How To Design an Irregularly Shaped PCB

The complete PCB we envision is usually a regular rectangular shape. While most designs are indeed rectangular, many require irregularly shaped boards, which tend to be less easy to design. This paper describes how to design an irregularly shaped PCB.

Today, the size of the PCB is shrinking, and the functions in the board are more and more, coupled with the increase in clock speed, the design has become more complex. So, let's look at how to deal with the more complex shape of the board.

Simple PCI board shapes can be easily created in most EDA Layout tools. However, when the board form factor needs to fit into a complex housing with high limitations, it is not so easy for the PCB designer, because the functions in these tools are not the same as those in a mechanical CAD system. Complex circuit boards are mainly used in explosion-proof enclosures and are therefore subject to many mechanical limitations.

Trying to reconstruct this information in EDA tools can take a long time and not be productive. Because, most likely, the mechanical engineer has already created the housing, board shape, mounting hole location, and height restrictions that the PCB designer needs.

Because there are radians and radii in the board, even if the board shape is not complex, the reconstruction time may take longer than expected.

However, from today's consumer electronics, you'll be surprised at how many projects try to fit all the functionality into a small package that isn't always rectangular. Smartphones and tablets come to mind first, but there are plenty of similar examples.

If you return a rental car, you may see an attendant reading the car's information with a handheld scanner and then communicating wirelessly with the office. The device is also connected to a thermal printer for instant receipt printing. In fact, all of these devices feature rigid/flexible circuit boards, where traditional PCB boards are interlinked with flexible printed circuits so that they can be folded into small Spaces.

How to import the defined mechanical engineering specifications into the 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 the PCB Layout software using DXF, IDF, or ProSTEP format. Doing so will both save a lot of time and eliminate possible human error. Next, we'll look at each of these formats.

DXF is a long-standing and widely used format for exchanging data between the mechanical and PCB design domains primarily electronically. 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 cells that will be used in the exchange process.

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

While the DXF format includes board size and thickness, the IDF format uses the X and Y positions of the components, the component position number, and the Z-axis height of the components. This format improves the ability to visualize the PCB in a three-dimensional view. The IDF file may also include additional information about prohibited areas, such as height limits 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 the DXF parameter Settings. If some components do not have height information, IDF exports can add missing information during creation.

Another advantage of the IDF interface is that either party can move a component to a new location or change the board shape and then create a different IDF file.

The downside of this approach is that the entire file representing the board and component changes needs to be re-imported, and in some cases it can take a long time due to file size.

In addition, it is difficult to determine what changes have been made with the new IDF file, especially on larger boards. Users of the IDF can eventually create custom scripts to determine these changes.

In order to better transmit 3D data, designers are looking for an improved way, STEP format came into being. The STEP format can convey board size and component layout, but more importantly, the component is no longer a simple shape with a height value.

The STEP component model provides a detailed and complex representation of the component 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 making changes.

To improve the STEP file exchange, we introduced the ProSTEP format. This format moves the same data as IDF and STEP and has great improvements - it can be changed and also provides the ability to work within the discipline's original system and review any changes after establishing benchmarks.

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 group that never existed before.

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 come with complex electromechanical designs.

What's more, engineers can save time by creating a complex board form factor with additional limitations and then transmitting this information electronically to avoid someone incorrectly reinterpreting the board size.