Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
1998-06-30
2001-01-09
Gaffin, Jeffrey (Department: 2841)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C361S760000, C361S779000, C361S783000, C257S738000, C257S786000, C174S260000, C174S261000, C438S108000, C438S128000, C438S129000, C029S840000, C029S850000, C228S180210, C228S180220
Reexamination Certificate
active
06172879
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to printed circuit board assembly processes. More particularly, the present invention relates to a method for isolating a pin of a ball grid array device mounted on a printed circuit board and routing the signal carried by the isolated pin to a new location by attaching a rework wire to the isolated pin.
There has been a steady increase in the densities of circuit assemblies on printed circuits boards (PCB). This increase has been fueled by an ever increasing demand for faster circuit speeds which require minimum distances between circuit packages, increased complexity of functional circuitry, and miniaturization of circuit devices with higher pin counts. Increased densities of PCB assemblies have in turn made it more difficult to perform PCB rework tasks which are frequently required to incorporate design changes, or to correct faults detected during testing or to increase the reliability of PCBs. The increased complexity of performing rework tasks has in turn increased both the total cost and time required to fabricate PCBs. Thus, the efficiency and effectiveness of a rework process directly affects the process yield and reliability of the PCB assembly and fabrication process.
Conventionally, rework processes involving pin isolation and signal routing, are performed entirely on the PCB itself. This is usually accomplished by performing a trace cut on the PCB and attaching a rework wire or an engineering change (E/C) wire to the trace cut or rework bond pad on the PCB. The rework wire is then used to route the signal from the integrated circuit to an alternate PCB location. This method, however, is not feasible for newer devices, such as Ball Grid Array (BGA) ASIC devices, or other BGA devices in which the spacing between the device pins is reduced so much as to prevent the use of rework bond pads. The only alternative in such situations is to perform a trace cut on the PCB, solder the rework wire directly to the trace on the PCB and then route the wire to the new location.
The phrase “BGA device” as used in this specification refers to a BGA package with the IC housed in the package. The word “pin” as used in the BGA device context refers to a device pad on the BGA package which has a ball of solder attached to it.
The flow chart in
FIG. 1
depicts a conventional method for isolating BGA device pins and routing the pin signals to alternate locations. As shown in
FIG. 1
, at step
2
, a trace cut is made on the PCB at the BGA site. At step
4
, the solder layer covering the trace is peeled off to allow soldering of a rework wire to the trace. Next, at step
6
, a rework solder wire is attached to the BGA trace close to the pad on the PCB using a high temperature solder. The rework wire is not allowed to touch the pad because the signal which is being carried by the PCB device requires the solder ball to be in contact with the pad. The rework wire is soldered to the trace using a high temperature solder which will not reflow when the BGA device reflows.
At step
8
, the rework wire is routed on the PCB between the BGA pads. At step
10
, the rework wire may optionally be secured by attaching it to the PCB. The BGA device is then screen printed at step
12
. At step
14
, the BGA device is placed in a BGA reflow system and reflowed. Finally, at step
16
, the rework wire is used to route the signal at the repaired location on the PCB to an alternate location on the PCB.
FIG. 2
depicts a PCB
26
illustrating the results obtained by using the prior art method, described above, for performing BGA device pin isolation. As shown in
FIG. 2
, signal trace
24
on PCB
26
is cut and a rework wire
20
is attached to trace
24
, close to PCB pad
22
, using a high temperature solder. As discussed above, rework wire
20
is not allowed to actually touch PCB pad
22
. Rework wire
20
is soldered to trace
24
with a high temperature solder such that the rework wire bond does not reflow when the BGA package reflows.
The above described rework process has several disadvantages which lower the process yield and reduce the reliability of the PCB rework process. In particular, since the rework wire is generally located on the PCB in the BGA pad matrix, the PCB can no longer be screen printed. This is because, on many occasions, the PCB is reworked before any assembly takes place and hence the entire PCB cannot be screen printed due to the rework wire attached to the PCB pad. As a result, each reworked BGA device has to be screen printed individually, leading to loss of time and increased cost of the PCB rework process. The process is further complicated by the need to use solder paste when placing a new BGA device on the PCB. Screen printing a device lowers the volume of solder paste because the solder balls protruding through the stencil lower the deposition volume. Lower paste volumes can cause open circuits due to the volume of solder and flux which make up the solder paste.
Thus, it is desirable to provide a effective and efficient method for performing pin isolation and signal routing for closely spaced pins of a BGA device which also increases the process yield and reliability of the rework process.
SUMMARY OF THE INVENTION
According to the present invention, a method is disclosed for isolating a BGA device pin or other type of BGA pin on a printed circuit board (PCB) and routing the signal carried by the isolated pin to an alternate location by attaching a rework wire to the isolated pin device pad. The method according to the present invention provides an effective and efficient technique for performing pin isolation and signal routing for closely spaced pins of a BGA device while increasing the process yield and reliability of the rework process.
According to one aspect of the present invention, the need to perform a trace cut on the PCB to achieve BGA pin isolation is obviated. The present invention accomplishes pin isolation by removing the solder ball surrounding the device pad on the BGA package for the BGA pin to be isolated. A rework wire is then soldered to the exposed BGA device pad, rather than to a trace on the PCB. The rework wire is then routed to the outside of the BGA device package along the underside of the BGA device. The rework wire is then used to route the signal carried by the isolated BGA device pin to an alternate location.
According to another aspect of the present invention, BGA pin isolation is achieved with minimal process modifications to downstream processes related to removal and replacement of BGA devices. The present invention allows screen printing of the PCB rather than screen printing each device. Furthermore, downstream processes for reworking a BGA device, such as using a stencil for printing the BGA pads, or placement of a BGA device using localized heating and device reflow processes, do not have to be changed.
Further understanding of the nature and advantages of the invention may be realized by reference to the remaining portions of the specification and drawings.
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Cilia Michael C.
Dayal Gurpreet S.
Nguyen Don
Gaffin Jeffrey
Sun Microsystems Inc.
Townsend and Townsend / and Crew LLP
Vigushin John B.
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