Teach you how to quickly design irregularly shaped PCBs

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

Today, PCBs continue to shrink in size, and as more and more functions are packed into the boards, coupled with increases in clock speeds, designs become more complex. So, let’s take a look at what to do with boards with more complex shapes.

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

Figure 1: Common PCI circuit board appearance.

However, when the board form factor needs to fit into a complex enclosure with height constraints, it’s not that easy for the PCB designer because the functionality in these tools is not the same as in a mechanical CAD system. The complex circuit board shown in Figure 2 is primarily used in explosion-proof enclosures and is therefore subject to many mechanical limitations. Trying to reconstruct this information in an EDA tool can take a long time and be unproductive. Because, the mechanical engineer has most likely already created the enclosure, board outline, mounting hole locations, and height constraints that the PCB designer requires.


Figure 2: In this example, the PCB must be designed according to specific mechanical specifications so that it can be placed in an explosion-proof container.

Because of the curvature and radii in the board, even if the board shape is not complex (as shown in Figure 3), the rebuild time may be longer than expected.

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

These are just a few examples of complex circuit board shapes. However, looking at today’s consumer electronics, you’d be surprised how much engineering is done to try to fit all the functionality into a small package, and that package isn’t always rectangular. Your first thoughts might be about smartphones and tablets, but there are many similar examples.

If you return a rental car, you may be able to see the attendant reading the car’s information with a handheld scanner, which then communicates wirelessly with the office. The device is also connected with a thermal printer for instant receipt printing. Virtually all of these devices utilize rigid/flexible circuit boards (Figure 4), where traditional PCB circuit boards are interconnected with flexible printed circuits to allow them to be folded into small spaces.

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Figure 4: Rigid/flex circuit boards allow maximum utilization of available space

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

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

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

Figure 5: PCB design tools (such as PADS introduced here) need to be able to use the DXF format to control the various parameters required.

A few years ago, 3D capabilities began to appear in PCB tools, and a format was needed that could transfer 3D data between machinery and PCB tools. From this, Mentor Graphics developed the IDF format, which has since been widely used to transfer board and component information between PCBs and machine tools.

While the DXF format includes board dimensions and thickness, the IDF format uses the component’s X and Y position, the component tag, and the component’s Z-height. This format greatly improves the ability to visualize PCBs in 3D views. The IDF file may also include additional information about the no-go areas, such as height restrictions for the top and bottom of the board.

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


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

Another advantage of the IDF interface is that either party can move components to a new location or change the board outline and 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. Additionally, it can be difficult to determine from a new IDF file what changes were made, especially on larger boards. Users of IDF can eventually create custom scripts to determine these changes.

In order to better transmit three-dimensional data, designers are looking for an improved way, and the STEP format came into being. The STEP format can convey board dimensions and component layout, but more importantly, components no longer have a simple shape with only a height value. STEP component models provide detailed and complex representations of components in three dimensions. Both board and component information can be transferred between the PCB and machinery. However, there is still no mechanism for tracking changes.

To improve STEP file exchange, we introduced the ProSTEP format. This format moves the same data as IDF and STEP, but with great improvements – it can track changes and also provides the ability to work in the discipline’s original system and review any changes once a baseline has been 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 created an ECO (engineering change order) between ECAD and the mechanical group that never existed before (Figure 7).

Figure 7: Propose a change, view the change on the original tool, approve the change, or suggest a different change.

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 arise with complex electromechanical designs. What’s more, engineers can create a complex board outline with additional constraints and then communicate this information electronically to save time by preventing someone from incorrectly reinterpreting the board dimensions.

If you have not yet used these DXF, IDF, STEP or ProSTEP data formats to exchange information, you should check their usage. Consider using this electronic data exchange and stop wasting time recreating complex circuit board outlines.

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