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Technical Articles

Technical Articles

Flexible PCBs (Part II) - Difference between Rigid & Flexible PCBs in Material, Technical Specs, Design Rules and Manufacturing Technologies

When comparing Rigid PCBs with Flexible PCBs, there are fewer similarities and more differences. Whether you are a design engineer or a purchasing official, it is important that you know the basic differences in material, processes and design parameters so that you gain the most from your flexible PCB project.

Material

Criteria Rigid PCB Flexible PCB
Base Layer FR-4 - most common
FR-3, FR-2, CEM-1, CEM-2 etc.
PI (Polyimide) – most common
PET (Polyester), PEN (Polyethylene Nphthalate), PTFE, Aramid etc.
Adhesive Layer Limited to chemical and thermal properties only Apart from chemical and thermal properties, mechanical properties are also important to allow stretching and bending
Current Carrying Capacity High Average
Temperature Resistance Average High (between -200° C and 400° C)
Resistance to Chemicals, Radiation and UV exposure Average High
Resistance to Shock and Vibration Low High

Technical Specifications & Design Rules

Criteria Rigid PCB Flexible PCB
Board Thickness 1.6mm (most common)
0.27mm (thinnest)
0.1mm - 0.6mm (according to the number of layers)
Board Thickness Tolerance +/-10% +/-0.05mm
Drill Size (minimum) 0.2mm (mechanical)
0.1mm (laser)
0.1mm (mechanical)
Hole/Drill Tolerance +/-0.075mm for PTH & +/-0.05mm for NPTH +/-0.05mm for PTH & NPTH
Hole Wall Thickness 18-23 micron 8-18 micron
Copper Isolation/ Track Spacing (minimum) 0.076mm (3 mil) - 1oz (special design)
0.127mm (5 mil) - 1oz (most common)
0.178mm (7 mil) - 2oz
0.05mm (2 mil) - 1/2oz
Track Thickness 0.076mm (3 mil) - 1oz (special design)
0.127mm (5 mil) - 1oz (most common)
0.178mm (7 mil) - 2oz
0.05mm (2 mil) - 1/2oz
PTH/NPTH slot width (minimum) 0.65mm for PTH & 0.8mm for NPTH 0.6mm-0.65mm for PTH & NPTH
Rout size (minimum) 0.8mm 2.0mm (standard) – laser cutting/Punching
0.0mm – laser cutting

General Aspects

Criteria Rigid PCB Flexible PCB
Bending Radius Not Applicable Critical
Manufacturing Cost Low to Average High
Stability for component placement/insertion No Problem Requires Stiffener to provide mechanical support
Process Complexity/Skill Level for PCB Assembly Normal High (requires special jig/fixture for SMT assembly)
Number of Device Interconnects High Low

Manufacturing Technology

Criteria Rigid PCB Flexible PCB
Material Cutting Same Same
Drilling Same Same
Copper Plating Same Same
Dry Film (Imaging & Development) Same Same
Etching Same Same
Protective Layer Solder Mask Coverlay
Profiling Routing Punching/Pressing Or Laser Cutting

Applications

Use of Flexible PCB is ideal when your product demands

  • Wiring solution that fit where rigid PCBs cannot
  • 3 dimensional packaging geometry
  • Ultra-compactness and high performance (e.g. tablets, GPS units, smart phones etc.)
  • Low number of device interconnects
  • Survival in extreme conditions like human body (e.g. Pharmaceutical and medical applications)
  • Survival in extreme temperature and exposure to harsh chemicals (e.g. borehole measurements in the oil and gas industry and industrial sensor technologies)
  • Resistance to high temperatures, UV exposure and ability to control impedance (e.g. high density applications)

PCB Design Tips

When designing a PCB, try to limit the amount of draws you use. Draws use a lot of memory and slow the programming time. Use a flash for pads instead.

More tips...
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