First Pass Success by Defining PCB Design Constraints

What You Can Takeaway

• There are many different rules and constraints in your PCB design tools, and they have specific purposes.

• Not only are the rules in place to help your design perform electrically, but also to be manufactured without problems as well.

• Some ideas for how to manage different sets of design rules and constraints.

These PCB design constraints caught the collision of an inductor and a connector in the 3D layout

The first time I was ever pulled over by a police officer was when I was 12 years old. I was on my bicycle and had pulled out in front of a car without looking. The officer decided to give me the whole show. He turned on the lights on the patrol car along with the siren, and then followed up with a command over a loud-speaker for me to pull over. I still remember being so completely terrified by that incident that I never pulled out onto a street without looking ever again.

Fortunately, that incident taught me the importance of the rules and constraints of the road, and how by following them it would help protect me. When we lay out a printed circuit board design, the rules and constraints in the CAD tools are there to keep us from making mistakes that can also cause problems. Here’s a look at the importance of defining PCB design constraints and how following them can help you to get your board manufactured quickly and efficiently without any problems.

The Different Types of Rules and Constraints in Your PCB Design Tools

When PCB design tools first came out, their ability to define and check your design was extremely limited. Over time, however, these rules evolved into the advanced constraint systems in use today. Here’s how the different rules and constraints are separated out in a CAD system today:

• Electrical: The basic design rule checks in PCB design tools started with simple metal to metal clearances to guard against shorting one net to another. Protecting against shorts is still the priority, but your constraints can do a lot more than that. For instance, the tools will also give you multiple constraints so that you can set up different clearances, trace widths, and power plane parameters for a specific net, or a group of nets. You can even set up multiple trace widths within a single rule depending on the type of trace that is being routed.

• Technology: Along with the standard electrical rules is the ability to set up additional constraints for different design technologies. These can be for power applications, RF circuitry, or high-speed designs. As an example, you can set up routing topology configurations for high-speed nets. There are also rules for minimum and maximum net lengths which will work in conjunction with your trace routing to create serpentine patterns for length control.

• Manufacturing: Although component clearances are usually included with basic electrical rules checking to prevent unintentional shorts, it is also important for manufacturing. To this end, these rules are often expanded into their own category to give greater control over the parts placement on the board. You can often specify different clearances for the same types of components on the top of the board then what they are set at for the bottom.

• Miscellaneous: You might think that there isn’t much more on a printed circuit board design that needs to be checked, but there actually is. Unplated tooling and mounting holes usually don’t have any electrical connections, and yet you still need to check that your components aren’t too close to them. It is also very handy to find out if you have silkscreen shapes that are hidden by components or other objects.

Yes, there’s a lot on a PCB design that needs to be controlled with constraints. Let’s take a look at some of the problems you might see if these constraints aren’t in place.

Setting up the physical design constraints to govern trace routing widths

Manufacturing Problems from Not Defining PCB Design Constraints

The need for PCB design rules and constraints to prevent electrical shorts and promote the best operating performance of the board is obvious. What isn’t always as obvious though is how important designing for manufacturing (DFM) is. Without adhering to the proper DFM rules and constraints, your circuit board may end up with some real problems when it is being built.

For wave soldered boards, the orientation and placement of the components are especially critical. Parts that are too close to each other or larger parts that proceed smaller parts into the wave may shadow the trailing part. This can result in the trailing part not achieving good solder joints which could lead to intermittent connections or outright opens.

Boards that are going through the solder reflow also have their own concerns. If the amount of metal is uneven between the two pins of a surface mount discrete part, a thermal imbalance may cause the component to stand up on end during solder reflow. This condition is known as “tombstoning,” and is an example where the trace sizes of the electrical rules need to also consider the needs of assembly.

Another problem that can spring up during manufacturing is if the proper clearances between components haven’t been designed into the board. Parts that are too close to each other may affect whether or not the automated assembly equipment can place them. Components that are too close can even affect whether or not the manufacturing technicians can access them for rework, debug, and test.

Setting up the design for assembly package to package spacing in a constraint manager

Putting the Rules in the Tools

The important thing is to get these rules and constraints into your PCB design so that you can benefit from the protection that they offer. You can of course manually enter all of these rules in, but there are usually templates and wizards in the tools that can help you to apply constraints across multiple nets, parts, or groups. Another way that you can help yourself is by keeping a collection of your previous rules somewhere for easy access. With a library of existing rules to work with, you will save yourself a lot of time and effort when setting up the constraints on your next design.

It is also very helpful to work with a PCB design system that has multiple design constraint features and capabilities built into it. A good example of a PCB design tool that helps you with setting up your design rules and constraints is OrCAD PCB Designer. With OrCAD you have a full-featured constraint manager that allows you to set up the greatest amount of control over all of the design rules and constraints that you will need.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.