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The rigors of pass-fail are not just the
pressure-cooker of classroom angst in the world of education. In the
business world, the difference between 'Pass Inspection' and
‘Failing to Meet Standards' could amount to thousands of millions of
dollars. One commercial pioneer, Electronic Packaging Company (EPC),
has developed a new system that considerably reduces the size of the
proverbial 'scrap heap.' EPC's particular application involves the
repairing of 'failed' printed circuit boards (PCBs); usually because
the technician is unable to locate the short. Using their
conventional repair stations, an average of 65% of the boards were
classified as 'irreparable.'
Now, using a highly accurate CNC Gantry, from Techno-isel, New Hyde
Park, NY as the base platform, EPC's new computer-aided PCB repair
system has significantly reduced scrapped boards. IBM, for instance,
recovered the cost of the repair station in only six months from
salvaging scrapped boards. EPC's new station, the Model 5500,
projects any 'problem traces' onto the board with an LCD projector,
which indicates the most likely areas or faults and it also detects
recurring production errors. The Techno CNC Gantry allows the
projected images from the LCD to be positioned with accuracy and
precision that is essential in today’s densely packed PCBs.
EPC, Dallas, Texas, was founded in 1982 to produce products that aid
circuit board manufacturers in the analysis and repair of errors.
The company now offers a long list of tools for the electronic
products industry including computer-aided repair systems, inner
layer inspection tools, loaded board systems, and an audible
resistance meter. EPC's customers range from Fortune 100 companies
to the smaller, family-owned shops.
Old repair method
PCB manufacturers test finished boards
for the soundness of the electrical connections on a device called a
bed-of-nails tester. The bed-of-nails is a test fixture that has
probes (that resemble nails) arrayed so that each will contact a
grid point on the board simultaneously. The fixture is connected to
automated test equipment that is programmed to iteratively check
each network for opens and shorts. If a board fails, the test system
produces a report indicating where the short is located. "The report
gives two x-y coordinates," explains Evan Evans, engineering manager
at EPC. "For example, it might say that the nail at point X23, Y19
shorted to the nail at point X99, Y87." In the past, repair
technicians took the artwork used to create the board and identified
the two traces involved in the short. Then they followed them on the
actual board to find where the two wires might be touching. That was
not difficult when boards consisted of just a single layer and it
was possible to see entire traces by turning the board from one side
to the other. After locating the problem spot, the technician
scratched it with an exacto knife to break the connection. But as
circuit boards grew more complex, it became impossible to follow the
traces visually. "In today's multi-layer boards, traces go in one
hole, then wander all through the inside of the board before coming
out the other side," says Evans.
As Evans explains, PCB manufacturers do
a lot of testing of inner board layers before they are sandwiched
together, so the inner layers are good most of the time. "Eighty
five to ninety percent of the shorts are on the outer layers," he
says. "There are many more processes on those layers.
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" he says. "There are many more
processes on those layers. Along with the etching, there is also
soldering to create pads and electrical plating of through holes."
But even when a problem is on the outside of the board and it should
be possible to repair, if the technician can't follow the traces to
find the short, he has to scrap the board. "These boards are
designated as having internal shorts and scrapped," Evans adds.
"When we asked one board manufacturer to show us all of its scrapped
boards with internal shorts, we found that 65 percent of them
weren't internal shorts at all. They were just a case of the
technician not being able to follow the traces and find the fault."
The other limitation of the past repair systems was that they gave
no indication to management of recurring problems. In one example, a
company had been building a particular board for three years at
100,000 boards per year. They were unaware that one out of 12 boards
was shorted in the same place because there were 20 different repair
people and no way to compile their experience to find this sort of
trend.
