Pre-production planning of PCB manufacturing.
What does the design process look like from beginning to end?
Tools that can assist and empower designers.
PCB engineering and design requires several work hours, even for what appears to be a straightforward board.
Any engineering process is an involved set of interlocked steps, with the final product requiring multiple hours of trial and error to achieve the design goals stated at the outset. It’s important to realize how rigorously a design must be probed and tested because it is inevitable that any mistakes or oversights the development team is unable to catch, end users will. The PCB engineering and design process is as complex as just about any system, and considering the ubiquity of electronics in modern industry, it’s no surprise it must incorporate the knowledge of several disciplines to achieve the desired performance and reliability.
Preparations Before Manufacturing
Design sounds straightforward: revise an existing design to improve performance and increase functionality, or devise something entirely new using existing theory, manufacturing technology, and best layout practices. In actuality, design comes in a variety of flavors. Not every layout necessarily proceeds to manufacturing, and even if it does, manufacturing itself may differ intensely depending on the production quantities. As engineering, design, and manufacturing are a group of symbiotic relationships, it naturally follows that as design changes, so will the demands and goals of the other processes.
Develop Proof of Concept
For new designs, the first step is developing a proof of concept to establish viability. With the global supply chains still recovering from shutdowns and exceptional backlogs, the need for rounds of evaluation is more important than ever. Under normal circumstances, this portion of design also evaluates theoretical performance which, when used in conjunction with analysis tools like circuit simulators and technical data, provides an exceedingly accurate estimation of the board’s parameters. What’s more important at this stage of development is not necessarily material cost (with the possible exception of boards with extreme requirements like aerospace), but a time-cost review to project the timeline for research and development to an assembled board.
Rome wasn’t built in a day; the same is true of any PCB. While significant efforts are invested across the entire production spectrum to avoid mistakes, the sheer complexity of a project marrying a range of engineering topics and disciplines is very nearly bound to encounter some unforeseen setbacks. This is a perfectly normal outcome of any engineering process. Changes may even arise due to the living nature of most designs: due to emergent trends in customer demands, market availability, or a host of other inputs, alterations will need to be made that are no fault of any portion of the product development team.
Anticipate Error and Communicate Clearly
With that said, a good design team anticipates some common errors and enacts practices to minimize their prevalence and significance. Communication is king in any collaborative process; eliminating ambiguity reduces the chance of misinterpreting documents and helps maintain design intent from beginning to end.
PCB development is an immensely collaborative process.
Establish Design Constraints
Especially for interdepartmental groups, engineering, layout, and fabrication need to devise a design rule list that encapsulates the performance of the board’s most demanding devices and features while adhering to the sophistication of the manufacturing equipment. One important exception is a board that is only slated for testing. Here, design constraints can be set to maximize layout speed and efficiency, as the design (or at least this stage of it) is not intended for manufacturing. Layout designers can flout some common minimum rules that would result in unproducible features or require processing far in excess of acceptability for most on a per-board basis.
A section of design constraints is due to the reliability needs of the final board. Different sectors of industry can be more permissible of particular defects in the manufacturing process. In absolute terms, this represents a cost escalator across different quality classes of the board, as the equipment and processes remain constant but yield decreases. These classes are numbered one through three, with a higher-numbered class representing more stringent reliability as well as the overall value of the board itself. For example, a class one board is typically viewed by manufacturers as disposable whenever the board experiences failure, but it may be more cost-effective for a class two or three board to undergo repair in that instance.
Design for Manufacturability
Design for manufacturability (DFM) counters design for testing by focusing on the abilities and sophistication of the manufacturing equipment as the deciding factor in designing the ruleset of the board. This is a forward and reverse process in that sense: manufacturing capability defines the operations of the board and the needs of the board help push the manufacturing capabilities to their achievable extreme performances. DFM acts as a much longer-lasting development cycle due to the constant ability to revise and improve the performance of the board or improvement to the yield, whereas design for testing only needs a single board file so long as the netlist remains the same. Because the design for testing is completely isolated from any manufacturing concerns, the bulk of its work focuses on the simulations that will help guide the actual settings for manufacturing guidelines.
