Method of producing circuit board

A printed circuit board having a selectable circuit routing configuration comprises a substrate, a plurality of electrical traces mounted to the substrate for interconnecting electrical components, and at least one fusible connector mounted to the substrate. Each of the fusible connectors has a fuse line formed from a conductive layer of the printed circuit board and forms a fusible connection between at least two of the electrical traces. The circuit routing configuration can be selected by application of a predetermined current through at least one of the fusible connectors to break the fusible connection formed by the fusible connector.

FIELD OF THE INVENTION 
The present invention pertains to the field of printed circuit boards. More 
particularly, the present invention relates to reconfiguring the 
electrical routing configuration of a printed circuit board after 
completion of the manufacturing process. 
BACKGROUND OF THE INVENTION 
Printed circuit boards are commonly used in the computer industry and in 
numerous other areas of technology to implement electronic circuitry. A 
printed circuit board typically consists of one or more substrate layers 
made of a dielectric material, on which electronic components are mounted. 
One or more layers of conductive material, such as copper, are mounted on 
and between the substrate layers to form the wiring, or "traces", that 
interconnect the electronic components. The traces may be formed by fixing 
a solid metal plane onto the surface of a substrate layer and then etching 
the metal plane according to a pattern. The traces are typically covered 
by an insulative protective coating, with the exception of certain 
portions which are left exposed for the purpose of providing bonding 
contacts. 
Because there are so many potential uses for printed circuit boards, it has 
become desirable to provide the capability to mass produce a printed 
circuit board which has a single generic routing configuration, but which 
later can be tailored to have one of multiple specific configurations. 
Various solutions have been developed to accomplish this goal. For 
example, a "zero-ohm" resistor, or jumper, can be used to electrically 
connect two traces on a printed circuit board to modify the routing 
configuration. This solution is disadvantageous, however, because it is 
not cost effective to purchase and maintain an inventory of separate 
components for this purpose. One way to overcome this disadvantage is to 
use solder instead of a zero-ohm resistor to connect two traces and 
thereby modify the routing configuration. There are many situations, 
however, in which it may be desirable to adjust the routing configuration 
of a printed circuit board during the post-manufacturing test phase. Yet 
both of the above-mentioned solutions may be impractical to implement once 
the manufacturing process has been completed. 
One situation in which the post-manufacture configuration of a printed 
circuit board may be desirable is the production of personal computer (PC) 
motherboards. A PC motherboard, which contains the central processing unit 
(CPU) of the computer, may also contain a dedicated power supply for the 
CPU. The CPU requires a regulated power supply which will provide a very 
precise voltage; often a tolerance of less than 1% is required. However, 
power supply voltages tend to vary depending upon load, temperature, 
noise, and manufacturing tolerances. Consequently, the "set point" of the 
regulated power supply must be carefully adjusted. The "set point" is the 
regulated output voltage at a steady load and temperature. Typically, the 
output voltage of a power supply is measured during testing. Power is then 
removed, and the set point is adjusted according to the measured output 
voltage. 
One way to permit the adjustment of the set point is to provide a feedback 
circuit comprising a resistor divider network, as illustrated in FIG. 1. 
FIG. 1 is a block diagram of a regulated power supply 1 having a feedback 
circuit 20 comprising a simple resistor divider network which is 
well-known in the prior art. The resistor divider network consists of a 
resistor RX having a fixed value and a variable resistor RY. The value of 
resistor RY is varied manually to adjust the output V.sub.FB of the 
divider network to adjust the set point of the power supply. This solution 
is disadvantageous, however, because the resistor RY requires mechanical 
adjustment, and because its value may tend to drift once the motherboard 
is installed and operating in a PC. 
Therefore, it is desirable to provide for the mass production of a printed 
circuit board having a single routing configuration which can be easily 
reconfigured to have one of multiple different routing configurations. It 
is also desirable to make reconfiguration practical and convenient even if 
performed after the manufacturing process has been completed. It is 
further desirable to provide a means for configuring the trace routing on 
a printed circuit board such that a separate inventory of parts need not 
be maintained for that purpose. In addition, it is desirable to provide a 
regulated power supply which has a set point that is adjustable during 
post-manufacture testing and which incorporates the aforementioned 
features. 
SUMMARY OF THE INVENTION 
A printed circuit board having a selectable circuit routing configuration 
is described. The printed circuit board comprises a substrate, a plurality 
of electrical traces mounted to the substrate for interconnecting 
electrical components, and at least one fusible connector mounted to the 
substrate. Each of the fusible connectors comprises a fuse line formed 
from a conductive layer of the printed circuit board and forms a fusible 
connection between at least two of the electrical traces. The circuit 
routing configuration is selectable by application of a predetermined 
current through at least one of the fusible connectors to break the 
fusible connection formed by the fusible connector. 
The fusible signal connector comprises a first contact pad located at an 
end of a first signal trace on a printed circuit board, a second contact 
pad located at an end of a second signal trace on the printed circuit 
board, and a fuse line. The fuse line is formed from the conductive layer 
of the printed circuit board and is fusibly coupled between the first 
contact pad and the second contact pad. The predetermined current driven 
through the fuse line causes the fuse line to fuse open. 
Other features of the present invention will be apparent from the 
accompanying drawings and from the detailed description which follows 
below.

