a plurality of circuit paths or conductors (not shown) required to connect together the various electronic components (also not shown) that will later be mounted on the PWB 12 and soldered to the conductors to complete the electronic circuit. The circuit region 32 of printed wiring board 12 may also comprise additional contact pads 23 located on the opposed or lower surface 44 of PWB 12, as best seen in Figures 4 and 5. The waste region 14 of PWB 12 comprises a plating tie bar or plating bar 18 that will later serve as the connection point for the voltage source (not shown) required to accomplish electroplating of the contact pads 22, 23.
The plating bar 18 and contact pads 22, 23 are electrically connected by a plurality of internal tracks 34 and electrically conductive vias 40. Methods for forming electrical vias are well-known in the art. The internal tracks 34 may be located on one or more of the internal c r~ M~ 1 n q ~l n ~

2~629~1 layers 50 52 comprising PWB 12, as best seen in Figure 4.
Whether the internal tracks 34 are provided on any single internal layer 50 or on several internal layers 50, 52 depends largely on the overall complexity of the circuit as well as whether sufficient space is available on a given layer for all of the tracks 34. Consequently, the present invention should not be regarded as limited to any one particular arrangement of internal tracks 34 on a single layer 50 or on multiple layers 50, 52.
The electrically conductive internal tracks 34 may comprise copper conductors and are aligned with the plating bar 18 and the contact pads 22, 23, as best seen in Figures 4 and 5. It should be noted, however, that the material comprising the conductors of the PWB, e.g., the electrically conductive internal tracks 34, contact pads 22, and plating bar 18, is not limited to copper, and could comprise any of a wide range of conductive materials, such as aluminum, silver, gold, etc. However, copper is by far the most commonly used material for PWB conductors. In one preferred embodiment, the internal tracks 34 may be created on the internal layers 50, 52 at the same time that the other circuit paths or conductors are being laid down on the internal layers 50, 52. After the internal layer 50 or layers 50, 52 have been fabricated, they are then pressed together with a suitable bonding material (not shown) in a manner well-known in the art to form the single, unitary PWB 12.
The electrically conductive vias 40 connecting the internal tracks 34 to the plating bar 18 and the contact pads 22, 23 are best seen in Figure 5 and may be provided during the same process steps that are used to provide other electrically conductive vias (not shown) required to connect the circuit conductors on the internal layer 50 to the circuit conductors on the upper and lower surfaces 36 and 44. In one preferred embodiment, the vias 40 may comprise thru-vias that extend through the entire PWB 12, r~.O No. 1()94034-1 21 ~29~ 1 as best seen in Figure 5. While many processes exist for constructing such electrically conductive thru-vias, a thru-via 40 may be constructed by drilling a hole through the plating bar 18, one end 46 of internal track 34, and all the way through the PWB 12 and an optional opposed plating bar 19 on the lower surface 44 of PWB 12. Any one of a number of plating processes may then be used to plate the inside of the hole with a conductive material, thus electrically connecting the internal track 34 with the plating bars 18, 19. The electrically conductive thru-via 40 connecting the other end 46 of the internal track 34 with the contact pads 22, 23 may be constructed in a similar manner.
After the electrically conductive vias 40 have been constructed, the contact pads 22, 23 may be plated in a conventional manner. For example, the contact pads 22, 23 may be immersed in a suitable electroplating solution (not shown) and a voltage source (also not shown) connected between the plating solution and the plating bar 18. After the plating operation is complete, each contact pad 22, 23 will be encapsulated by a thin layer 42 of the plating material, such as a gold alloy, as best seen in Figure 6.
The thin layer 42 also covers the ends 20, 21 of each contact pad 22, 23 to prevent it from being oxidized by exposure to the atmosphere. A suitable device, such as a router (not shown) may then be used to remove the waste region 14 of the PWB 12, exposing a new edge 16. The edge 16 may also be chamfered to remove sharp edges and prevent splintering of the PWB 12. The ends 20, 21 of the contact pads 22, 23 are not exposed by the routing operation, and remain protected by the thin layer 42 of plating material.
Of course the contact pads 22, 23 on the upper and lower surfaces 36 and 44 of the PWB 12 remain electrically connected together by the thru-via 40, as best seen in Figure 6.
Another embodiment 110 is shown in Figure 7 that uses Case No. 1094034-1 216294~

a plurality of blind vias, 140, 141, which allow the contact pads 122, 123 on the upper and lower surfaces 136 and 144 to remain electrically isolated. More specifically, the embodiment 110 shown in Figure 7 comprises a pair of blind vias 140 to electrically connect the plating bar 118 and contact pad 122 on the upper surface 136 to an upper internal track 134 located on an upper internal layer 150. Another pair of blind vias 141 are used to electrically connect a plating bar 119 and contact pad 123 on the lower surface 144 to a lower internal track 135 located on a lower internal layer 152.
The use of blind vias, such as vias 140 and 141, has the advantage of allowing the contact pad 122 on the upper surface 136 to remain electrically isolated from the contact pad 123 on the lower surface 144, yet still be connected to a plating bar 119 on the lower surface 144.
It is contemplated that the inventive concepts herein described may be variously otherwise embodied and it is intended that the appended claims be construed to include alternative embodiments of the invention except insofar as limited by the prior art.

