AutoCAD Key to Reverse Engineering Flex Circuits
by Tom Woznicki
Every now and then, a call like this comes into the CAD cave:
ďIíve got a broken flex circuit and I need to buy a replacement, but I
donít have any drawings and I have no idea who made it.Ē
Time for reverse engineering!
I love working on these projects--each one is a mystery to be
solved. As Agatha Christieís detective Hercule Poirot would say, reverse
engineering exercises ďthe little grey cells.Ē In this article Iíll share
the eight steps I use for reverse engineering a flex circuit.
I use AutoCAD, Electronics Packaging Designer (EPD), Photoshop, a
flatbed scanner, an ohmmeter, a good magnifier and two calipers--a digital one
that measures in both inches and millimeters, the other a dial caliper that
measures in inches.
I scan at 600 DPI and then use Photoshop to improve the images,
increasing the contrast and adjusting the brightness so I can clearly see the
features of the flex circuit.
Figure 1: Carefully measuring features for
scaling the image.
AutoCAD is commonly used in the flex circuit world. One especially
useful feature is its ability to import JPG or other graphic images into the
The process is then repeated for the bottom image, with the
additional step of mirroring the image so it will overlap the top image
properly. I place the bottom image on a separate reference layer so I can turn
each image on or off as I need to.
At this point in the operation, I try to figure out whether the
circuit was designed in inches (mils) or millimeters and set up my database
accordingly. It makes it easier when drawing traces and constructing vias,
components and other features. For example, if there are non-plated holes that
measure close to 78 mils, I can safely assume that they are 2 millimeter holes
and the circuit was designed in millimeters. Next I pick a feature to use as my
datum point. A non-plated hole is best, but if there is no such feature the
intersection of two straight edges will do. Using the select feature, I move the
images to the zero-zero point in the database.
Using the reference images, I draw the outline features, trace
widths, lands and vias on the appropriate layers in AutoCAD. Then I measure the
actual circuit features for verification.
I check the circuit carefully for shorts and opens. Sometimes this
involves some selective destruction of the circuit--usually scraping away some
of the coverfilm to expose a trace or via. For example, if there are traces with
a via between two pads that are obviously connected but show no connectivity
when probed with the ohmmeter, itís likely that the via is defective. Scraping
the coverfilm off can help determine where and what kind of defect it is--it may
be the trace is cracked in a bending area, or the via is cracked. Knowing where
the defect is will help improve the design.
By carefully measuring the circuit thickness in areas containing
copper and other areas with no copper, I can figure out the copper weight and
the thicknesses of the polyimide and adhesive. Sometimes this involves
destruction of a small part of the circuit by pulling some of the coverfilm off
to measure the thickness.
Careful examination can reveal other attributes of the materials
and manufacturing. I look carefully to see if I can pick out any grain
structure. In areas that are exposed by coverfilm openings I look to see if the
base material is adhesive-based or adhesiveless, and I then examine the vias to
see if it was button-plated. The thickness of any stiffeners is measured.
EPD will create intelligent vias and components in AutoCAD. I
export the reference netlist that I will use in the next step.
Full disclosure: I have used EPD for almost 20 years and I find it
to be a very useful CAD tool because it runs on top of AutoCAD to make it
perform PCB design functions and create Gerber files. Last year Flex Circuit
Design Company became a minority shareholder in CAD Design Software, the folks
who write EPD. What can I say? I liked EPD so much I bought a part of the
company! (My apologies to Remingtonís Victor Kiam.)
That said, there other ways to proceed without EPD. Heck, you
donít even need AutoCAD! The key is using a mechanical CAD program that
will import images. AutoCAD LT, the scaled-down version of AutoCAD, will do it.
SolidWorks will also import images for reference, and I suspect others do as
well. Draw the outline and trace layers and export them as DXF files.
All PCB design programs I know of can import DXF files. Set up your
PCB database with the same units as your mechanical CAD database (inches or
millimeters). Create vias and components, and then import the DXF files for the
trace layers and outline. Note: The higher-end CAD tools, such as Cadence
Allegro or Mentor Expedition, require a schematic and netlist first. So youíd
have to create the components to establish pin numbers and draw the schematic
based on the recreated trace layout.
In addition to fixing whatever made the circuit fail, as long as I
have the design open I clean up any flex design rule violations. These circuits
often have no stiffeners under solder joints, vias in bending areas, no pad
capture, no teardrops or fillets, etc. Having the reference netlist makes sure I
have everything hooked up properly when Iím done.
In the documentation, I specify the info gleaned from step 5,
changing only those things that will improve the design.
A Reverse Engineering Case Study
Thermal Conductive Bonding here in Silicon Valley (www.tcbonding.com)
provides bonding solutions for high-tech companies, with a specialty for joining
materials with both elastomer and Indium bonding systems. A customer of theirs,
a Gen 10 LCD manufacturer, asked TCB to refurbish an electrostatic chuck used to
hold the glass during the manufacturing process. These chucks have a very large
flex circuit bonded to a thick aluminum plate with a gelpad elastomer. These
chucks have to be replaced every so often because the flex circuits get damaged
when a sheet of glass breaks.
These replacements are very expensive to purchase from the OEM, so
the customer asked TCB to refurbish the chuck by bonding a new flex circuit onto
it. TCB was confident that, if they could get the flex circuit designed and
built, they could strip off the old flex circuit and elastomer from the plate
and then bond the new flex circuit in place for a much lower price than the
customer was paying for replacements from the OEM.
Right from the start this circuit was a challenge. The metal plate
was about 29 inches square and the flex circuit covered the entire surface. The
flex also had a narrow tail about 9 inches long that wrapped around the plate. I
didnít want to remove the flex from the plate--I felt this would stretch and
distort it, making it harder to measure features accurately. With the flex
attached to the plate it wouldnít fit on my flatbed scanner, even in sections,
and I couldnít find any artwork, blueprint or graphics company that had a
scanner that was big enough!
Fortunately, I found a commercial photographer who took digital
pictures of eight different sections and pieced the eight pictures together with
Photoshop to provide us a full image of the circuit (Figure 2). Other parts were
easy; the pattern is actually only two conductors, so there was no need to
create a reference netlist or probe the circuit with the ohmmeter.
I mentioned in Step 2 that itís a good idea to determine if the
design was done in millimeters or mils, and it surely helped with this circuit.
After I took several measurements, it was pretty clear that the design was done
in millimeters and there were repeating patterns.
Figure 3: Traces (blue), holes and outline (red)
drawn over the image.
This circuit also had an additional twist: if you look
closely at Figure 2 it appears to be made of polyimide material, but the color
of the copper appears natural, not tinted orange as it would be if polyimide
coverfilm was applied. Using a hobby knife I peeled away the coverfilm in
one corner and found it was made with polyimide basefilm and polyester coverfilm,
an unusual combination, but understandable given the large size of the circuit.
Polyimide is much more expensive than polyester, and if a polyester coverfilm
will do the job it reduces the cost of the finished circuit.
Figure 4 shows the newly created flex circuit over the plate it
will be bonded to.
Figure 4: Wayne Simpson, President of TCB, with
the reverse engineered flex circuit.
These techniques obviously work best with two-layer flex circuits
and two-layer PCBs. I suppose it could be used with three- or four-layer flex
circuits if you can see clearly through them. Could it work on a rigid-flex
circuit? I suppose it could, if you had access to equipment that could carefully
remove layers once theyíve been scanned. If you try it and succeed, let me
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