How to handle irregular PCB design challenges

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

Today, PCBs are shrinking in size, with more and more features on the board, and with increasing clock speeds, designs are becoming more complex. So let’s take a look at how to deal with more complex shaped boards.

Simple PCI board outlines can be easily created in most EDA Layout tools.

However, when the board outline needs to fit into a complex enclosure with height restrictions, it is not so easy for PCB designers because the functions in these tools are not the same as those in mechanical CAD systems. Complex boards are mainly used in explosion-proof enclosures and are subject to many mechanical restrictions. It can take a long time and be unproductive to recreate this information in an EDA tool. Because the mechanical engineer has most likely already created the enclosure, board outline, mounting hole locations, and height restrictions required by the PCB designer. Because of the curves and radii in the board, rebuilding can take longer than expected, even if the board is not complex in shape.

However, looking at today’s consumer electronics, you’d be surprised at how much engineering is put into a small package that isn’t always rectangular. Smartphones and tablets come to mind first, but there are many more examples.

If you return 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 also has a thermal printer connected for instant receipt printing. Virtually all of these devices use rigid/flex circuit boards, where a traditional PCB board is interconnected with a flexible printed circuit to be able to fold into a small space.

Then the question is, “How do you get the defined mechanical engineering specifications into the PCB design tool?” Reusing this data in the 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 saves a lot of time and eliminates the possibility of human error. Next, we will look at each of these formats.

DXF is the oldest and most widely used format 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.

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 been 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-axis height. This format greatly improves the ability to visualize the PCB in a 3D view. Other information about keep-out areas may also be included in the IDF file, 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, 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.

Another advantage of the IDF interface is that either party can move components to new locations or change the board shape 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, it can take a long time due to the file size. In addition, it is difficult to determine what changes have been made from the new IDF file, especially on larger boards. IDF users eventually created custom scripts to determine these changes.

STEP and ProSTEP:

Designers were looking for an improved way to better communicate 3D data, and the STEP format was created. The STEP format can communicate board dimensions and component placement, but more importantly, components are no longer a simple shape with only a height value. 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 was still no mechanism to track changes.

To improve STEP file exchange, the ProSTEP format was introduced.

This format moves the same data as IDF and STEP, but with a big improvement – ​​it tracks changes, as well as providing the ability to work in the discipline’s original system and review any changes after 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 creates an ECO (Engineering Change Order) between ECAD and Mechanical groups 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. More importantly, engineers can create a complex board outline with additional constraints and then transfer this information electronically to avoid someone incorrectly reinterpreting the board dimensions, saving time.

Summary:

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