One of the most common questions that can arise when working with old boards is recovering the design data. When you have a physical board, and you want to recover the design data from the physical device, one might wonder with all of our technology whether this is really possible. Indeed it is possible to do, but it is time-consuming and prone to potential problems. You also have to be a bit of an artist to do this correctly.
In this article, we’ll outline two ways to recover design data from an old assembly. For some products, the old fabrication data can also be used to recover design data and help you get something into a PCB layout file for a CAD tool. From there, you are well on your way to recreating a PCB layout from an old PCBA or from old fabrication files.
Regenerating Design Files From Real Boards
In order to recreate a PCB layout file from a set of design files, you will need to start with the physical PCBA, or the Gerber files and BOM. It helps if you also have the schematics. To create the design database, you will need to start by recreating the schematics. This could require regenerating the following list of files, in part or in full:
- A manually compiled netlist
- The BOM, which might require testing passives with an LCR meter
- Replacements for obsolete components
- Schematic files in your PCB design tool’s file format
Once these items are determined or created, net connections need to be determined from the physical PCB. These will eventually need to be recreated into a PCB layout.
Determine Where Things Are Routed
Routing on the surface layers is generally very easy to recreate; you can take a photograph of the PCB surfaces and see the routing directly. If you’re familiar with image manipulation programs like ImageJ, you can even recreate the width and length limitations of these routes. Other image programs will allow export of the image to a DXF file, where it can then be imported into a layer in the PCB layout file.
DXF file imports show copper fills, but these are not trace or polygon objects. The traces and polygons still need to be recreated manually.
What if there are internal layers? In this case, you have two choices for exposing the connections on internal layers: by manually probing and guessing the internal connections, or by destructively inspecting the internal layers. Manual probing with a multimeter to find opens/shorts takes a lot of time, but it will not destroy the PCB. If you are willing to destroy the PCB, then it is possible to remove layers in order to see the internal copper connections.
Destructive Inspection and Imaging
If you want to precisely determine the connections in internal layers, you have to remove the top layers from the PCB. First, an inspection with a microscope along the edge of a board can be used to spot the number of dielectrics used to build a stackup. In some cases, the layer count can be seen with a high-resolution camera.
In the case where the number of layers cannot be determined easily, it may be necessary to physically remove each dielectric in the PCB to expose the inner copper. This could be done thermally, or by cutting away the top layer with a grinding tool.
If you can expose the internal copper, you can recreate it in a PCB layout application.
Once exposed, photographs of inner copper can be taken and converted into DXF files showing the inner copper. Once converted to DXF files, the data can be imported back into a PCB layout file.
Gerber-to-DXF Workflow for Copper Layers
Sometimes, with older boards, the Gerber files are also intact, and the drill positions can be pulled out of a drill drawing. In this case, there is a simple workflow that allows the Gerber files to be imported back into a PCB layout tool.
If you have the original Gerber files, you can use the following process to create PCB design files with DXF files as an intermediary:
- Import the Gerber files for copper layers into a viewer or CAM tool
- Export the Gerbers to DXF files
- Verify the DXF file scale in a drafting tool based on a reference length
- Create a blank PCB project and import the DXF files into a mechanical layer
- Transfer the drawn polygons into your copper layers
The same process can be used with silkscreen layers. When elements from the mechanical files are transferred to the signal layers, essentially all of your routing will be in the form of fills or polygons. However, because these objects are copper, they can still be assigned to nets and can make connections between components. Use the paste mask layer to line up component placement and verify you are assigning the correct net connections.
In the example below, the components were initially defined in the schematics and imported into the reverse-engineered PCB. These components can then be placed by following guidance on the paste mask layer, and the net definitions will fill in automatically. The data from the imported DXF files can then be transferred to the copper layers to make the required net connections.
Whenever you need to create or reverse engineer PCB layout data from an old PCBA, use the complete set of CAD tools in OrCAD from Cadence. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.