Implementation Strategies for Design for Manufacture and Assembly Constraints

Have you ever had one of those days when you beat the alarm clock and jump out of bed ready for the day! You glance out the window and the sun is just smiling at you. So, you head out the door, jump in your car and set a course for work. The day could not be better...until...out of nowhere a car turns into your lane with no signal. You slam on the brakes, but it is too late. You hit the car in the back. BAM! The next thing you know, you are in pain watching little birds fly around your head. And off in the distance, there is the sun, just a faint glow slowly setting. The day could not be worse. 

This accident was an avoidable rear-end collision. Avoidable, because if the driver had signaled the intent to turn I would have slowed down and left ample room for the car to move into the lane ahead of me. This lack of good judgment or foresight even when it is known that the outcome may be bad is far too pervasive on the road and also during PCB design.

In order for your board design to be manufactured, your contract manufacturer (CM) must receive a complete design file package that includes all specifications for the design. In order for your board to be manufacturable, you must ensure that your design specifications fall within your CM’s design for manufacture (or manufacturing) rules or constraints. Similar constraints exist specifically for PCB assembly. Let’s take a look at why these rules and constraints are important and then discuss implementation strategies for design for manufacture and assembly for your PCB design.  

Why is PCB Design for Manufacture (DFM) and Assembly (DFA) Important?

Before answering the question of importance it is probably helpful for us to define design for manufacture (DFM) and design for assembly (DFA) in terms of PCB development. The development process for circuit boards consists of three distinct stages: 1) design; 2) manufacture or manufacturing; and 3) testing or bring up. The manufacturing stage is itself divided into two major activities: 1) fabrication, where the bare board is constructed without components; and 2) assembly during which the components are soldered onto or through the board. Now, for PCB development 

Design for manufacture or manufacturing (DFM) refers to making design decisions; including material selection, PCB layout, stackup and via options for the ease of fabrication and assembly.

Design for assembly (DFA) refers to design decisions; including component footprint spacing, clearances, board edge spacing, solder masking, silkscreen layout and panelization to facilitate good PCB assembly or PCBA.

Designing for Assembly and Fabrication can be simple with the right software

DFA rules are a subset of DFM rules that is focused on ensuring that the components are securely attached to the bare board. These rules or constraints are very important as they directly determine the manufacturability of your board. Ill-defined constraints may not prevent your board from being made; however, multiple problems may arise during operation that may require maintenance or even replacement. To prevent any problems that may shorten or upset operation or shorten your board’s normal lifecycle you should use the best available source for DFM and DFA from the list below.   

Sources for DFM and DFA

• CM Specific

This is the best source as it ensures that your specifications will be in-line with what is possible by your CM. 

• General Industry Standards

If you cannot obtain direct DFM and DFA rules and/or guidelines from your CM, then following general industry standards is the next best option; although, you should be prepared for some back and forth with your CM and design changes before a manufacturable design is achieved.

• Program Defaults

Simply, using your design program’s default values is not the best source. However, if DFM and DFA are not readily available, it can be a good starting point. As with using industry standards, it is highly likely that you will need to modify your design prior to getting any boards spun.

The benefits of good DFM and DFA are many. Not the least of which are a better design and faster bring up. Whether you began there or not, your final design constraints will reflect your CM’s equipment capabilities and tolerances; otherwise, your board cannot be made. Just as there are multiple sources for DFM and DFA, there are different implementation strategies. Let’s take a look at the options.

Implementing Design for Manufacture (DFM) and Assembly (DFA) for PCBs

The importance of using DFM and DFA for your PCB development cannot be overstated. However, the method of implementation can impact your design. Specifically, if the specifications are flawed or incorrect they can cause you to make other design decisions that may negatively affect the quality of your design and its manufacturability. For example, by choosing trace widths that are too narrow to be etched will force your CM to return your design for corrections. Subsequent increases in trace widths may cause spacing and clearance violations that must also be fixed. This could mean changing component placements or redoing your PCB layout altogether. 

If you are using one of the more capable PCB design software programs then you will probably have at least three options for implementing design rules and constraints. These are creating design rule check (DRC) rules from scratch, editing the existing default rules and uploading a DRC file created by your CM. These three strategies are compared in the table below.

As shown above, uploading your CM’s DRC file is the simplest, fastest and most precise of the implementation strategies for design for manufacturing and assembly constraints. Implementing this strategy also means that you are utilizing the best source for DFM and DFA. Your CM.

The utilization of accurate DFM and DFA from your CM is essential for your design to be manufacturable. However, the implementation strategies employed can make the difference between slow and possibly inaccurate integration into your PCB design software or quick and precise incorporation, such as when using the DesignTrue DFM ecosystem by Cadence.

OrCAD, the PCB design and analysis package from Cadence amplifies the advantages of integrated DFM and DFA. And innovative PCB layout and routing tools allow you to visualize design constraints for DFM and DFA in real time and avoid errors that cause delays and increase manufacturing costs. 

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