Flexible Thinking: Designing Flex Circuits for Dynamic Reliability
Global PCB Connections: Rigid-flex and Flexible PCBs—The Backbone of Modern Electronics
Happy’s Tech Talk #29: Bend-to-Install Semi-flex FR-4
Every type of component can be soldered to flexible circuits with confidence. Through-hole components, SMT components, wires, switches, BGAs, etc. Some require more skill than others to be attached, but they all can be mounted reliably to flexible circuits. Some may be soldered automatically like through-hole or SMT components and others may have to be attached manually like wires or cables. The use of a microscope is necessary in assembly today.
Figure 4: Surface mount components and through-hole components.
The components get smaller each year and we are now in an era where a component that measures .020” by .010” is common. That’s not much bigger than a flake of black pepper. Most manual assembly and inspection, therefore, is done under a microscope or Automated Optical Inspection (AOI) device. With components that are too small to see with the naked eye, imagine the thought of identifying a non-compliant solder joint that is a fraction of the size of those small components. It’s not of significant importance though. A skilled assembler armed with soldering tools and a quality microscope will be able to attach any component to a flexible circuit.
Protection of Solder Joints After Assembly
The concerns do not end with the attachment of components. Products developed with flexible circuits are intended to be flexed. Although the material is flexible, the solder joints are not! If components or solder joints are in, or near a bend area, then it is wise to protect the solder joints. If not, the joint may fracture and cause intermittent issues that are difficult to identify.
A flexible epoxy or conformal coat may be added to the solder joints after the product has been tested and confirmed. This will keep the bends and flexing away from the solder joint and in the material where it is intended to be. This added safety feature will add robustness to flexible circuit assemblies and likely reduce the risk of failures in the field.
Figure 5: Epoxy covered solder joints.
Conclusion
Flexible circuits have many advantages. They’re lightweight, thin and flexible. This allows products to be lighter, smaller and thinner as well. Although the typical circuit assembly is not intended to be bent, formed or even dynamic, it can be done confidently with flexible circuits. The added time spent on the design of fixtures to assist in the assembly process is time well spent and will allow the circuit to flow through the assembly line smoothly and consistently. The result will be a robust flexible circuit assembly.
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