Beyond Design: High-speed PCB Design Constraints


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Digital design has entered a new realm. Modern high-speed design (HSD) not only requires the designer to continuously break new ground on a technical level but also requires the designer to account for significantly more variables associated with higher frequencies, faster transition times, and higher bandwidths. Ignoring signal and power integrity and electromagnetic compatibility invites schedule delays and increases development costs and the possibility of never succeeding to build a functional product, which is a career-limiting strategy.

The key methodology is to understand the underlying high-speed design issues and then translate these into corresponding design constraints that will be adhered to during the entire design process. It is best to develop these high-speed design constraints based on pre-layout simulation.

We had a few critical nets to manage in the past, but now, it seems that a significant number of interconnects are critical. Also, each design requires a specific set of constraints based on the technologies used. Sure, we can port basic design rules for trace width, clearance, etc., from a previous design to the next, but individual constraints still need to be established. Constraint reuse is also limited by net and group naming conventions. If you are consistent, then porting is much easier.

To begin with, every designer needs a set of well-established design rules to base the constraints on. IPC has provided the electronics industry with guidelines for designing and manufacturing PCBs compiled over the years with the support of both committee and industry members.

The IPC-2220-FAM: Design Standard for Printed Boards series is the bible for PCB designers. The series is built around IPC-2221B—the base document that covers all generic requirements for PCB regardless of materials. From here, the designer chooses the appropriate sectional standard for a specific technology.

The IPC-2220-FAM series includes:

  • IPC-2221B: Generic Standard on Printed Board Design
  • IPC-2222A: Sectional Design Standard for Rigid Organic Printed Boards
  • IPC-2223C: Sectional Design Standard for Flexible Printed Boards
  • IPC-2224: Sectional Standard for Design of PWBs for PC Cards
  • IPC-2225: Sectional Design Standard for Organic Multichip Modules (MCM-L) and MCM-L Assemblies
  • IPC-2226A: Sectional Design Standard for High-density Interconnect (HDI) Printed Boards

This series provides coverage on material and final finish selection, current-carrying capacity and minimum electrical clearances, test-specimen design, guidelines for V-groove scoring, dimensioning requirements, and conductor thickness requirements.

To read this entire column, which appeared in the May 2019 issue of Design007 Magazine, click here.

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