DETAILED DESCRIPTION 
A printed circuit board having a selectable circuit routing configuration 
is described. In the following description, for purposes of explanation, 
numerous specific details are set forth in order to provide a thorough 
understanding of the present invention. It will be evident, however, to 
one skilled in the art that the present invention may be practiced without 
these specific details. In other instances, well-known structures and 
devices are shown in block diagram form in order to avoid unnecessarily 
obscuring the present invention. 
FIG. 2 shows a regulated power supply 2. The regulated power supply 2 
consists of a regulator 10 which receives an input voltage V.sub.IN and 
which generates an output voltage V.sub.OUT. The output voltage is input 
to a feedback circuit consisting of a resistor divider network 25. The 
resistor divider network 25 outputs a feedback voltage V.sub.FB to a 
comparator 30. The comparator receives the feedback voltage V.sub.FB and a 
reference voltage V.sub.REF and outputs to the regulator 10 an error 
voltage ERROR which is proportional to the difference between the feedback 
voltage V.sub.FB and the reference voltage V.sub.REF. The functions 
performed by the combination of the comparator 30 and the reference 
voltage V.sub.REF may be implemented using well-known integrated circuits, 
such as the TL1431 or the TL431. 
The resistor divider network 25 consists of six resistors R1 through R6. 
Set Point resistor R1 is coupled between the output V.sub.OUT of the 
regulator 10 and the V.sub.FB input of the comparator 30. Set Point 
resistor R2 is coupled between the V.sub.FB input of the comparator 30 and 
ground. Resistors R3 and R5 are coupled in parallel with Set Point 
resistor R1, while resistors R4 and R6 are coupled in parallel with Set 
Point resistor R2. Each of resistors R3 through R6 is coupled to the 
V.sub.FB input of the comparator 30 through a separate one of four fusible 
resistors 40. 
The regulated power supply 2 has a set point which represents the output 
voltage V.sub.OUT at a constant load and temperature. The set point may be 
adjusted by adjusting the feedback voltage V.sub.FB input to the 
comparator 30 for a given output voltage V.sub.OUT. In other words, the 
set point may be adjusted by changing the transfer function V.sub.FB 
/V.sub.OUT of the resistor divider network. This is accomplished by 
breaking the current path through one or more of resistors R3 through R6. 
Consequently, the set point of the regulated power supply 2 may be 
adjusted by selectively fusing open one or more of the fusible connectors 
40. 
Referring now to FIG. 3, a section 60 of a printed circuit board is shown. 
The printed circuit board consists of a substrate 62 and multiple copper 
traces, including traces 42 and 44. Traces 42 and 44 are terminated in 
copper contact pads 46 and 48, respectively. Coupled between contact pads 
46 and 48 is a fuse line 50. A fusible connector 40 comprises the contact 
pads 46 and 48 and the fuse line 50. In the preferred embodiment, the 
contact pads 46 and 48, the fuse line 50, and the traces 42 and 44 are all 
formed simultaneously from a single copper layer by the same etching 
process. Although the traces on the printed circuit board 60 are generally 
covered by a protective layer of insulating material, the contact pads 46 
and 48 are exposed to allow electrical contact with a test probe or other 
similar instrument. 
The fuse line 50 may be fused open by applying a predetermined voltage 
between contact pads 46 and 48 to cause a fusing current to flow through 
the fuse line 50. The width and shape of the fuse line 50 are calculated 
such that the fuse line 50 will fuse open at the lowest possible fusing 
current and provide the cleanest break upon fusing, subject to current 
manufacturing capabilities. A fuse line 50 having a kinked shape as shown 
in FIG. 3 may be suitable for this purpose. In addition, the length of the 
fuse line 50 is as short as possible according to current manufacturing 
capabilities. The contact pads 46 and 48 and fuse line 50 are sized and 
shaped in order to provide negligible resistance. 
The present invention may also be embodied on a multiple layer printed 
circuit board, as shown in FIG. 4. The fuse line 50 is formed on one 
substrate layer 63 of the printed circuit board while the traces 42 and 44 
are formed on a different substrate layer 64. Connection between the fuse 
line 50 and contact pads 46 and 48 is made through vias 52 and 54. 
Hence, the routing configuration of the printed circuit board 60 can be 
easily altered after the manufacturing process has been completed (e.g., 
during the testing phase) by selectively fusing open any of the fusible 
connectors 40. By applying a predetermined voltage between contact pad 46 
and contact pad 48 to cause a predetermined current to pass through and to 
fuse open the fuse line 50. Furthermore, if this process is performed 
during the testing phase, the open/short condition of the fusible 
connector 40 can be easily tested, since the printed circuit board is 
already connected to the test equipment. Therefore, the printed circuit 
board 60 can be mass produced to have a single routing configuration that 
can be easily reconfigured into one of multiple different routing 
configurations. It is not necessary to maintain a separate inventory of 
parts for the purpose of performing such reconfiguration. Furthermore, the 
fusible connector 40 can be implemented in a regulated power supply such 
that the set point of the power supply can be easily adjusted during 
testing without the need for mechanical adjustment of a variable resistor, 
and such that the set point has minimal variance during operation. 
Although the present invention has been described with reference to 
specific exemplary embodiments, it will be evident that various 
modifications and changes may be made to these embodiments without 
departing from the broader spirit and scope of the invention as set forth 
in the claims. Accordingly, the specification and drawings are to be 
regarded in an illustrative rather than a restrictive sense.