It’s important to understand DFM comes in many different flavors as the design matures over the revision process. What begins as a relatively simple prototype quickly evolves into a refined form as better layout techniques are employed based on feedback from the production line. Ultimately, DFM culminates into an extremely optimized build and layout for high production volumes where poor yields or inefficiencies can quickly become costly; but before then, prototype boards are also used by the engineering team to test software. The beauty of electronics is that almost any problem can be broached as purely hardware, software, or some hybrid solution. The development team will not only want to test software for validation purposes but also to determine whether the iterative design process can be improved through that avenue.
Production: Where Engineering and Design Come to a Head
The design process workflow is a tool to help understand the individual tasks that make up the work on a board project, but also the chronology and why these work items are arranged in the manner they are. At the moment, the schematic passes from the engineering team to the layout team, and a complex set of steps must be followed in order to design a board conducive to improved yields. Though this list is not comprehensive, as individual board needs or structures may require additional processing steps, it provides a good basis of what to expect during the design process:
- Land pattern design - Working from the BOM, designers (or perhaps a dedicated librarian) will need to use wizard tool part generators or manufacturers’ datasheets to create accurate patterns for component solderability. In the case of polarized components, these patterns will also include indicators for the rotation of the part, which is necessary for accurate routing.
- Schematic capture - The extraction of the netlist from the schematic. While EDA has mercifully made this process achievable within a couple of keystrokes or clicks, it is still a very complicated process on the back end: extracting the associated land, part reference, and other parameters to provide layout designers with a graphic for manipulation in the board file. This data extraction from the schematic to the board also provides a link between the two documents and can be used to update the board file from the schematic (in the case of any schematic updates), or from the board file to the schematic. These annotation styles, known respectively as forward and backward, underscore the importance of establishing a controlling document to serve as a restoration point in case of a change in a project or to reverse a mistake.
- Synthesis - It may be quicker or easier to describe the functionality of a circuit using a high-level programming or logic language and convert the description into a schematic file that maintains the electrical relationships. One advantage to approaching a project from the software side is that it is readily translatable to target devices such as microcontrollers or FPGAs for rapid testing and refinement before a single board is manufactured.
- Simulation - Alongside software representations of the circuit, simulations offer early feedback and targeting prior to manufacturing. Designers can quickly probe a circuit and inject waveforms for analysis using extremely accurate models.
- Layout - The bulk of the designer’s work will be dedicated to arranging land patterns, routes, and copper features to not only satisfy the netlist, but also the thermal and electrical demands of devices. First, designers need to focus on arranging the components by sub-circuit, including any associated “nearby” devices like decoupling capacitors. There needs to be enough room between components to allow for routing and via fan out; especially for double-sided assemblies, consider fan outs for both sides of the board unless some more advanced via features are in use (via in pad, micro vias, etc.). Fill in any copper features on the top and bottom layers as well as shapes on the power planes for the power busses. Then begin routing, starting with the most critical signals first to prioritize their integrity. Now, all that’s left for manufacturing is cleaning up the board’s visual data and outputting artwork and fabrication/assembly files.
- Manufacturing - Here is where the rubber meets the road: it is expected during the early prototypes of the board that optimization and changes will come fast, but eventually, a board will be produced that is a good representation of the final product. This board will undergo testing to evaluate its performance and verify manufacturing steps, and any additional refinement of the design can work with physical system feedback.
Luckily, paper schematics are mostly a relic of the past, but the transformation is striking!
Cadence Tools Can Assist Product Development
PCB engineering and design is a complex manufacturing topic that invokes multiple disciplines; however, the workflow process is straightforward, and given a dedicated team and enough time, the manufactured board will reflect its intended functionality and characteristics. With any manufacturing production, it’s easy to overlook small mistakes that can eventually add up to costly revisions and rework delays in time-to-market, or other consequences of an unsatisfactory design process. Luckily, Cadence’s PCB design and analysis software provides a wealth of tools for validation to ensure your design intent is maintained. Included with OrCAD PCB Designer is the robust Constraint Manager, which saves time and frustration by automatically checking placed features for rules adherence.
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