When my youngest son was very little, he didn’t understand that we had put rules into place to protect him. Even though he had been told not to play in the kitchen when someone was cooking, he decided that he needed to find out why for himself. Fortunately, the stove burner wasn’t completely hot, and he didn’t injure himself. He found out that day, though, that sticking to the rules was actually a good thing and that you should never put your hand on a hot stove.
In PCB design, we have rules to follow as well that are intended to protect us from creating PCB designs that can’t be assembled. Here’s a look at some of those PCB trace design rules and how by using them, you can help yourself design a better board.
Manufacturing Problems That Can Happen If Design Rules Aren’t in Place
PCB design tools usually contain a lot of design rules that can be configured and enabled for many different areas of the board design. These rules can govern most everything from how wide a trace is to how close components can be to each other. By using these rules, you can protect yourself from many problems that could eventually cause problems during the manufacturing of the board. Here are some of the problems your board may experience during manufacturing if these rules aren’t followed:
- Component location: In some instances, components may need to be restricted from being placed in certain areas of the board due to clearances or to help with manufacturing. If these restrictions aren’t followed, there may be problems assembling these components onto the board.
- Component spacing: Components that are placed too close together can be at risk of poor solder connections. Their location and rotation may also pose problems for automated wave or reflow soldering processes, as well as manual assembly.
- Testpoint availability and placement: Testpoints must be placed with enough clearance to interface with automated test equipment. If these rules aren’t followed, the board may not be fully testable.
- Component land pattern size: Although not a CAD tool design rule, land pattern sizes that don’t adhere to industry standards can cause a lot of problems. Pads that are too small may not form a good solder joint to their pins during automated soldering. Pads that are too big may cause the part to float out of position.
- Trace routing: Trace sizes can cause problems for manufacturing if they don’t follow recommended manufacturing rules. Sharp corners in traces can create acid traps where fabrication chemicals will continue to etch the metal trace further than desired. Wide traces used for power or ground routing into chip components may create a thermal imbalance during soldering, causing the part to pull away from one of the pads.
To avoid these problems, there are several different design rules that you can set up in your PCB design tools.
The Different PCB Trace Design Rules that Can be Set Up
We obviously can’t describe here the configuration of all of the design rules in each CAD system. We can, however, give you a general description of the design rules you will likely be working within your tools for trace routing:
- Default width and spacing: You will be able to assign a trace width and spacing for all nets on the board.
- Specific width and spacing: Unique width and spacing rules can also be assigned to specific nets or groups of nets. This is useful for routing wider traces for power and ground nets or specifying width and spacing parameters for a controlled impedance line.
- Area rules: You will be able to set up areas on the board where the regular rules don’t apply. This is handy when you need to route thinner traces out of a BGA, which will then widen back up when the trace exits the area rule.
- Keepout areas: You will also be able to designate certain areas of the board where you don’t want any routing to go through.
- Layer specific rules: Your tools will be able to designate specific layers to certain nets or groups of nets, as well as setting up layer routing directions.
In addition to these general design rules and constraints, most PCB design tools will give you many advance rules and constraints to work with as well. These can include the following:
- Measured lengths: You can set up a specific length and tolerance for the trace that you are routing, such as 3.5 inches with a tolerance of 0.1 inches (3.4 to 3.6 inches).
- Length matching: To maintain the high-speed timing constraints of clock and data lines, trace lengths usually must be matched together. To do this, the CAD tools will allow you to set a specific trace length, which is usually the length of the longest net, and all the other traces in the matched group will need to increase their length to match it.
- Differential pairs: To accurately route diff pairs together, you can assign them to diff pair rules that will route the two lines as one. Combined with measured and matching length rules will give you precise control over these pairs.
- Trace topologies: You can also set up rules that will guide you in routing specific trace patterns or topologies. You may designate a net to be routed as a daisy chain, while others may want to be routed as a starburst.
- Signal Paths: High-speed design often requires measuring the signal from the source, through various components, and then terminating at the destination. Since this could involve multiple nets, the single net measured lengths and length matching rules aren’t helpful. By assigning signal path design rules, you can control the length and topologies of the entire signal path instead of only each net segment.
These are just a sampling of the trace routing rules that PCB design CAD tools offer today. Additionally, you will find design rules that will govern component placement and power and ground plane creation. The key is knowing which values to set these rules.
Your PCB Contract Manufacturer: A Source for Design Rule Data
Fortunately, you already have a partner in your board design that can help you with questions such as setting up your design rules. When you work with a quality PCB CM, they should have a vast amount of experience building circuit boards and understand all of the data you need.