What is Flex PCB?
A pattern of copper traces bonded on a flexible substrate with or without flexible coverlay is known by a term called flex circuit or flexible PCB. These flexible circuits can be fabricated using same components that are used in fabrication of rigid electronic circuits, the only difference is that flex circuits can be molded into any desired shape depending on the packaging of the product without affecting connectivity.
Flex PCB’s have found their fame in modern electronics where along with performance, size and weight is of great importance. Portable electronic devices like hard disk drives, desktop printers and wearables are the most prominent examples of flexible circuits. Flexible printed circuit boards are also ideal for high temperature and high-density applications like different sensors used in oil and gas industry and sensors used for measurement of moisture, humidity and pressure because flex circuits use polyimide or similar polymer as a substrate. These materials dissipate heat faster than other rigid PCB materials and thus can withstand extreme temperatures.
Designing flex PCB as per IPC-2223 rules:
Using same software for rigid and flex PCB designing is not a good practice. Engineers should use flex designer suits of computer aided software to design flexible circuits to check that whether component placement, case assembly and PCB clearances meet all mechanical requirements or not. Few rules that need to be considered while designing flex circuits as per IPC-2223B are:
Hole to edge distance:
The final residual material should not be less than 0.5mm while choosing the minimum distance between exterior edges and the edge of non-plated interior through holes (NPTH) and cutouts.
Points need to be considered while determining number of conductor layers required are:
- Number of signal traces required across flexible portion.
- Spacing for voltage isolation
- Impedance required
- Voltage drop requirements
- Line widths depending on current carrying capacity
Bend area considerations for conductor:
Where there is any sort of discontinuity in the cover coat or termination of plating, flex printed wiring shall not be flexed or formed in that area. Conductors in the bend area should follow these considerations:
- Should be perpendicular to the direction of bend
- Should be maximized across bend area
- Should be uniform in width
- The number of layers should be minimum in bend area
- Should be evenly spaced
Minimum bending estimation for single sided and double-sided flex:
The formula for bending estimation of single sided flexible circuit is
R= (C/2) [(100-EB)/EB] – D
And the formula for double sided flexible circuit bend estimation is
R= (d/2 + c) [(100-EB)/EB] – D
R= minimum bending radius (mm)
C= copper thickness (mm)
D= dielectric thickness (mm)
= copper deformation amount (%)
Incase of double-sided flex “d” is for total thickness i.e 2x coverlay thickness + base material thickness + Copper thickness.
Flexible circuit overlay:
A lamination that is installed on the flex printed circuit board to encapsulate and protect the external circuitry is known as coverlay. The polymide and adhesive coverlay are laminated through proper heat and pressure, heat helps the adhesive to flow and fill the air gap between the trace and pads. The two types of pads are individual bared pads and group bared pads. Group bared pads are more preferred because it prevents the solder from cracking during bent or flex because it provides stiffness to the holes and larger area between bent position and individual pads.
One should consult IPC-2223B rules before designing flexible circuits to avoid any electrical and mechanical failures in the design.
Flex PCB’s vs Rigid PCB’s
Flexible circuits reduce assembly errors and saves time and cost through reduction of manual labor during assembly process as that required in rigid circuits assembly. The automated procedure eliminates human error and high cost of soldering wires, wrapping and routing. Flex circuits are not restricted to two dimensions, they can be cut or molded into any configuration depending on the application like in RayMing PCB and Assembly, one side of the flex PCB can be 250 mm while second side is limitless, you can design as long PCB’s as you want. Like rigid PCB’s, flex PCB’s can be multilayer. RayPCB feels proud in taking the most complex of design challenges and can provide maximum 10 layers flexible PCB’s.
Flex PCB’s have multiple advantages like:
Improved heat dissipation:
Because of the polymer substrate and streamline design of the printed circuit board, flex PCB’s provide better airflow and heat dissipation.
The flex PCB’s can flex up to 500 million times in the designs having moving parts without failure. Due to the thermal stability of polyimide, these flex PCB’s serve as a better base for surface mounted components.
Rigid printed circuit boards are usually rectangular and bulky due to the thickness of substrate used. Flex PCB’s use polymer as a substrate and at RayMing we provide flex PCB’s with a maximum thickness of 0.3 mm with a maximum copper thickness of 2oz. These properties of flex PCB’s make them a favorable solution for compact and sensitive applications.
Flex PCB’s provide better electrical performance because their low dielectric constant allows the electrical signal to transmit quickly and reduces the noise factor. Higher heat transfer properties make the assembly work easy and fast.
Flexible circuits decrease the assembly cost by replacing multiple single circuits with only one single flex assembly. This saves the assembly time and material as well. There is no limit to the length of flex sheet that’s why multiple circuits can be assembles in series at a time.
Rigid flex PCB:
Flex PCB’s paved the way for rigid flex PCB’s. Combining the best of both flexible circuits and rigid boards together into one circuit has yielded major devices in military and aerospace industries. Rigid flex PCB’s come into play when interconnection between two rigid boards or a board with any other device is needed. The flex part in rigid flex PCB’s eliminates the jumper or wiring portion of the circuitry and helps it in reducing size and become vibration resistant.
Limitations of Flexible circuits
“Flex PCB’s have so much advantages why not shift completely to them?”, the answer to this question is simple that each and everything has its own pros and cons associated with it. Flexible PCB’s are easy to assemble and reduces manufacturing time but it increases the overall cost of the final product. The substrate polymer material used for flex circuits is costly as compared to rigid boards. Moreover, rigid boards can be manufactured through man force but flex PCB’s require automation that increases its cost. Making amendments in the design or repairing once the assembly is done is very costly for flex PCB’s because removal of coverly from the conducting material and then recovering it is not easy at all. The most important thing is the sensitivity of flex PCB’s, during component mounting procedure, the engineers need to be well trained for handling this material otherwise it can cause serious loss.
One major limitation of electronics using flexible circuits is that they can not pass higher currents. The maximum copper width provided on polymer substrate is 2oz because beyond this level flex PCB’s will not be flexible as copper is a hard material. Applications with higher current requirements use rigid PCB’s.
At RayMing we have state-of-art machinery for flex circuit assembly and our engineers are always ready to help costumers in helping with circuit designs to reduce overall cost and maximize efficiency. You can send your designs or can contact anytime by writing us at www.raypcb.com