Case No. 1094034-1

Claims

1. A method of electrically connecting a contact pad (22) and a plating bar (18), both the contact pad (22) and the plating bar (18) being located on an external surface (36) of a printed wiring board (12), comprising the steps of:
forming an electrically conductive track (34) internal to the printed wiring board (12), said electrically conductive track (34) having a first end (46) positioned at about the plating bar (18) and a second end (48) positioned at about the contact pad (22);
forming a first electrically conductive via (40) between the plating bar (18) and the first end (46) of said electrically conductive track (34), said first electrically conductive via (40) electrically connecting the plating bar (18) and said electrically conductive track ( 34); and forming a second electrically conductive via (40) between the contact pad (22) and the second end ( 48) of said electrically conductive track (34), said second electrically conductive via (40) electrically connecting the contact pad (22) and said electrically conductive track (34).

2. The method of claim 1, wherein the printed wiring board (12) comprises a multi-layer printed wiring board having at least one internal layer (50) and wherein step of forming an electrically conductive track (34) internal to the printed wiring board (12) is performed during the time the electrical conductors for the internal layer (50) are being manufactured.

3. The method of claims 1 or 2, wherein the steps of forming the first and second electrically conductive vias (40) are performed during the time other vias in the printed wiring board (12) are being made electrically conductive.

4. A printed wiring board (12) having two opposed surfaces (36, 44), comprising:
a first contact pad (22) located on one of the two opposed surfaces (36, 44);
a first plating bar (18) located on one of the two opposed surfaces (36, 44);
a first electrically conductive track (34) internal to the printed wiring board (12), said first electrically conductive track (34) having a first end (46) positioned at about the first plating bar (18) and a second end (48) positioned at about said first contact pad (22);
a first electrically conductive via (40) extending between said first plating bar (18) and the first end (46) of said first electrically conductive track (34), said first electrically conductive via (40) electrically connecting said first plating bar (18) and said first electrically conductive track (34); and a second electrically conductive via (40) extending between said first contact pad (22) and the second end (48) of said first electrically conductive track (34), said second electrically conductive via (40) electrically connecting said first contact pad (22) and said first electrically conductive track (34).

5. The printed wiring board (12) of claim 4, wherein the printed wiring board (12) comprises a multi-layer printed wiring board having at least one internal layer (50) and wherein said first electrically conductive track (34) is located on the internal layer (50).

6. The printed wiring board (12) of claims 4 or 5, wherein said first contact pad (22), said first plating bar (18), and said first electrically conductive track (34) comprise copper.

7. The printed wiring board (12) of claims 4, 5 or 6, wherein said first and second electrically conductive vias (40) comprise thru vias that extend through the printed wiring board (12) and through said respective first contact pad (22), first plating bar (18), and first electrically conductive track (34).

8. The printed wiring board (12) of claims 4, 5, 6, or 7, wherein said first and second electrically conductive vias (40) comprise blind vias.

9. A method of plating a contact pad (22) located on an external surface (36) of a printed wiring board (12), comprising the steps of:
electrically connecting the contact pad (22) with a plating bar (18) located a spaced distance from the contact pad (22) with an electrically conductive track (34) internal to the printed wiring board (12), the electrically conductive track (34) having a first end (46) positioned at about the plating bar (18) and a second end (48) positioned at about the contact pad (22), the first end (46) of the electrically conductive track (34) being electrically connected to the plating bar (18) by a first electrically conductive via (40) and the second end (48) of the electrically conductive track (34) being electrically connected to the contact pad (22) by a second electrically conductive via (40);
immersing the contact pad (22) in an electroplating solution containing a plating material and connecting a voltage source between the plating bar (18) and the electroplating solution, wherein the plating material is deposited onto the surface of the contact pad (22); and removing the contact pad (22) from the plating solution.

10. The method of claim 9, further comprising the step of removing a waste region (36) of the printed wiring board (12), the waste region (36) including the plating bar (18).