What we expect of a complete PCB is usually a neat rectangular shape. While most designs are indeed rectangular, many require boards with irregular shapes, which are not always easy to design. This paper introduces how to design PCB with irregular shape.

Today, PCBS are getting smaller and more and more functions are added to the boards, which, coupled with the increase in clock speeds, make designs more complex. So, let’s look at how to deal with a circuit board with a more complex shape.

As figure 1 shows, simple PCI board shapes can be easily created in most EDA Layout tools.

Figure 1: Appearance of common PCI circuit board.

However, when board shapes need to be adapted to complex enclosures with high limitations, it is not easy for PCB designers because the functions in these tools are not the same as those in mechanical CAD systems. The complex circuit board shown in Figure 2 is designed primarily for explosion-proof housing and is subject to many mechanical limitations. Trying to reconstruct this information in EDA tools can take a long time and be unproductive. It is likely that the mechanical engineer has already created the housing, circuit board shape, mounting hole location, and height limits required by the PCB designer.

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

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

Because of radians and radii in the circuit board, reconstruction may take longer than expected, even if the circuit board shape is not complex (as shown in Figure 3).

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

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

These are just a few examples of complex circuit board shapes. However, from today’s consumer electronics, you’d be surprised how many projects try to cram all the functionality into a small package that isn’t always rectangular. Smartphones and tablets are the first things that come to mind, but there are plenty of examples.

If you return a rental car, you may be able to see an attendant using 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/flexible circuit boards (Figure 4), where conventional PCB boards are interlinked with flexible printed circuits so that they can be folded into small Spaces.

Figure 4: Rigid/flexible circuit board allows maximum use of available space.

Figure 4: Rigid/flexible circuit board allows maximum use of available space.

The question, then, 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 information into PCB Layout software using DXF, IDF or ProSTEP format. This saves a lot of time and eliminates the possibility of human error. Next, we’ll take a look at each of these formats.

Graphics interchange format – DXF

DXF is one of the oldest and most widely used formats for electronically exchanging data between 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 interchange. DXF imports/exports require additional functionality to control the layers, different entities and units that will be used in the exchange process. Figure 5 is an example of importing very complex circuit board shapes in DXF format using Mentor Graphics’ PADS tools:

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

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

A few years ago, 3d functionality began to appear in PCB tools, and there was a need for a format that could transfer 3D data between machines and PCB tools. From this, Mentor Graphics developed the IDF format, which has since been widely used to transfer circuit board and component information between PCBS and machine tools.

While the DXF format contains the board size and thickness, the IDF format uses the X and Y positions of the component, the component bit number, and the z-axis height of the component. This format greatly improves the ability to visualize a PCB in a 3D view. Additional information about forbidden areas, such as height restrictions 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 contained 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, IDF exports can add missing information during creation.

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

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 the component to a new location or change the board shape, and then create a different IDF file. The disadvantage of this approach is that you need to re-import the entire file representing changes to the board and components, and in some cases it can take a long time because of the file size. In addition, it can be difficult to determine from the new IDF file what changes have been made, especially on larger boards. Users of IDF can eventually create custom scripts to determine these changes.

STEP and ProSTEP

In order to better transmit three-dimensional data, designers are looking for an improved way, STEP format came into being. The STEP format can transmit circuit board dimensions and component layouts, but more importantly, components no longer have a simple shape with only a height value. STEP component model is a detailed and complex representation of components in three – dimensional form. Both circuit board and component information can be transferred between the PCB and the machine. 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 and has a big improvement – it can track changes and also provide the ability to work within the discipline’s original systems and review any changes once a baseline has been established. In addition to viewing changes, PCB and mechanical engineers can approve all or individual component changes in layout, board shape modifications. They can also suggest different board sizes or component locations. This improved communication creates an ECO (Engineering Change Order) between ECAD and the mechanical team that never existed before (Figure 7).

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

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

Today, most ECAD and mechanical CAD systems support the use of the ProSTEP format to improve communication, saving a lot of time and reducing costly errors that can result from complex electromechanical designs. What’s more, engineers can save time by creating a complex circuit board shape with additional constraints and then transmitting that information electronically to avoid someone misinterpreting the circuit board’s dimensions.

conclusion

If you have not already used any of these DXF, IDF, STEP, or ProSTEP data formats to exchange information, you should check their usage. Consider using this edi to stop wasting time recreating complex board shapes.