Method for reducing shorts on a printed circuit board edge connector

An edge connector for a circuit board having a top layer and at least one layer underlying the top layer to form a composite structure. The edge connector comprises a first set of metallic tabs formed on the top layer in a row spaced from an edge of the top layer, with each of the first set of metallic tabs having a plating spoke associated therewith formed in the one layer underlying the top layer. A second set of metallic tabs is formed on the top layer in a row adjacent the edge. A metallic tab of the first set is positioned between a pair of metallic tabs in the second set, however, the surface area of the top layer between each pair of metallic tabs in the second set is substantially free of a plating spoke.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The present invention generally relates to techniques for improving the 
alignment between electrical contacts on a printed circuit board and an 
associated socket connector to thereby reduce shorts when the board is 
plugged into the socket. 
2. Description of the Related Art 
A typical computer is highly modular. During a computer's useful life, one 
or more components are replaced either due to maintenance or system 
upgrades. To this end, most computer components are now designed to be 
"field replaceable" and are supported on a "pluggable" printed circuit 
board. The board typically includes a so-called "edge connector" having 
metallic "tabs" that mate with spring contacts located within a connector 
socket, which in turn is supported on a motherboard. Each tab should mate 
with a specific spring contact when the printed circuit board is inserted 
into the connector socket. Because of the dense spacing between tabs, 
however, when the board is inserted into the connector, it is quite common 
for a tab to "short circuit" by mating with the wrong connector contact. 
This causes an undesirable electrical connection. 
In the past, this problem has been addressed through the use of so-called 
"card guides" that reduce the amount of play between the pluggable card 
and the connector socket. While the use of a card guide helps to position 
the board at a precise position in the connector, this approach does not 
always prevent shorts. Moreover, in many applications it is not practical 
to build a card guide that can maintain the tight tolerances required to 
properly mate the edge connector and socket. 
SUMMARY OF THE INVENTION 
It is a primary object of this invention to reduce the possibility of short 
circuits when an electrical connection is made between a pluggable 
component card and a connector socket. 
It is another primary object of the invention to improve the alignment 
between a pluggable component card and an electrical connector socket. 
It is a further important object of this invention to improve the 
registration between metallic tabs of a printed circuit board (PCB) card 
edge and spring contacts of a connector socket. 
It is another important object to decrease the effective "density" of 
metallic tabs on an PCB card edge to thereby reduce the likelihood of 
"shorts" when the card is plugged into an electrical socket connector. 
A more specific object of the invention is to provide a method for reducing 
shorts on printed circuit board edge connectors while at the same time 
allowing more flexibility in the mating of the connector and the connector 
socket. Even if the board is not inserted precisely in alignment with the 
connector socket, shorts are less likely to occur because the "effective" 
distance between at least some of the metallic tabs is increased. 
Yet another more general object is to improve the mechanical and electrical 
performance of pluggable component cards in a computer. 
These and other objects of the invention are provided in an edge connector 
for a printed circuit board preferably having a top layer, one or more 
intermediate layers, and a bottom layer. A first set of metallic tabs are 
formed on the top layer in a row spaced from an edge of the top layer, and 
each of the first set of metallic tabs has a plating spoke associated 
therewith formed in the intermediate layer. A second set of metallic tabs 
are formed on the top layer in a row adjacent the edge, and each metallic 
tab of the first set is preferably positioned between a pair of metallic 
tabs in the second set when the first and second sets are viewed in plan. 
Preferably, the surface area of the top layer between each pair of 
metallic tabs in the second set is substantially free of a plating spoke 
because the plating spokes are "buried" in the intermediate layer. Each 
metallic tab in the first set is connected to its associated plating spoke 
by a via or other connecting means. 
