A Flexible PCB is defined as an arranged pattern of integrated circuits and components that uses flexible based materials. Here, the same components used to produce rigid printed circuits boards might be used to fabricate these flexible electronic assemblies, but they should allow the flexibility of the board during its application.
Some of the advantages provided by flexible PCBs include; low mass, greater reliability, high ductility, and space savings. But also, inventors/designers must be ready for their complexity and complications.
Except for the fact that the designer must be accountable for the mechanical complexity related to a flex circuit, there’s no great difference between flexible PCBs and rigid boards during the designing phase. For example, if a flexible PCB is flexed above its capability, it is going to tear during installation.
Hence, creating a mechanical model of the PCB and testing it for a proper fit is important before going ahead with an electrical design. Also, this will include the test for any misalignments, servicing, and ergonomics of the installation. Additionally, designers are required to know the different available flex circuits that exist and how they work.
Types of Flex PCBs
Based on their application, there are different types of existing flexible PCBs. Among them, the flex, high-density-interconnect (HDI) flex, and rigid-flex, are the most prominent.
- Flexible PCBs
These PCBs are flexible and resistant to vibrations making them unique and different from others. They are the flexible version of the most common existing rigid PCBs. Their additional features are accompanied with the usual repeatability, high density, and reliability which are already provided by rigid PCBs.
Flex circuits have the capacity to assume a three-dimensional configuration which is the major advantage they have over rigid PCBs. One of the flexible PCBs’ most common applications is as being a wire harnesses replacement.
These PCBs are a combination of both the flex and rigid. While adding some unique abilities that cannot be owned by either flex and rigid when separated, they provide the best of both constructions. For example, a series of rigid PCBs derived from a typical rigid-flex configuration will be linked by an integrated flex circuit. Therefore, designers can greatly improve their circuits design capability by integrating rigid parts as an addition to the flexible regions.
- High-density-Interconnect (HDI) Flexible PCBs
When there are no adequate options provided by typical flexible circuits, HDI PCBs come into play by incorporating fine features like micro-vias, better layout, construction, and design. In addition to the above features, other features include; highly dense flex circuitry, Increased functionality, and smaller form factor.
HDI technology offers improved electrical performance, access to advanced IC package use, and better reliability though it makes use of thinner materials.
Benefits of Flex Circuits
As for a ribbon cable or discrete wiring replacement, customized repeatable routing supplied by flex circuits trails all through the assembly. Because of their greater reliability, they can reduce service calls.
Polyimide is a dielectric layer that covers and offers protection to the circuit more than the simple sold mask. This layer is used by flex boards to cover the conductors of flex circuits. Other based and cover materials are used by manufacturers so as to handle a wider range of harsh surroundings and ambient conditions.
Flex boards are capable of weathering flexing long duty cycles though they can be very thin. To resist and being able to carry millions of flexing cycles, these boards can be strengthened by using appropriate design materials. This is to ensure that there are no interruptions while carrying power and signal.
When facing high acceleration and/or vibration, the high ductility and low mass of flex circuits is of great advantage. When under the same working conditions, the impact and stress on a rigid PCB and its solder joints and components are far more than that faced by the solder joints on a flex PCB
The advantages of flex circuit boards make them perfectly suitable for numerous applications in areas like Medical, Consumer Electronic, Military, Aerospace, Industrial, Automotive, Transportation, and communication.
Uses of Flex Circuits
Flex circuits are shaped by designers to fit into areas that any other type of PCB cannot. While flex circuits are exhibiting their individual advantages, someone can think that they are a hybrid combination of round wire and the ordinary PCB. Regular PCBs density, precision, and repeatability can be retained by someone, and with the help of flex circuits, the unlimited freedom for packaging geometry can still be achieved.
Usually, the wiring harness is replaced by flex circuits. In one operation, this permits a single flex circuit to displace several cables, hardboards, and connectors. Because it wraps them in bundles and eradicates the need to color-code wires, the assembly will go on faster than usual. During in-service failures and assembly, there will be lower chances of rejects and the volume of production levels will increase while the cost of installation drops.
Wire routing repeatability increases when flex circuits replace wire harnesses. During wire routing, this eliminates errors thereby reducing rework, test times, and rejections. Because round wires cannot carry more current and dissipate heat better than flat foil conductors of the same cross-sectional area, the connections are stronger. There will be a better control of noise, crosstalk, and impedance as the designer makes a decision on a more uniform conductor pattern in flex circuits.
In addition, the space and/or weight of conventional wiring can be reduced to about 75% by flex circuitry. The recurring costs of flex circuits are lower compared to the use of wire harnesses. The cost for replacement and repair of flex circuits are far lower as compared to the ones incurred for hard boards because they are much more resistant to shock and vibration. However, the components of surface-mount can be mounted easily on flex boards by putting bonded stiffeners at required regions.
Benefits of Rigid-Flex Circuits
Because a rigid-flex circuit takes the place of the maximum number of components, using it is beneficial when reviewing the whole installation for the total cost of ownership. Rigid-flex circuits provide the greatest maximum vibration resistance and capability as they make it possible to mix the best abilities of the resistant rigid areas with the resilient flex areas in addition to the mounting ability of surface-mount components on either side of the board. When mounting a component with a high mass, this is the best combination option as it provides a better and smoothest transition between flex and rigid regions while preserving their benefits.
Defining the appropriate stackup for a multilayer PCB is one of the most important facts in its initial design. It is important for large, dense PCBs having many pin-count BGAs, particularly when stackups of the standard laminate are insufficient with regards to performance goals and costs. If properly designed, HDI stackup is a good substitute for a great number of layers and it will provide low costs along with higher performances.
There are three types of stickups for boards with high pin-count BGAs. They include; sequential lamination with buried and blind vias, buildups with micro-vias, and standard lamination with through-vias. Amongst these three, buildup with micros-vias is that which is primarily used by HDI boards because it provides many advantages:
- Patterns of micro-vias can be used more efficiently. With this, more routing channels are opened resulting in potentially fewer layers.
- For high-density boards, it provides the lowest cost.
- Fewer layers and higher routing density are results of the smaller size feature of the vias and traces.
- Micros-vias is the only practical method used to design several large BGAs having a pitch lower than 0.88mm.
- An appropriate definition of stackup results in an enhanced powerful and signal integrity.
- They contain suitable materials for procedures that must meet the standards of RoHS.
- At lower costs, there is the availability of newer materials for higher performance and they might not be appropriate for other lamination types.
Manufacturers of eminent PCB defined HDI PCB stackups as those with 16 layers and having an overall board thickness of only about 66 ± 7 Mils. They have laser-drilled micro-vias and needs a sequential build-up (SUB).
In general, though rigid PCBs are less expensive than flexible circuits, cost increases with the count of the layer. Hence, to minimize cost, some considerations must be done. For example, using one four-layer circuit may be more expensive as compared to a two double-layer circuit.
In favor of flex circuits, some other factors might reduce the overall cost. For example, layers and space could be saved by folding a flexible circuit. Depending on the condition, the investment of time in the assessment of a project has potentially led to substantial general savings.
RayMing Technology offers detailed and completed printed-circuit-board development resources.