As board materials pass through production, how does a manufacturer know whether a board is performing suitably and as expected? Depending on the techniques, materials, equipment, labor, and components, PCB manufacturing can incur high costs per panel. It is in a shop’s best interest to sniff out defects and failures early. Intuition certainly plays a part; more tangibly, testing can confirm individual processes and overall functionality. However, these tests alone provide an immediate snapshot of the board’s status and offer only minimal predictive ability.
Board reliability is established through the Institute of Printed Circuits (IPC), which creates and manages PCB quality standards worldwide. With these standards, manufacturers can gauge the performance of their processes for quality control purposes and as a feedback mechanism. High reliability is no longer a preference for certain operations but a design specification.
A PCB Quality Standards Checklist Includes Reliability and Producibility
The overarching measurement of reliability by IPC is the class of the board, indicated with ascending reliability from one to three. Unless indicated otherwise, all criteria for a board’s class are minimum requirements. In other words, a board that meets Class 3 specifications in all but one aspect would not qualify as a Class 3 device.
Classes are delineated by how critical the continuous operation is to a device’s performance: Class 1 is consumer electronics that are seen as disposable if defects are encountered. Class 2 is found in medicine, where service disruptions must be minimized. Class 3 is for aerospace and other uses where device downtime is entirely unacceptable. Paralleling reliability, board features can also be regarded in ease of producibility. Level A is the simplest and cheapest to produce, while Level C has the greatest performance.
From planning to production, IPC provides a checklist for designers and manufacturers to follow to ensure compatibility with the desired Class. In general, the standards can be grouped thusly:
- Component package. The lead style will greatly influence the solder joint and profile, and certain package styles are accompanied by additional standards for the associated land patterns.
- Material selection. The choice of prepreg, copper-clad laminate, solder mask, and surface finish will influence the final board properties. Different materials may be more (or less) amenable to particular processes, and material subsets such as flex and rigid-flex comprise wholly distinct printed circuit solutions to standard board and cable assemblies.
- Solder. Stencil design is paired with automated soldering (wave or reflow), and flux application and cleaning will be encountered with all soldering methods, manual included. RoHS adherence has eliminated the use of lead in consumer-level electronics to route heavy metal accumulation from electronic waste. Still, lead-free solders require higher temperatures and are prone to whiskering which can cause shorts. Class 3 electronics can be exempt from the leaded solder restriction, and eutectic lead-tin solder is still used due to its excellent operating characteristics.
- Assembly. Beyond soldering, assemblies may call for an additional pass for repairs and gauge interface between board and enclosure or cable assembly.
A Closer Look at Board Features and Promoting Their Reliability
Although applicable standards can differ greatly, a core foundation of printed circuit best practices is detailed in IPC-2221A. The document serves as a baseline for interconnect technology but does not provide specifications without the appropriate board structure standards. PCB parameters can be meticulously modified to suit the board’s needs best. However, reinforcement of performance in one area naturally comes at the cost of another (or possibly multiple). Continuing the focus on reliability, these are a few metrics designers and manufacturers can emphasize to bolster continuous operations:
- Trace integrity: Widening traces in the plane and thickening traces that are normal to the plane provides the extra material to prevent current crowding and heat dissipation. Most often, thicker traces are used for power and ground connections to ensure a low impedance path from component to via to plane. Conversely, a reduced CTE value in the plane can pose a long-term risk to boards experiencing significant thermal loads/cycling due to an expansion mismatch.
- Via integrity: Though their processing is more involved, vias function similarly to traces through the board’s width instead of within the plane. All plated holes begin as drilled holes. Therefore, the aspect ratio plays an outsized role in via barrel durability: the smaller the ratio of board thickness to hole diameter, the better the plating coverage on the interior surface of the hole. Excessive aspect ratios can lead to incomplete plating on the surface hole interior, leading to poor continuity or complete open circuits.
Additionally, a thinner layer of deposited copper inside the via barrel will have less resistance to z-axis CTE mismatches. This may cause open circuits only after a board has experienced minimum thermal aging.
Other factors, such as solder joint integrity and land pattern adhesion, gravitate to design and production choices that fortify anisotropic measurements in a favored direction. Electrical isolation, meanwhile, performs best when features are maximally spread to prevent coupling; designers must balance this constraint against general miniaturization trends of boards, and high-density interconnect (HDI) layouts.
Your Contract Manufacturer Can Tackle the Most Challenging PCBs
PCB quality standards are demanded by the role and environment of certain boards. Still, any product is better served by minimizing downtime and extending service life. In either case, pairing with an experienced PCB assembler grants numerous benefits: defect detection methods within and beyond standards are well-established, and a well-honed shop is better prepared to make tight deadlines like time-to-market.
At VSE, we’re a team of engineers inspired to build electronics for our customers. Coupled with our professional manufacturing partners, we deliver life-changing products with sterling performance and reliability.