- How designers can weave reliability best practices into the board’s pre-manufacturing stages.
- Maximizing reliable design choices during manufacturing.
- IPC standards governing circuit reliability, namely classification and the use of coupons.
Circuit reliability includes analyzing complex logic diagrams for functionality
Besides being unsafe, the worst property electronics can have is unreliability. Devices are complex productions of labor and materials, therefore diagnosing reliability issues can quickly become time-consuming. While reliability issues are frustrating for everyone, there are some critical operations where reliability issues can mean something more sinister, even directly correlating to safety. In these instances, designers and manufacturers must be overwhelmingly positive that every effort has been made to ensure uninterrupted performance.
At some level, circuit reliability governs every design, though its prominence may be more pronounced in situations where even temporary failure or malfunction can lead to severe consequences. As such, it permeates every step of PCB production, and every member of a design team needs to understand and commit to best practices to ensure continuous operation.
Circuit Reliability Begins at Design Inception
Ensuring the proper function of a board, as well as high performance, involves every step of product development, from design all the way through manufacturing. Importantly, each stage up until mass production runs provides valuable feedback on choices that came before it, allowing engineers, ECAD teams, and others to slowly refine the board over multiple revisions.
- Design - As arguably the most involved of any of the pre-manufacturing steps, there are numerous instances of reliability in design related to power design and EMI prevention.
- The design of a stackup can make or break reliability depending on the proximity of signal to plane layers as well as the position of ground planes for noncircuitous return paths.
- Power and ground fan out traces should be granted extra width to avoid creating hot spots that can choke current flow and starve components.
- EMI can radiate energy, which can cause instability in low-power designs. To avoid this, best mitigating practices should be followed, such as avoiding routing over gaps in reference planes.
- Decoupling capacitors are multifunctional. Foremost, they can be thought of as secondary power sources that bear some of the load and ensure the individual power concerns of every active device are met. This prevents a situation where the draw of one device affects the operation of another. They also act as a shunt for transients directly to the ground, due to the inherent lowpass nature of an RC circuit that ensures steady DC continues to feed the circuit, while higher-frequency AC is filtered.
- Simulation - Simulation gives designers a way to quickly and accurately evaluate circuit topology without having to rely on devices or the design and construction of a physical board. This can include an analysis of the analog waveform and its associated properties or a more abstract study of the digital logic of the circuit. It might also include ensuring physical constraints, such as signal rise/fall times or skew, are properly accounted for in the design.
- Validation - Design needs to be checked to ensure the functions it is purported to perform align with the intent. This includes disparate modules of the design, including the logic, timing, and even the physical form of the board.
- Preparation - The board will need a second step of verification after assembly to ensure both the design and manufacturing process were able to correctly produce the board as outlined by the design team. To that end, logic structures known as built-in self-tests are engineered to continually test a design to aid in the early detection of faults or malfunctions.
Manufacturing Practices Can Make-Or-Break Circuit Reliability
Manufacturing is the culmination of every design process to produce the physical board, and as such, it possesses the largest impact on the final product. To frame it differently, the best manufacturing processes will be able to catch and correct instances of bad design (impossible-to-manufacture gaps between copper, features in excess of tool availability, etc.), but the best design will simply result in unreliable performance, potential reworks, and costly scraps if bad manufacturing processes are at play. That said, designers should still support fabricators by following some key design-for-manufacturing practices:
- Confer with the shop - The biggest mistake a design team can make is failing to bring in the manufacturers as early into the process as possible. This is less likely to occur with in-house manufacturing, but in either case, it is important to determine whether the board is driving the manufacturing or vice versa. The former case can encounter significant performance issues but may be necessitated by the demands of a cutting-edge board, in which case optimization during prototyping may be less of an issue. On the other hand, when applicable, having technology drive the design rules will result in a manufacturing process that proceeds smoothly, with the added benefit of the design team being able to ensure the performance of the board remains to a suitably high standard.
- Copper balancing - During the lamination process, the elements of the stackup undergo elevated temperatures in a press meant to liquefy the epoxy resin (or other similar material) in the prepreg to flow over the layers of the board and ultimately set the design in place. However, fabricators must be mindful of the coefficient of thermal expansion, or CTE of the materials, within the press. Because there can be a vast difference between the intrinsic properties of the nonmetal elements of the prepreg and the metal of the foils and cores, efforts must be made to create a homogenous distribution of the materials wherever possible. In practice, this means layers that may require only a small portion of the total area for copper features will instead flood the layer to provide a better thermal response. Balancing helps inhibit board warpage that can place large amounts of stress on non-surface layer interconnections.
