Estimated reading time: 3 minutes

Target Condition: The 5 Ws of PCB Design Constraints

Have you ever sat down to define PCB design constraints and found yourself staring at a settings window with more checkboxes than a tax form? You’re not alone. For many designers—especially those newer to the layout world—the task of setting up design constraints can feel like trying to write a novel in a language you just started learning.

It’s tempting to jump into the layout tool and start connecting traces, only to hit a wall of design rule checking (DRC) violations and second-guessing. That’s where we need to pause and reframe. We need to restart with the end in mind. As I have in the past, I suggest using one of my favorite frameworks for this task: the 5 Ws.

Let’s explore how these five simple questions—who, what, when, where, and why—can lead you toward PCB design constraint clarity and success.

Who? Know Your Stakeholders
Before entering your first clearance or trace width value, ask yourself: Who are you designing this for? Is this board going to a production fabrication shop with limited process capabilities? A local prototype shop? Will it be assembled by hand or by automated pick-and-place machinery?

Stakeholders include more than just an end-use customer. They also encompass PCB fabricators, assembly teams, electrical engineers, and even our test engineers. Design constraints should reflect the capabilities and expectations of everyone involved.

What? Define the Assembled Product, Not Just the Bare Board
You’re not just connecting the dots with copper on a laminate material. You’re designing a functioning electronic system. What are the electrical requirements? Will this board assembly operate at high frequency? Will it carry high current? What about creepage and clearance in high-voltage designs?

The “what” question zooms out from the PCB itself to the intent of the design. Constraints like trace width, spacing, impedance control, and layer stack-up all start with this question.

When? Time is a Constraint Too
Design constraints don’t live in a vacuum. Timing pressures often dictate how conservative or aggressive you need to be. Got a quick-turn prototype due in three days? You might need to relax certain constraints and communicate that to your team. Working on a Class 3 aerospace board? Better plan extra time for reviews and validation.

A timeline is a constraint in itself. It can force compromises or call for tighter controls. Early clarity around the project schedule lets you tune your design rules appropriately.

Where? Know Your Environment
This question has two environmental facets: fabrication and assembly environment and operating environment. Where will this board be built? Will it be a low-volume lab or a high-volume production line? Manufacturing constraint settings will differ drastically based on manufacturing capabilities.

Where will it be used? In a cleanroom, a car engine bay, or maybe outer space? Some conditions require thermal constraint considerations, mechanical support, solder mask clearances, and more. A good rule of thumb: If the environment can affect the board, it should influence the design constraints.

Why? The Purpose Behind the Parameters
This is perhaps the most important W. Why are you defining these constraints at all? The answer isn’t “because the tool requires it.” It's because constraints protect the intent of your design. They enforce boundaries that ensure your layout matches the electrical, thermal, and mechanical goals of your project.

Addressing this question can be an education for newer PCB designers. When you know why a differential pair needs 100-ohm impedance or why a 20-mil clearance is critical for 600-volt isolation, you don’t just follow the rule—you understand it. That’s how good designers become great.

Summary: Constraint Context Must Match End-Use Requirements
Setting up design constraints in your layout should feel like a handshake between you and your project team stakeholder. It should be a fulfilling conversation—between design and manufacturing, and between function and feasibility. When you start defining rules with the 5 Ws in mind, you will begin to see constraints not as obstacles but as enablers of quality.

This column originally appeared in the July 2025 issue of Design007 Magazine.