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.

Step 1
Use the flatbed scanner to capture front and back images of the flex circuit. 

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.

Step 2
Import images into AutoCAD, rotate and scale the images. 

 

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 database.
After the top image is imported and placed on a reference layer, I draw some reference lines and rotate the image until itís orthogonal (or as close as I can get it). Then using the magnifier and calipers (Figure 1) I measure the length and width of key areas and adjust the X and Y scaling of the image so it matches as closely as possible to the actual measurements of the circuit.

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.

Step 3
Draw the outline and circuitry. 

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.

Step 4
Verify the finished trace layout with an ohmmeter. 

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.

Step 5
Carefully examine the circuit to determine materials, copper thickness and manufacturing processes. 

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.
Step 6 
Use EPD to build intelligence into the AutoCAD database and export a reference netlist. 

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.

Step 7
Improve the design.

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.

Step 8
Create fabrication and assembly drawings.

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!



Figure 2: Top image of the flex circuit, created by combining eight different digital pictures. 

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.
I drew the patterns in AutoCAD and dimensioned the circuit outline, trace layout and hole locations I created. Then I measured the circuit itself and verified the dimensions; it matched almost perfectly (Figure 3).

 

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.

Closing Thoughts

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 know!

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