When prepping for a backpacking trip, list-making is the first tool I turn to for organization. By running through and ensuring that every item is present and gear has been checked and functioning as intended, I can relax rather than worry. After all, there’s nothing quite like being 10,000 feet above sea level and several miles from civilization (or even the next closest human) to realize you overlooked something.
A similar anxiety can creep over designers when preparing for manufacturing. After all, PCB designs are intricate, with features existing in all three dimensions. It’s easy for small, innocuous mistakes to result in a board that requires rework, or worse, is scrapped due to critical failure. PCB quality checklists form a strong defense against errors that can occur during the design or manufacturing process. With a few simple guidelines, it’s easy to have design and manufacturing in lockstep throughout the production process.
Starting your PCB Quality Checklist During Design
The design will include controlling documents that provide the agreed-upon elements between customer and designer. Besides functioning as a roadmap, these items can provide a legal basis for any negligence disputes between the two parties. As such, any engineering change order requests must include a digital paper trail. It is best to have customers directly apply any ECOs to the controlling documents before re-submitting them to the PCB designers. However, this can be averted so long as the joint request/approval of the ECO is included within the aforementioned paper trail. Changes should be performed immediately once approved to ensure work does not progress further to the point where adjustments could take considerable time to roll back or be forgotten during the design phase.
Land pattern verification will be necessary for any design that introduces new components that aren’t part of a pre-approved internal library. Leave standard land pattern designs to a part creation software
wizard when applicable. For land patterns that require a more hands-on approach from the designer, ensure datasheets are official documents rendered by the manufacturer. Often it’s possible to obtain 3D models from manufacturers that function as a good check to determine whether a datasheet was correctly interpreted into the land pattern. 3D models provide vertical clearance information for boards within enclosures, stacks, or similarly confined areas.
The design rule system embedded within the layout software of choice will also prove indispensable against an accidental infringement on manufacturing capabilities. This system may be fully furnished by the customer or a collaboration between the customer and design team to hatch the best solution for realizing the board. In either case, the design rules will ensure layout does not lead to a design that is unreproducible in the shop while also serving as the determining factor for several properties of the board (impedance, trace width spacing, etc.).
Material selection will also play a role in the ultimate specifications, but this is usually a more cut-and-dry matter through the lens of the design team. Design rule implementation should occur no later than before the outset of the layout. There is no point in placement, routing, etc., if the features do not adhere to manufacturing limitations.
Steps to Ensure Fabrication Proceeds Flawlessly
Fabrication will be the first of two major manufacturing stages, and it is far more intensive and sensitive to error. While the design is complete from the perspective of connections, placement, and features, most shops are given significant leeway in operation for hitting targeted values. For example, while a stackup callout on design documentation may have been devised using sophisticated field solvers, it is common to distribute the overall thicknesses of substrate/core to the fabricators. The reason for this is simple: the fabrication team is more well-versed in manipulating the material during the actual production, and so long as no other design features are infringed upon, it’s better to defer to their expertise over that of software.
For a less experienced designer, consulting with fabricators is likely to be one of the first steps taken while working with a customer’s design constraints. While the designer should be somewhat knowledgeable regarding some basic design concepts and the IPC guidelines covering them, cutting-edge features or exceptionally demanding constraints should always have the fabricator offer feedback on what is and is not physically feasible. Some considerations common to almost every modern board include:
- Copper foil thickness: High electrical or thermal demands may require a thicker than standard application copper foil for increased current density. However, this affects the minimum width of traces (not necessarily unwelcome concerning current density) and may frustrate routing or placement and the impedance profile.
- Via diameter: Designers are looking for any edge to increase the routable area of a board, and vias are an obvious candidate due to their plentiful nature. However, the ratio of the thickness of the board and via diameter should rarely exceed 8:1 except for particular circumstances. Heeding this aspect ratio helps prevent downstream production issues where via barrels can crack due to thermal expansion.
- Documentation clarity: Ambiguity without due diligence can be a silent killer in engineering products like PCBs. Designers should take care to call out unorthodox features to manufacturers and use symbols that are visually distinct and easily digestible without the aid of excess magnification. Remember that what exists on a computer monitor with extremely versatile zoom functionality may be provided to machine operators as a paper document. By anticipating potential issues raised by a lack of clarity, designers can reduce the burden of interpretation in later production steps.
A completed bare board will undergo a battery of tests to confirm the success of the overall fabrication stage. A flying probe or bed-of-nails machine will ensure the interconnects of the board correspond to the netlist, while X-ray testing will verify inner layers match with output GERBER data. While these verifications can’t fix a board that has incurred some defect during the manufacturing process, they will prevent the board from accumulating additional materials or labor costs. These steps also keep failing boards out of the hands of end-users.
Finish Strong With An Error-Free Assembly
Following fabrication, boards are ready to be put together with the individual components that comprise the logic and functionality of the unit. At this stage, the major difficulties of design are completed, and to its benefit, assembly is usually an easily rectifiable process should an error arise at this point. However, the best rework is no rework at all.
There are steps the designer can undertake to prevent their inclusion:
- Silkscreen and designators: Having a clear silkscreen where reference designators, pin 1 designator, and other installation information isn’t blocked by solder mask, or other features of the board will help assemblers hand-soldering components too large for a pick-and-place machine. Polarized components such as diodes and electrolytic capacitors as well as ICs and connectors that would be able to solder down in a rotated configuration should have their silkscreen designators and markers take precedence over passive components that have multiple correct installations. As with fabrication, designers should aim to view assembly documents through the eyes of the operators to eliminate any opportunity for ambiguity.
- Bill of materials: The BOM contains all part information and points to any necessary datasheets for land pattern creation through the manufacturer’s part number. Designers should alert customers when searches for provided MPNs prove fruitless to come up with a replacement component. The current supply chain crunch may necessitate a pivot to alternate packages to avoid excessive delays in sourcing components.
- Single-side placement: When possible, designers should attempt to perform all the placement on a single side of the board. This option eliminates the need for repeated passes through solder reflow or wave soldering, depending on the technology employed.
Your CM Can Provide a High Level of Quality Assurance and Control
PCB quality checklists require a concerted effort that must permeate through each stage of the design and manufacturing process. While this list is not all-inclusive of every quality validation that presents an opportunity to eliminate errors during production, it serves as an excellent base on which to build from:
- Consult early and often with the fabrication team for nonstandard designs to aid in design rule formatting. When in doubt, let the technology guide the upper limit of feature resolution.
- Ensure ECOs are quickly executed, and there is a reliable form of communication to show customer and designer consent.
- Confirm land patterns are properly translated from datasheets. When in doubt, a fresh set of eyes helps.
- Documentation for manufacturing processes and board symbols should prioritize legibility and provide directives, callouts, etc., straightforwardly.
Your design calls for the care and dedication of an engineering team whose job is to build electronics for customers. We here at VSE take on that responsibility as our core mission to our clients. As your CM, we’ll partner with expert fabricators to ensure that every board we help produce is held to the highest scrutiny and exacting standards that you’d expect of a design carrying your name.
