I love lists. Mentor Graphics just released a free version of a design rule checker with a list of the top 22 potential problems to watch out for in your next design. The list is free to view and an automated DRC checker with the first eight in the list is available for free download. An automated tool with all 22 is available for purchase.
It’s both a handy reference guide and an automated checker for any layout format.
No matter how well you understand the design principles, and no matter how much experience you have in designing boards, it’s easy for some problems to slip through the cracks because you just forgot to check that feature, or your design is just too big to manually check. This is where a design rule checker comes in.
Many EDA layout tools have one built in. They are sometimes called an expert system, in that it’s like having an expert sitting next to you, nudging you in the ribs when it notices a feature in your design on its list for which you might want to give a second look.
In the Mentor Graphics HyperLynx DRC, each rule has a short video with a description of what the rule checks and how to set up the constraints.
Here is their list. Even if you don't use this DRC tool, it’s a handy list to pin to your wall.
- Impedance target, based on the cross-section geometry
- T-fork, special to DDR routing, looking for equal length branches
- Differential impedance target, based on the cross section
- Differential pair- coupling and skew
- Differential pair phase matching- adequate length matching compensation
- Decoupling capacitor placement- keeping them close to the IC pins
- Metal islands- floating metal created during a copper fill
- Nets crossing gaps- should have been #1 in the list
- Edge rate- identify nets with fastest edge rates and most sensitive to SI problems
- Guard traces- making sure the guard traces have high enough density of ground vias
- Long nets- watch out for the longest traces that may need terminations.
- Long stubs- should be #2, if routing stub is too long. Can be a killer problem at higher data rates.
- Multiple vias- if there are too many via transitions in a signal path.
- Termination check- is the termination scheme adequate, and close to the RX
- Cross talk coupling- is the coupled length too long for the coupled spacing
- Edge rate to period- is the rise time too long compared to the period?
- Topology: star- if you use this topology, make sure legs are equal lengths
- Via stub length- especially important in very high-speed serial links > 5 Gbps
- Topology: fly-by- typical in DDR3 or DDR4. Checks for the lengths of each segment
- Decap order- (ed note: I think this is a misleading rule)
- Power/gnd width- makes sure the width of power traces are wide enough for the current
- Signal supply- makes sure the correct supply lines go to the correct IC pins