Teach you how to design irregular shaped PCB

The complete PCB we envision is usually a neat rectangular shape. While most designs are indeed rectangular, many designs require irregularly shaped boards, which are often not easy to design. This article describes how to design irregularly shaped PCBs.

Today, PCBs are shrinking in size, with more and more features packed into them, and with increasing clock speeds, designs are becoming more complex. So, let’s look at how to handle circuit boards with more complex shapes.

As shown in Figure 1, a simple PCI board outline can be easily created in most EDA Layout tools.

Figure 1: The outline of a common PCI board.

However, when the board shape needs to fit into a complex enclosure with height restrictions, it is not so easy for the PCB designer because the capabilities in these tools are not the same as those in mechanical CAD systems. The complex board shown in Figure 2 is primarily for an explosion proof enclosure and therefore has many mechanical constraints. Recreating this information in an EDA tool can be time consuming and unproductive. This is because the mechanical engineer has most likely already created the enclosure, board shape, mounting hole locations, and height constraints that the PCB designer needs.

Figure 2: In this example, the PCB must be designed to certain mechanical specifications so that it can fit inside an explosion-proof container.

Even if the board shape is not complex (as shown in Figure 3), the rebuild time may be longer than expected due to the curvature and radius in the board.

Figure 3: Designing multiple arcs and different radius curves can take a long time.

These are just a few examples of complex circuit board form factors. However, looking at today’s consumer electronics, you’d be surprised at how much engineering goes into trying to fit all the functionality into a small package, and that package isn’t always rectangular. Smartphones and tablets might be the first examples that come to mind, but there are many more.

If you've ever returned a rental car, you've probably seen the valet use a handheld scanner to read the car's information and then communicate wirelessly with the office. The device is also connected to a thermal printer for instant receipt printing. Virtually all of these devices use rigid/flex circuit boards (Figure 4), where a traditional PCB board is interconnected with a flexible printed circuit to allow it to be folded into a small space.

Figure 4: Rigid/flexible circuit boards allow for maximum utilization of available space.

So, the question is, “How do you get the defined mechanical engineering specifications into your PCB design tool?” Reusing this data in mechanical drawings eliminates duplication of work and, more importantly, human error.

We can solve this problem by importing all the information into the PCB Layout software using DXF, IDF or ProSTEP formats. Doing so can save a lot of time and eliminate possible human errors. Next, we will look at each of these formats one by one.

DXF is one of the oldest and most widely used formats for electronically exchanging data between the mechanical and PCB design domains. It was developed by AutoCAD in the early 1980s. This format is primarily used for 2D data exchange. Most PCB tool vendors support this format, and it does simplify data exchange. DXF import/export requires additional features to control the layers, different entities, and cells that will be used in the exchange process. Figure 5 shows an example of importing a very complex board outline in DXF format using Mentor Graphics’ PADS tool:

Figure 5: PCB design tools, such as PADS presented here, need to be able to control the various parameters required using the DXF format.

A few years ago, 3D capabilities began to appear in PCB tools, and a format was needed to transfer 3D data between mechanical and PCB tools. As a result, Mentor Graphics developed the IDF format, which has since become widely used to transfer board and component information between PCB and mechanical tools.

While the DXF format includes board dimensions and thickness, the IDF format uses the component's X and Y position, component reference number, and component Z-height. This format greatly improves the ability to visualize the PCB in a 3D view. Additional information about keepout areas, such as height limits on the top and bottom of the board, may also be included in the IDF file.

The system needs to be able to control what will be included in the IDF file in a similar way to the DXF parameter settings, as shown in Figure 6. If some components do not have height information, the IDF export can add the missing information during the creation process.

Figure 6: Parameters can be set in the PCB design tool (PADS in this example).

Another advantage of the IDF interface is that either party can move components to new locations or change the board outline and then create a different IDF file. The disadvantage of this approach is that the entire file representing the board and component changes needs to be re-imported, and in some cases this can take a long time due to the file size. In addition, it can be difficult to determine what changes were made from the new IDF file, especially on larger boards. Users of IDF eventually created custom scripts to determine these changes.

Designers were looking for an improved way to transfer 3D data, and the STEP format was born. The STEP format can transfer board dimensions and component placement, but more importantly, components are no longer simple shapes with only height values. STEP component models provide a detailed and complex representation of components in 3D. Both board and component information can be transferred between PCB and mechanical. However, there is still no mechanism to track changes.

To improve STEP file exchange, we introduced the ProSTEP format. This format moves the same data as IDF and STEP, but with a big improvement – ​​it tracks changes and also provides the ability to work in the discipline’s original system and review any changes after a baseline is established. In addition to reviewing changes, PCB and mechanical engineers can approve all or individual component changes in layout, board outline modifications. They can also suggest different board sizes or component locations. This improved communication creates an ECO (Engineering Change Order) between the ECAD and Mechanical groups that never existed before (Figure 7).

Figure 7: Propose changes, review changes on the original tool, approve changes, or make a different suggestion.

Most ECAD and mechanical CAD systems now support the use of the ProSTEP format to improve communication, saving significant time and reducing costly errors that can occur with complex electromechanical designs. More importantly, engineers can create a complex board outline with additional constraints and then electronically communicate this information to avoid someone incorrectly reinterpreting the board dimensions, saving time.

If you are not already exchanging information using these DXF, IDF, STEP or ProSTEP data formats, you should check out their use. Consider using this electronic data exchange and stop wasting time recreating complex board outlines.