- Temperature dissipation - In general, heat is the enemy of circuit operation, and long-term exposure to elevated temperatures is likely to degrade materials and expedite failure. To counteract this, designs need to consider mitigation techniques to effectively dissipate heat, especially from areas of rapid generation (namely power). To begin, pins connected to power and ground nets should be widened to provide extra surface area for the high current draw. Power circuitry in particular will require large copper pours with multiple thermal vias to guide heat away from the source. More extreme methods to prevent the buildup of heat may include heat sinks or more active cooling measures such as fans.
- Via structure - In general, a larger drilled via is more structurally sound and is less likely to experience issues during soldering that can lead to cracking and disconnects. HDI designs and spacing on FPGA pins are likely to force designers to adopt a smaller via hole (so long as it does not violate the maximum aspect ratio) and may necessitate the need for more intensive via designs such as blind, buried, or laser-drilled.
A representative cross-section that shows some valuable properties to promote structural reliability
Relating Reliability to Industry Standards
No discussion of reliability would be complete without covering the class of a PCB. As outlined by IPC, the PCB classification looks at the general responsibility of the end product. As class increases, producibility decreases to meet tighter manufacturing specifications:
- General Electronics - These boards do not need to exceed the life of the product (which may include planned obsolescence) and in the case of early failure, are a nuisance at most. Boards of this class are highly replaceable and disposable, but due to the relatively lax production qualifications, are cheap and unlikely to experience significant yield losses when manufactured correctly.
- Dedicated Service Electronics - Products of this level are expected to perform at a high level over long periods of time, with interruptions to the function ideally minimized, but allowable within reason. The consequences of a board failing at this level are more dire than a Class 1 board, but may not necessarily cause or aggravate injury. As the midpoint of the classes, producibility standards are more stringent than Class 1, leading to reduced yields, but not so much as Class 3.
- High-Reliability Electronics - These boards operate in environments where even minor malfunctions or downtime can result in severe and immediate injury or death. Manufacturing tolerances are tight and specific to reduce the chance that any non-compliant board is able to reach an end-user. As such, yield is reduced even further than that of a Class 2 production lot.
The assessment of a fabricated board begins with the coupon, which determines the conformity of the material to established standards. The coupon is an ideal test subject because it exists as part of the same panel the bare board resides on, and destructive testing of the coupon doesn’t result in any losses to the lot’s yield. Coupons do not just exist as pieces of the final bare board; test process coupons allow manufacturers to assess the overall fabrication in a piecewise fashion. By closely tracking the parameters that can lead to fluctuations in the final board, manufacturers ensure the production proceeds in an expected and repeatable manner. To that end, coupons can come in a variety of styles for the different processes they serve to validate:
- A/B - Used to determine the drill and plating process for the spectrum of plated through holes that exist in the design as well as how the plated holes respond to thermal stressing.
- D - Test for a plated hole and via reliability after exposure to thermal stress.
- E - Serves as an evaluation of the laminate’s moisture content as well as the resistance of the insulating material.
- G - Confirmation of solder mask adhesion to surface (including the width of traces).
- H - Determination of any lingering residue from processing or handling on the insulation resistance.
- P - Looks at the peel strength and plating adhesion of the surface metal foils applied to the laminate.
- S and W - Concerned with the solderability of through holes and surface mount lands, respectively.
- X - This coupon checks the overall flexibility of the laminate for flex boards.
- Z - Responsible for the impedance of controlled structures on the board.
Tooling coupons, as seen bottom- and center-left in the image, provide representative material for destructive testing
Circuit reliability forms a core tenet of PCB design and manufacturing, and development must work to integrate this concept throughout the course of production. While the topic of reliability exists across a variety of design spaces and can be difficult to follow for those unfamiliar with the process, working with manufacturers will help guide the effort. For the aspects of reliability that fall upon the design team, Cadence’s suite of PCB design and analysis software provides a comprehensive and easy-to-use CAD system with supporting tools for even the most granular of analyses. A robust Constraint Manager, just one of the many included features of OrCAD PCB Designer, will also promote optimal performance to keep your boards running smoothly in the field.