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IPC Class Standards: Defining Reliability

Key Takeaways

  • Why specific devices require more intensive manufacturing.

  • IPC class standards and producibility levels are distinct but related.

  • How the standards impact manufacturing processes like drilling, plating, and surface finishes.

Test equipment scanning PCB for manufacturing defects

Automated and manual inspection techniques verify adherence to IPC class standards.

All electronics are not made equal. While the design and manufacturing process do not change drastically from the boards found in toys to those used in aircraft, the precision differs to an exceptional degree. Intuitively, most people would be disappointed over a piece of consumer technology exhibiting poor performance or service life; however, devices where failure would prove hazardous or lethal, contain much less room for error. IPC class standards exist to cover the spectrum of design for manufacturing (DFM) criteria for all electronics. 

Generic IPC Design Specifications

IPC-2221 - Generic Design
An overview of generic design principles.

IPC-2222 Rigid
Design requirements for single-sided, double-sided, or multi-layer rigid boards.

IPC-2223 Flex
Design requirements for flexible materials (insulating films, reinforced / non-reinforced dielectrics, metallic materials).

Design standards for single-sided, double-sided, or multilayer boards with various via structures.

IPC-2225 MCM-L
Design standards (electrical, mechanical, material properties) for interconnecting chip components.

Design standards for the sectional design of high-density interconnect boards.

IPC Class Standards at a Glance

IPC class standards set the acceptable manufacturing quality for PCBs dependent on the reliability requirements of their installation, which is a consequence of the potential safety or operational hazard the board’s failure (or any constituents) would present:

  • Class 1 assemblies are indicative of general products, also called consumer-level electronics. The requirements for these assemblies are minimal; therefore, production focuses on high yield with a relatively high tolerance for imperfections so long as they don’t detract from baseline performance. Service interruptions are acceptable from a standard point of view, but DFM will want to minimize these for higher customer satisfaction.
  • Class 2 assemblies represent stricter manufacturing tolerances and design constraints. Electronics in this class are designated dedicated service products, including sophisticated industrial equipment and instrumentation. Uninterruptible service is a preference but not a requirement. While less accepting of defects than Class 1, there is still some leeway.
  • Class 3 assemblies are high-reliability or harsh environment products where any downtime is unacceptable due to their life-saving or risk-averting role. Rejection is high during manufacturing to weed out electronics deemed unsuitable, and as a result, yields suffer, and costs rise.
  • Class 3A assemblies are the highest manufacturing quality. Found in aerospace missions with more challenging operating conditions than Class 3.

If the board and the assembly Class inspection requirements don’t match, the inspection deems the total product lower than the two classes. In other words, even a theoretically perfect Class 1 bare board assembly quality would only be a Class 1 product. A related topic, board producibility, echoes IPC class standards but is not a design requirement; the board producibility levels relay the difficulty in realizing a particular design. However, class standard and board producibility likely align a Class 1 board with general design requirements or a Class 3 board with high design requirements.

How Standards Impact All Corners of DFM

The designer experiences the effects of IPC class standards while setting the board’s design rules. Mutually agreed upon between the manufacturer and the designer, they set the framework for violations to prevent unproducible features in the layout. However, what the manufacturer establishes as feasible for their operations must still be realized. Three essential and interlocking aspects form these decisions:

  • Design feasibility/manufacturing capacity - Designers place features in their theoretical position. Exact does not exist in manufacturing, so processes allow for tolerances that account for slight variations and system perturbations, with more expensive equipment and techniques able to approximate the true position better. The accuracy impacts the formation of annular rings with different production criteria across the separate IPC classes. Primarily the distinction between classes is due to minimum annular ring requirements and processing defects that limit the quality of the ring, such as pits, nicks, or edge roughness.

  • Process control - The most critical processes for final quality control are those concerning drilling, plating, and the surface finish. Within reason, thicker plating indicates more reliable vias. Depending on the surface finish, solder can wick away from the pad and migrates elsewhere on the PCBA surface, potentially shorting terminals or plated through holes (PTHs); the acceptable excursion length for solder decreases from Class 1 to Class 3. Some voiding and cracking are permissible in the laminate, provided it remains within the Class criteria and does not violate minimum dielectric spacing rules. However, voiding requirements are more stringent for Class 3(/A) boards, with a complete restriction on plating voids in PTH barrels.

  • Verification - Industry standards are only helpful if manufacturers check products for adherence. Testing must establish that processes and products comply with and do not violate the requirement criteria for a Class. Verification and process control collaborate to maximize quality: process control instills a quality foundation in manufacturing systems, while verification confirms the process control results. Iteration in quality control from process to verification to process (and so on) is necessary to keep pace with heightening fabrication and assembly demands.

Meeting particular Class standards – especially Class 3 or 3/A – involves synchronization among many moving parts and disciplines. While manufacturing’s primary concern will be meeting the criteria of the desired class, it’s equally important not to overspecify the requirements. A board meant for Class 1 designation gains little value from the improved reliability of the more stringent classes, as the products are highly replaceable and impermanent. Further, as the standards rise, manufacturing yield falls, increasing per-board production costs that need justification at the point of sale.

Meet and Exceed DFM Standards With Cadence Solutions

IPC class standards enforce the expectations desired by consumers and industries to implement safety and reliability. Designers will want to discuss product goals early with manufacturers to shape the design rules that influence the physical layout and associated production costs. This conversation will align production sectors and reduce the prototyping phase by focusing DFM at the design's nascent point. Cadence’s PCB Design and Analysis Software contains the powerful Constraint Manager to configure design requirements for any environment or specification. Together with the operational ease of OrCAD PCB Designer, designs come together faster without forgoing quality.

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