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Hetero PCB, how do you design it?

date:2018-04-09

The complete PCB we envision is usually rectangular in shape. While most designs are truly rectangular, many designs require irregular shape boards, which tend to be less easily designed. This article describes how to design a PCB with irregular shape.

Today, the size of PCB is shrinking, and the function of the circuit board is more and more. With the increase of clock speed, the design becomes more and more complicated. So, let's look at how to deal with a more complicated circuit board.

As shown in figure 1, simple PCI circuit boards can be easily created in most EDA Layout tools.


Figure 1: common PCI circuit board configuration.

However, when the circuit board shape need to adapt to the complex shell is highly restricted, for PCB designer is not so easy, because of these tools function and the function of mechanical CAD system is not the same. The complex circuit boards shown in FIG. 2 are mainly used for explosion-proof housing and are therefore subject to many mechanical limitations. Rebuilding this information in the EDA tool may take a long time and is not productive. Because mechanical engineers are likely to have created the shell, circuit board profile, mounting hole position and height restriction required by PCB designers.


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

Because of the radian and radius of the circuit board, even the circuit board is not complex (as shown in figure 3), and the reconstruction time may be longer than expected.


Figure 3: designing multiple radians and different radius curves may take a long time.

These are just a few examples of complex circuit boards. However, from today's consumer electronics, you'll be amazed at how many projects try to add all the functionality to a small package that is not always rectangular. Your first thought should be smartphones and tablets, but there are plenty of examples.

If you return a rented car, you might see a waiter reading the car information with a handheld scanner and then wirelessly communicating with the office. The device also connects to a thermal printer for instant receipt printing. In fact, all of these devices use rigid/flexible circuit boards (figure 4), where the traditional PCB circuit board is interlinked with the flexible printed circuit so that it can be folded into small Spaces.


Figure 4: rigid/flexible circuit boards allow maximum use of available space.

The question, then, is "how do you import a defined mechanical engineering specification into a PCB design tool?" The reuse of these data in mechanical drawings eliminates duplication and, more importantly, eliminates human error.

We can solve this problem by importing all the information into the 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 look at these formats one by one.

Graphics interchange format - DXF.

DXF is one of the oldest and most widely used formats for exchanging data between mechanical and PCB design domains. AutoCAD was developed 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 the data exchange. The DXF import/export requires additional capabilities to control the layers, different entities, and units that will be used in the exchange process. Figure 5 is an example of using the PADS tool of Mentor Graphics to import very complex circuit board shapes in DXF format:


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

A few years ago, three-dimensional functions began to appear in PCB tools, requiring a format for transferring three-dimensional data between mechanical and PCB tools. In this way, Mentor Graphics developed the IDF format, which was then widely used to transmit circuit boards and component information between PCB and mechanical tools.

Although the DXF format includes circuit board size and thickness, the IDF format USES the X and Y position of the element, the element bit number, and the z-axis height of the element. This format greatly improves the ability to visualize the PCB in a 3d view. The IDF file may also include additional information about the exclusion zone, such as the height restriction at the top and bottom of the circuit board.

The system needs to be able to control the contents contained in the IDF file in a similar manner to the DXF parameter, as shown in figure 6. If some components do not have a high level of information, the IDF export can add missing information during the creation process.

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

Another advantage of the IDF interface is that either party can move the components to a new location or change the shape of the circuit board, and then create a different IDF file. The disadvantage of this approach is that you need to re-import the entire file that represents the changes in the circuit board and components, and in some cases, the file size may take a long time. In addition, it is difficult to determine which changes have been made through the new IDF file, especially on larger boards. IDF users can eventually create custom scripts to determine these changes.

STEP and ProSTEP

In order to better transmit the 3d data, designers are looking for an improved method, and STEP format has emerged. STEP format can transmit circuit board size and component layout, but more importantly, components no longer have a simple shape with only a high value. The STEP component model has detailed and complex representation of the element in three dimensions. The information of circuit boards and components can be transferred between PCB and machinery. However, there is still no mechanism for tracking changes.

To improve the STEP file exchange, we introduced the ProSTEP format. This format is removable and IDF and STEP of the same data, and has greatly improved, it can track changes, can also provide in the system of subject original work and review any changes after establishing benchmark functions. In addition to viewing the changes, PCB and mechanical engineers can also approve all or individual component changes in layout, circuit board configuration changes. They can also suggest different circuit board sizes or components. This improved communication creates an ECO (engineering change sheet) that has never existed between ECAD and the mechanical group (figure 7).


Figure 7: suggest changes, view changes on the original tool, approve changes, or make different recommendations.

Now, most ECAD and mechanical CAD systems support the use of ProSTEP format to improve communication, thus saving a lot of time and reducing the costly errors that complex electromechanical design can bring. More importantly, the engineer can create a complex circuit board shape, with additional restrictions and then pass this information electronically, in order to avoid some mistakes to reinterpret circuit board size, so as to achieve the aim of saving time.

conclusion

If you haven't used these DXF, IDF, STEP, or ProSTEP data formats to exchange information, you should check their usage. Consider using this electronic data interchange to stop wasting time recreating complex circuit boards.


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