How to use Circuit Design Software for PCB DFM Guidelines
I once had the privilege of touring the USS John C. Stennis (CVN-74) courtesy of a family member who served aboard her. Although secure areas were off limits to me, I still was able to see a good portion of the ship. I saw everything from hangars and workshops to crew quarters and the galley. I even stood on the foremost part of the flight deck, right where Optimus Prime stood at the end of the second Transformers movie. Through it all, I was impressed with how everything onboard was designed to support the primary goal of the ship: the launch and recovery of its aircraft.
When we as PCB designers create a new board design, we should also have a primary goal in mind. We need to design our boards so that they can be manufactured as error-free and inexpensively as possible. To do this we need to design according to manufacturing guidelines that will best help the fabrication and assembly vendors to build the boards. These guidelines are known as “Design for Manufacturability,” or simply DFM rules.
If you haven’t spent much time working with PCB DFM guidelines before, then this will be a good introduction to the subject. We will look generally at what DFM rules are and how they can help you.
What is DFM?
Design for Manufacturability rules and guidelines are a collection of PCB design best practices to help your design be as easily manufacturable as possible. The easier it is to manufacture a PCB, the more likely it will be to reduce both manufacturing errors and costs. DFM rules have been developed by PCB manufacturing experts and they will vary depending on the design technology of the board, and the capabilities of the manufacturer.
Design for Manufacturability rules and guidelines have been around for as long as PCB manufacturers have been building boards. These rules have changed and grown over time as PCB technologies have become more complex. For example, 30 years ago the standard board had only thru-hole with 12 mil traces and spaces. This level of board technology didn’t require much in the way of DFM rules. The list of rules was short and the manufacturing process had a lot of tolerance in it. This was forgiving for designers unfamiliar with the PCB DFM guidelines. Today’s PCB technologies are a different story though. Fine pitch , micro-vias, and 3 mil traces require strict adherence to DFM rules to design a board that can be profitably manufactured.
Key DFM Rules for Your Printed Board Design
There are many different DFM rules that may apply to your PCB design. The specific rules are dependent on what type of board you are designing and how it is going to be manufactured. Here are some general ideas to begin your DFM exploration:
Consider the board form factor: The more complex your board shape is, the more involved it will be for the fab vendor to work with. This also includes cutouts and slots in the board. By keeping the board shape as rectangular as possible, you will save money on your fabrication costs.
Keep the components as uniform as possible: A PCB that has many different-sized components on it will slow down the manufacturing process. Each different component size requires different setups for the assembly line, so reduce these to reduce manufacturing time and expense.
Select the right manufacturer and involve them up front: Different manufacturers can have strengths and weaknesses just as any business can. Ensure that you have selected the best manufacturer for building your PCB. Once you have made that choice, it is important to also involve them as early as possible with your design. This way you can design your PCB to their DFM rules, instead of making changes later.
Using Design Software Tools to Meet PCB DFM Compliance Guidelines
The first step in ensuring DFM compliance on your PCB is to set up your DRCs. Design rule checks, or DRCs, are critical for designing your board. These rules should be set up for both circuitry requirements and the DFM rules. This ensures that your board adheres to specific trace and space widths, component spacing, and other clearance requirements. However, just because your board passes all of its design rule checks, doesn’t necessarily mean that it is fully DFM compliant.
PCB DFM guidelines encompass much more than just what you set up in your DRCs. Consider the soldering process for example. Your manufacturing vendor may require specific pad shapes and component placement orientations to prevent a phenomenon known as “tombstoning”. This is not something that DRCs are intended to look for, but if missed it would be a major DFM rules violation.
Just as I discovered how well the Stennis is designed for the goal of launching and recovering aircraft, you can design your PCB with the goal of being easily manufactured. By designing your board with PCB DFM guidelines in mind, you can help to reduce manufacturing errors and increase productivity.
Circuit design software, like Altium Designer®, has DRCs and other functionality built into it to work together with your DFM rules. This will help you to deliver a design that is DFM compliant to your manufacturer the first time.
Want to find out more about how Altium can help you to design a DFM compliant PCB? Talk to an expert at Altium.