Altium PCB Designer: Practical Design Constraints and Myths

Tuesday, July 24, 2018

Practical Design Constraints and Myths

Printed circuit board designs are constraint driven by customers and end use requirements.

Design Constraints Include:

Mechanical Constraints
Fabrication Costs
Assembly Costs
Design Time Costs
Time to Market
Proof of Concept Prototypes
Low / High Volume Production
Design for Manufacturing
Design for Test
Design for FCC or ETSI Compliance
Ease of Maintenance or Rework
Controlled Impedance Requirements
Signal Integrity
Power Distribution Networks
Thermal Considerations
Conducted and Radiated Emissions
Radiated Susceptibility

A plethora of design constraints are often heaped on designers. An overly constrained design can prove to be an exercise in futility for the PCB designer. Often times I see constraints that are not applicable or practical to apply to a specific design.

Designers often work with customers who have never actually designed anything more than a simple board. However the customers have either read an article or have been convinced by someone that some design practices are good and others are bad.

Designs are often overly constrained using what appears to be a cobbled together collection of best practices. Unfortunately a one size fits all set of rules and constraints does not apply well to PCB designs.

Let's consider a few designs that have widely varying end use requirements.

High-Speed Designs
RF Designs
Power Supplies
High Voltage Designs
Sensitive Analog Designs
Automotive and Medical Safety
Low Cost Commercial Designs
Industrial Grade Designs
Harsh Environment Designs

Myths still persist.

90 Degree Corners

Designers have been lead to believe that routes with square corners are a bad thing and they assume all square corners should be eliminated from a design.

I have some bad news for the no square corners camp. Every surface mount resistor placed on the board will introduce four sharp 90 degree turns in the signal path. I suggest they figure out how to design their boards without using resistors :)

Source: http://www.resistorguide.com/thin-and-thick-film/

45 Degree Corners for RF Designs

Regarding using arcs instead of 45 degree corners. After 20+ years of RF board design experience working mostly in the 2.4GHz and 5GHz I have concluded based on test results and experience that RF tracks with 45 degree corners are no worst than using smooth arcs.

At 10GHz or higher I would suggest using arcs vice 45 degree turns.

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