Designing for Adaptation
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In the world of hardware engineering, board re-spins are costly, time-consuming, and often avoidable with proper planning. Hunter Scott, Director of Hardware Engineering at Reach Labs, shared valuable insights on "Design for Hacking" (DFH) at Altium Live 2018 - strategies that make PCB modifications easier when inevitable design changes arise.
Three Core Principles
Scott organizes his approach around three fundamental techniques:
1. Isolate Subcircuits
Breaking your design into isolated modules creates flexibility and debugging opportunities:
- Use series resistors (precision small values are better than 0-ohm)
- Implement load switches for digital control of isolation
- Add removable jumpers and fuses
- Include connectors between major circuit blocks
This approach allows you to test modifications on specific sections without affecting the entire board and facilitates A/B testing of different implementations.
2. Give Yourself Options
The design phase is the perfect time to build in flexibility:
- Embrace "DNP" (Do Not Populate) components liberally
- Include footprints for filter configurations (pi/T networks)
- Add DNP RF and debug connectors
- Design footprint-in-footprint layouts that accommodate multiple component packages
- Consider castellated modules for easily swappable subcircuits
- Break out spare I/O pins and status/setting pins
- Incorporate spaces for shields that may be needed for EMC compliance
3. Make Changes Easy
When modifications become necessary:
- Use vias in pads strategically (but carefully)
- Place abundant and well-labeled test points throughout your design
- Use potentiometers instead of fixed resistor dividers for tunable settings
- Choose LDOs with high input voltage ranges for power flexibility
- Keep components on the same side when possible
- Leave adequate space for soldering iron access
- Avoid permanent materials (potting, conformal coating) until designs are finalized
Special Considerations for RF
RF circuits often intimidate designers, but several techniques make modifications more approachable:
- Implement RF jumpers for testing and isolation
- Use thin-gauge coaxial cable for modifications
- Include semi-rigid pigtails with proper ground pads
- Design with coplanar waveguides for easier probing
- Select antenna structures that accommodate adjustments
Practical Tips
The presentation emphasized practical design habits:
- Document thoroughly with commented schematics and labeled test points
- Use silkscreen effectively to mark pin numbers and component designators
- Consider using circuit tape for secure wire management during modifications
- Select component packages with external pins rather than difficult-to-modify QFNs
- Avoid ultra-small components (0201, 0402) when possible
By incorporating these defensive design strategies, hardware engineers can create adaptable boards that accommodate changes without requiring complete re-spins - saving time, money, and frustration throughout the product development cycle.
