Sometimes, there can be a gap between what designers request and what manufacturers can realize. Board design and manufacturing are both considerably complex topics, and while there is significant overlap, design and manufacturing processes may have competing goals. PCB design for manufacture (DFM) intends to bridge the gap between the practical and theoretical ends of boards, balancing optimal performance, defect-free manufacturing, and long-term reliability.
|Board Layout Analysis to Improve Yield|
|Plated holes and pads||Manufacturers examine the hole/pad size ratio and check all PTHs and annular rings for compliance with minimum values (as established by a drill chart)—similarly, quality control checks for the same ratio on internal antipads.|
|Non-plated holes||Manufacturers determine the size and location of holes and look for correspondence with fabrication artwork.|
|Signal layers||A confirmation of trace width and spacing will exhibit adherence to the targeted impedance value for each appropriate layer.|
|Acid traps||Traces exiting a pad at less than a 90-degree angle may trap acid during etchant processes, leading to later defects or board rejection.|
|General tolerance||Does the board support acceptable levels of deviance in manufacturing from true or ideal dimensioning?|
|Silkscreen and solder mask||Solder mask must be compatible with the application method and comply with IPC standards depending on pad size. Silkscreen must maintain a minimum distance with solder mask to prevent clipping.|
Does Every Project Need PCB Design for Manufacture Guidelines?
Not every board is set up for production quantities or even prototyping: the wealth of analytical features in today’s ECAD tools means board design may simply be a means to an end for more accurate simulation data. While it’s most often the case that designers use this iterative process as a stepping stone to a physical board, some designs may only progress to the layout stage for ECAD simulation software. This step allows for a robust analysis of impedance, timing, and EMI. Yet, board layouts operating purely as simulation inputs might take many design rule liberties that would be unproducible for an actual board. The issue mainly concerns design philosophy and time – if a board design has no intentions of reaching any level of production, why invest additional hours into routing and placement that will have a negligible impact on the simulation results?
The DFM Process, Step By Step
Of course, if simulation does give way to eventual production, designers can find themselves in a pickle. Manufacturers would take one look at their 1 mil traces used to expedite the BGA fanout before rejecting the job, for example. Designers must incorporate a DFM approach at the beginning of the layout to avoid common manufacturing defects from improper design:
- Pre-schematic – While choosing components for the circuit, designers must keep cost, availability, and package type in the forefront of their minds (other factors like weight, size, and temperature sensitivity may also play a role). While the assembly must meet the design intent of the project, e.g., prioritizing cost for high-volume production, the three factors are heavily correlated. Simply adding the cheapest package may complicate the assembly with a selective soldering process due to the presence of surface mount (SMT) and through-hole devices. Production schedules may also relax cost restrictions if supply stocks are dwindling or backordered. Rarely are component choices completely suitable on the first attempt, and board revisions will often be necessary to accommodate an evolving design; however, a thoughtful component selection will help to minimize placement/routing changes.
- Stackup – The stackup is the vertical construction of the board layer by layer. As such, it features alternating layers of a substrate (often a fiberglass weave with an epoxy resin to cure, set, and harden during lamination) and copper for signals and power/reference planes. Foremost, the stackup must meet the thickness requirements of the final board and the impedance of the various structures (microetched traces, stripline traces, single-ended, double-ended, etc.) Copper thickness (both foil and post-plating) will play a role, as aggressive etching can result in under-etched features that are more prone to mechanical failure and may miss targeted impedance values.
- Design rules – Design rules are at the heart of a DFM design – they are the handshake agreement between designers and manufacturers for the board’s producibility. To wring more space out of dense layouts, designers may exceed manufacturing capabilities or necessary process tolerances that imperil board quality in the short and long term. The via aspect ratio is a classic example: to prevent poor outcomes during via plating that render incomplete or insufficient barrel plate coverage, most fabricators will max out at a 10:1 ratio between the board width and hole diameter for mechanically drilled holes. Other common design considerations, such as trace width, spacing, and copper distribution, also require
- Electrical testing – Testing will occur throughout the fabrication and assembly processes to verify manufacturing outcomes, allow for rework (if applicable), and limit yield-related losses owing to materials and labor. Tests like ICT will check for continuity, the presence of opens or shorts, and basic circuit characteristics (e.g., capacitance) and compare this against generated files to assuage the correctness of the assembled board. Depending on the pin pitch, a flying probe test may be more apt, although the choice between these tests will also come down to production volumes.
High-density interconnects (HDI) design, though not technically a design stage, is commonplace enough to warrant discussion. HDI complicates manufacturing since many standard techniques are not viable when space is at an extreme premium. Alternate via structures like blind and buried microvias enable dense assemblies but add sequential drilling and lamination steps that add cost due to additional processing while reducing yield. The decision is a considerable cost driver and shouldn’t be made lightly, especially for non-prototype lot sizes.
Your Contract Manufacturer Ensures You’re DFM-Ready
PCB design for manufacture aligns the layout with manufacturer capabilities, ensuring the production proceeds smoothly and revisions related to mismatches therein remain at a minimum. Speaking to a manufacturer at the earliest possible stage of design can save both time and money by ensuring that design intent and technical requirements remain in lockstep. An experienced manufacturer can go beyond simply providing input for design rules: staff can weigh in on ways to optimize the stackup or layout that improves performance. At VSE, we’re a team of engineers, and we’re committed to building electronics for our customers that exceed expectations before, during, and after production. Along with our valued manufacturing partners, we’ve built our reputation on realizing life-changing and life-saving devices for over 40 years.