The foregoing has outlined some of the more pertinent objects and features 
of the present invention. These objects should be construed to be merely 
illustrative of some of the more prominent features and applications of 
the invention. Many other beneficial results can be attained by applying 
the disclosed invention in a different manner or modifying the invention 
as will be described. Accordingly, other objects and a fuller 
understanding of the invention may be had by referring to the following.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
The present invention is implemented in any apparatus in which a printed 
circuit board having electrical contacts is plugged into a connector 
socket. By way of example only, one such apparatus is a computer, for 
example, an IBM RS/6000 computer (a reduced instruction set of so-called 
RISC-based workstation) running the AIX (Advanced Interactive Executive 
Version 4.1 and above), or an Intel-based processor system running the 
Windows NT or OS/2.RTM. operating system. Other applications of the 
present invention include printed circuit boards or other "cards" used in 
communications equipment, audio equipment and games, among other types of 
electrical devices. Indeed, any application that requires a connector with 
many electrical signals in a small connection area may take advantage of 
the present invention. For illustrative purposes, however, the remainder 
of the Detailed Description relates to a pluggable component card for use 
in a computer. 
One such card 10 is illustrated in FIG. 1. It includes a main body portion 
12, which is typically made of an insulating material, having a plurality 
of electrical components (not shown) supported therein. A plurality of 
electrical conductors are usually etched on the insulating material and, 
in conjunction with the electrical components, form a so-called "printed 
circuit board" or PCB. The card 10 is typically "double-sided" and 
includes an edge connector 14 on each side including a plurality of 
metallic tabs. Card 10 may be single-sided as well. As is well-known in 
the art, the edge connector is designed to mate with an associated 
connector "socket" supported on, for example, a motherboard. A 
representative socket is shown in FIG. 6. Thus, for example, the connector 
socket is one manufactured by Amp, Incorporated of Harrisburg, Pa. under 
the Part No. 145090, and this socket includes a plurality of spring 
contacts, each of which is designed to contact a particular metallic tab 
thereby to establish an electrical connection between the PCB edge 
connector and the socket. The edge connector and the socket include two 
(2) distinct sections, but this is not a requirement of the invention. 
FIG. 2 shows a conventional layout of the metallic tabs of the edge 
connector 14 of the PCB according to a known prior art approach. 
Typically, the metallic tabs 15 are laid out in two rows, an upper row 16 
and a lower row 18. Although not meant to be limiting, the upper row of 
tabs 15 is normally used for address and data signals while the lower row 
of tabs is normally used for power and ground connections. As can be seen 
in this view, each upper tab includes an associated extension or "spoke" 
20 on the PCB surface that extends between a pair of tabs in the lower 
row. The spoke of each tab in the upper row is thus effectively 
"interleaved" between a pair of tabs on the lower row. The spoke is used 
during the manufacture process to "plate" the upper tab with an 
appropriate plating material, such as gold. Plating the metallic tab is 
advantageous for several reasons including reduction of corrosion and 
enhancing the electrical connection between the tab and the associated 
spring contact. However, after the plating process, the spoke is 
"vestigial" in that it serves no useful function. On the contrary, the 
spokes actually increase the effective "density" of the lower row 18, 
thereby increasing the likelihood of electrical shorts when the edge 
connector is placed in the socket. 
The present invention ameliorates this problem by "burying" the spokes 
under the top PCB layer, namely, the layer upon which the metallic tabs 
are etched. This has the effect of increasing the distance between each 
pair of tabs in the lower row without actual changing the placement of 
those tabs because the "interleaved" spoke is no longer present on the 
same layer. This contruction is shown in FIG. 3. In this view, the 
metallic tabs 15 are again laid out in two rows, an upper row 16' and a 
lower row 18'. However, as compared to FIG. 2, each spoke 20' is now 
buried under the top PCB layer and thus is illustrated in phantom. Thus, 
each upper tab now includes a spoke 20' underlying the PCB surface layer 
and that extends between a pair of tabs in the lower row. The spoke of 
each tab in the upper row is still "interleaved" between a pair of tabs on 
the lower row, however, the spoke is not present on the upper layer 
itself. This has the effect of decreasing the "density" of the tabs in the 
lower row or "widening" the distance between each pair of tabs. This 
construction reduces short circuits that otherwise occur when the spokes 
are present on the top layer. 
