PCB Assembly Instructions: Best Practices and Transparency

If you have ever tried to learn a new language—other than your first—then you know how important the correct word choice and syllable emphasis can be when you attempt to communicate with a native speaker. Even when you think you’ve got it, clarity can be elusive; misunderstanding and confusion are hard to overcome. Achieving clarity can pose an even greater challenge when communication is limited to text, symbols and graphics.

For PCBA manufacturing, all of the necessary information that describes how your board should be built—usually textual data and imagery—is passed to your CM via a design file(s). Design file clarity and comprehensiveness will determine if your boards are manufacturable without design changes. Likewise, that clarity will contribute to the turnaround time for building your boards.

The probability of a quote or manufacturing process delay can be significantly reduced by ensuring that your CM is clear on your board fabrication preferences, assembly instructions and design intent. For assembly and design intent, you can optimize manufacturing efficiency by following a set of best practices that employ transparency and completely convey all necessary details within your assembly drawing or PCB assembly instructions.

How Should Your Design File PCB Assembly Instructions Be Constructed?

Following the DFM rules and guidelines from your CM may be the single most important factor to ensure your boards can be built. Likewise, for the best PCB assembly, you should acquire and implement your CM’s DFA rules. Having your boards built, however, does not guarantee their quality, reliability or performance.

PCBA quality will depend on your CM’s manufacturing process, especially its adherence to industry standards. For assembly, adhering to industry standards requires compliance with IPC-A-610 and J-STD-001. And while PCBA performance and reliable operation are primarily design issues, structural integrity and component mounting—as well as the mechanical ability of your boards to remain intact over their projected operational lifecycle—are CM responsibilities that define board quality and contribute significantly to operational reliability. Following PCB assembly instructions best practices for your assembly drawing, including the requirements listed below, can help ensure board quality and longevity.

PCB Assembly Instruction Requirements

• Be sure to include the board revision and other identifiers, such as publishing date, that clearly distinguish this design.
• Specify the level of class of IPC (or other standard) inspection required.
• Specify solder alloy requirements, especially if RoHS standards must be met. 
• Specify any components or areas of your board that need special attention—for example, polarization, moisture sensitivity and keep-outs. 
• Ensure that any process or readability requirements for silkscreen layer reference indicators, labels and markings are included.
• Make sure that footprints on your PCB layout are accurate for the MPNs in your BOM component listing. 
• Include component staking (pre-SMT), underfill, material and hardness requirements.
• Include specifications for washing, cleanliness, conformal coating and encapsulation.
• Make sure imagery is comprehensive and includes any mechanical hardware additions, such as heat dissipation devices and radiation shielding elements.  
• Specify any additional processes clearly, such as drying or functional testing.
• Include instructions for depaneling.

Following the practices presented in the list above will help facilitate the building of your boards while ensuring they meet your reliability and performance objectives. And to ensure that your instructions meet your CM’s needs, you should follow a PCB Design and Development Checklist.

Why is Transparency Important for PCB Assembly?

For many designs today, especially for critical industries such as aerospace, medical devices and automotive applications, PCBA design complexity makes it difficult to effectively communicate design intent simply through a design file(s). For custom designs, this can be due to design option choices like solder mask color not aligned with the literal size of silkscreen markings. CM equipment limitations or process issues, such as component locations that impede thermal distribution during soldering, could also impede PCB assembly.

Nuances like these are almost impossible to identify through pre-manufacturing vetting efforts. Nevertheless, they must be addressed before a prototype design can be finalized and ready for full-scale production. Correcting these issues, however, may result in what can seem like an endless back-and-forth with your CM, extended turnaround times and additional unnecessary board spins—not to mention the possibility of missed schedule deadlines and NPI launch dates.