Despite the industry’s best efforts, a large gulf between PCB designers and manufacturers can exist. Often, most designers come from an academic background that provides broad electrical engineering knowledge that they must apply to the specific case of PCB design. On the other hand, manufacturers start from the specificity of equipment, processes, and regulations to build repeatable and high-quality circuit board manufacturing guidelines. Fortunately, IPC-2221A exists, and while it focuses more on the manufacturing side of board development, it provides the bulk of constraints for ECAD design rules.
| An IPC-2221A Section Summary | |
|---|---|
| Scope | Details the scope of IPC-2221A and identifies critical board descriptions like class and performance type. |
| Applicable Documents | Documents relevant to IPC-2221A from other industry standard bodies. |
| General Requirements | An overview of the workflow of PCB design, including the importance of testing and reviews. |
| Materials | Certain board materials perform better/worse than others depending on application and environment. |
| Mechanical/Physical Properties | Boards encounter tremendous stressors during manufacturing, leading to permanent damage or failure when operating. |
| Electrical Properties | Electronics must consider power distribution, circuit type/topology, and impedance controls. |
| Thermal Management | Boards need to manage heat generation by transferring heat away from its source. |
| Component and Assembly Issues | Assemblies that combine SMD and TH components encounter more processing/thermal wear. |
| Holes/Interconnections | The size, depth, and plating thickness of holes all contribute to their overall reliability |
| General Circuit Feature Requirements | Design rules that enhance performance and artwork. |
| Documentation | Instructions for how to best convey design information to the manufacturer. |
| Quality Assurance | Manufacturers confirm the quality of their products destructively and non-destructively. |
Breaking Down Relevant IPC-2221A Sections
IPC-2221A, or the Generic Standards on Printed Circuit Board Design, covers all of the essential design elements of PCB design for manufacturing (DFM). PCB design and manufacturing are disparate but intensely overlapping disciplines, so it should be no surprise that the industry standards overseeing both are vast and intricate. While the full details of IPC-2221A are beyond this piece’s scope, designers will gain an understanding of their requirements regarding PCB manufacturing to enhance their design and bridge communication with the manufacturer.
Materials
- Material selection – Designers can consider various materials depending on the circuit parameters/application and operating environment. These considerations include thermal susceptibility/resilience for soldering, electrical performance, and mechanical strength/rigidity. At its most basic, board materials typically include fiberglass weave with an epoxy resin prepreg and a copper foil on one or either side of the laminate.
- Laminate – The laminate provides the substrate for the copper features and soldered components. The heat generated by the operating circuit cannot exceed maximum temperature ratings, which could lead to delamination events or pose a hazard to the board’s surroundings.
- Conductive materials – The conductor is the basis for circuit interconnections between discrete components. While copper is nearly universal as the base material, additional plated metals form the plated (or plated-up) conductive features, such as the surface of the through-hole connections between horizontal signal/plane layers.
Mechanical/Physical Properties
- Product/board configuration – The board’s shape, size, and structure are essential when designing the enclosure or determining interfacing with a standalone or external electrical system. The board size within a panel/lot should be consistent to avoid multiple test fixture designs/fabrications and control processes (i.e., prevent bow, twist, and warpage of the board).
- Assembly requirements – Mechanical sub-products affixed to the PCB require additional mechanical support measures when at or exceeding 5 grams in weight. Systems exposed to high levels of shock/vibration require more attentive testing and design to limit the effects of mechanical wear. Be mindful of mechanical clearance – attached mechanical assemblies should not protrude more than 6.4 mm (~250 mils) below the board’s surface.
Electrical Properties
- Electrical considerations – Electrical performance is vital to electronic products. Appropriate processes like masking and coating are necessary to protect the board from degradation. Power distribution is critical to performance. Using bypass/decoupling capacitors ensures low-impedance pathways to all power pins. Keep power/ground traces wider to ensure low impedances. In high-density interconnect (HDI) designs, planes may be necessary to distribute power from power nets across the board.
- Electrical clearance – The proximity of conductive surfaces creates many nonidealities during runtime. Parallel surfaces lead to an unintentional capacitive effect, while signals can couple to adjacent lines, adding noise. Exceptionally high voltages (>200 V) require additional material and clearance considerations.
- Impedance controls – Today’s electronics operate on an established transmission line impedance assumption. When designing the board’s stackup, designers must determine the most effective arrangement of dielectric/conductor layers to create transmission lines that meet circuit requirements.
Thermal Management
- Cooling mechanisms – During circuit operation, heat transfer occurs due to conduction, radiation, and convection; designers can employ all three to maximize cooling capabilities. Primarily, copper pours act as thermal sponges to soak and divert heat from their source, and convection (passive or forced) can remove generated heat from the board.
- Heat dissipation considerations – Designers employ heatsinks due to the conductor’s low thermal impedance, but more exotic applications, like heat pipes and heatsink substrates, are available.
Component and Assembly Issues
- General placement requirements – Designers must remain aware of the limitations of automatic soldering processes, such as solder shadowing with wave solder. Mixed assemblies (that is, those using both surface mount devices (SMDs) and through-hole (TH) components) require multiple soldering passes with different solder processes, which can increase the overall thermal wear of the board materials.
General Circuit Feature Requirements
- Conductor characteristics – Conductor width and thickness will affect circuit performance, including impedance. Generally, thicker copper can withstand higher power applications, but etching becomes an issue due to the undercutting of the copper features.
Quality Assurance
- Conformance test coupons – While there are many nondestructive methods to investigate board quality, test coupons are an essential analysis that uses a part of the panel to check the internal quality of the board.
- Material quality assurance – Before entering production, manufacturers must inspect and analyze materials to ensure their quality independent of the vendor/supplier.
- Conformance evaluations – Manufacturing quality control can determine how effectively boards conform to acceptable industry regulations and internal standards.
Your Contract Manufacturer Breaks Down DFM to Improve Designs
IPC-2221A is the backbone of PCB DFM; while this piece covered some of the core topics mentioned in the standard, designers would do well to read through the standard and not be shy about referencing it. From these standards, manufacturers can develop core design rules that guide PCB layout and minimize revisions/rework to keep production on time and under budget. For designers looking to grow their understanding of how IPC-2221A affects their layout, VSE is here to help. Our engineers are committed to building electronics for our customers, including a complete assessment of design manufacturability. We’ve been realizing life-changing and life-saving devices for over forty years with our valued manufacturing partners.