A better approach
The Model 5500 is a significant
improvement over the old repair methods because it addresses both
limitations—the difficulty of following traces on multilevel boards
and the inability to detect recurring problems. When a failed board
is brought to this repair station, the operator enters the two x, y
coordinates from the test report. The system's fault prediction
software examines the CAD data used to manufacture the board and
checks all of the pads and traces associated with the two
coordinates to determine where the wires come in close proximity.
When the fault prediction analysis is complete, the repair station
uses a color LCD high-resolution video graphics projector to display
repair data (traces, pads, components, and other information)
directly onto the circuit board. The colors of the projected image
can be changed so that there is maximum contrast between the circuit
board and the graphic data.
The system does not just display images,
however. "It incorporates intelligence so that it actually walks the
operator through the process," explains Evans. "The system
highlights the first place to put the test probe, then the second
place. It presents the problem areas one after the other so the
operator can find the fault." The failure analysis software takes
into account whether there might be internal shorts so the repair
person does not waste time repairing these external faults only to
find that the PCB must be scraped because of a non-repairable fault.
A stereo zoom microscope is included as
part of the Model 5500 so that location, verification, and repair of
very small faults that could not be seen by the naked eye can be
performed on the repair station. The repair station also features
fault ranking software that collects a history of the faults that
are reported and uses this historical data to determine the
reparability of subsequent PCBs. |
A key feature of the system is the
accuracy with which it displays the images of the traces, pads, and
so on. Positioning accuracy is critical because of the close
proximity of the traces and components on today's PCBs. In designing
the Model 5500, EPC had the option of building a gantry system on
which to hand the LCD projector and the x, y positioning system. "We
had designed many such mechanisms in the past but decided not to
reinvent the wheel this time," says Evans. An advertisement in a
trade journal convinced them to use a Techno Gantry System III.
This servomotor controlled gantry system delivers 0.0005-inch
resolution and repeatability, and 0.003 inches/foot absolute
accuracy. It is constructed on steel stress-relieved bases with
hardened steel linear ways. Its shaft-and-bearing system produces
very smooth, play-free motion and is an extremely rigid system that
produces high-accuracy positioning. The gantry's design includes
heavy cast aluminum side plates supporting the y axis, giving extra
stiffness for accuracy in positioning. Anti-backlash ball screws and
nuts are standard. These screws have excellent power transmission
due to the rolling ball contact between the nut and screws, and this
type of contact ensures low friction, low wear, and long life.
The Techno gantry comes in a wide range of work surface sizes. This
allows EPC to offer its repair system in different sizes as well.
There are three versions of the Model 5500, built on three different
models of the Techno gantry. The EPC 5500 – 027 has an active board
area of 21.5 inches by 22.0 inches (54.6 cm by 55.9 cm) and can
repair a board of 32.0 inches by 27.0 inches. The EPC 5500 - 054 has
an active board area of 40.0 inches by 22.0 inches (101.6 cm by 55.9
cm), allowing a board of 64.0 inches by 27.0 inches (162.6 cm by
68.6 cm) to be repaired. The EPC 5500 - 130 has an active board area
of 50.0 inches by 42.0 inches (127 cm by 106.7 cm). It can handle
boards of 64.0 inches by 47.0 inches.
The Model 5500 is new but several companies are already using it
with excellent success. IBM, as one example, recovered enough failed
boards in the first six months of using the Model 5500 to pay for
the system. Another example is the company that was producing
hundreds of thousands of boards with one out of 12 being defective.
This manufacturer used the system's fault ranking software to
statistically analyze the historical repair data. This enabled them
to find the source of the problem, a smudge in the original artwork
that was creating a bridge across two traces. That single find alone
went a long way toward paying for the system.
The Model 5500 offers PCB manufacturers a significant improvement
over past repair methods. By helping technicians find more fixable
faults, it reduces the number of scrapped boards. And by allowing
manufacturers to see trends in failed boards, it helps fix
production problems before they cause failed boards. A highly
accurate gantry system ensures that the Model 5500 has the necessary
precision to accommodate today's densely packed PCBs.
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