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

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)

Contract Assembly

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