The present invention provides numerous advantages over the prior art. By 
using the "buried" spoke, the density of the metallic tabs on the lower 
row of the connector is significantly decreased as noted above. This 
allows much more flexibility in the mating of the connector and the 
socket. Even if the pluggable component board is not inserted precisely 
correct, shorts are not likely to occur because the distance between the 
metal tabs on the lower row is greater than in the prior art. Burying the 
spoke "internally" effectively increases the distance between the metallic 
tabs on the lower row. This substantially reduces the likelihood of shorts 
between the tabs and the spring contacts of the connector socket. 
Thus, according to the invention, the edge connector comprises a first set 
of metallic tabs formed on the PCB top layer in a row (namely, the upper 
row) spaced from the card edge. Each of the first set of metallic tabs has 
a plating spoke associated therewith formed in an underlying (e.g., 
intermediate or perhaps even a lower) layer. The phrase "underlying layer" 
should thus be broadly construed to cover any layer below the layer on 
which the tabs are etched. A second set of metallic tabs are formed on the 
top layer in a row (namely, the lower row) preferably adjacent the card 
edge, and each metallic tab of the first set is preferably positioned 
between a pair of metallic tabs in the second set when the first and 
second sets are viewed from the top or "plan" view. Preferably, the 
surface area of the top layer between each pair of metallic tabs in the 
second set is free or substantially free of a plating spoke because the 
actual plating spokes are "buried" in the intermediate layer. Each 
metallic tab in the first set is connected to its associated plating spoke 
by a via or other known connection means such as a conductor as 
illustrated in FIG. 4. 
In the preferred embodiment, the edge connector is a composite structure 
formed of a plurality of layers of insulating material laminated together 
in a known manner. In one illustrative embodiment shown in elevation in 
FIG. 4, which is a single-sided card, the composite structure comprises a 
bottom layer 30, intermediate layer 32, and upper or "top" layer 34. 
Multiple intermediate layers may be used. The first and second sets of 
metallic tabs are etched on the top layer 34 and the plating spokes are 
etched in the intermediate layer 32. The edges of the plating spokes are 
visible in this view. The means for connecting the spoke to the tab is 
preferably a via, although this connecting means may alternatively be any 
conductive element such as a wire. Preferably, the composite structure is 
manufactured by forming the first and second sets of metallic tabs on the 
first layer, forming the spokes on the intermediate layer, and then 
laminating the first and second layers. 
During this manufacturing process, the metallic tabs are plated by applying 
a plating current to a plating bar. FIG. 5A shows the prior art approach 
wherein the spokes are interleaved between pairs of metallic tabs in the 
lower row and etched on the top layer. After the sets of tabs are plated, 
a transverse cut is made to form the edge connector. In FIG. 5B, the 
spokes are buried and are thus represented in phantom. After the plating 
process, the transverse cut is made to form the edge connector. A side 
view of the connector (following the transverse cut) is illustrated in 
FIG. 4, as previously described. 
Although the present invention is preferably implemented in an edge 
connector of the type illustrated in the drawings, the use of "buried" 
spokes to facilitate decreasing the effective density of metallic edge 
contacts in an electrical connector has broader applicability. Thus, the 
teachings of the present invention should not be construed to be limited 
to the particular two-tiered metallic tab configuration illustrated in 
FIG. 3. Moreover, while the embodiment shown in FIG. 4 illustrates a 
single-sided card, the inventive concept of burying spokes is also useful 
in a double-sided carad wherein each edge connector (one per side) 
includes the buried spokes. The double-sided configuration is preferably 
for ease of manufacture. 
Although the invention has been particularly shown and described with 
reference to a preferred embodiment, it will be understood by those 
skilled in the art that various changes in form and detail may be made 
therein without departing from the spirit and scope of the invention.