Benefits of Flex and Rigid-Flex PCB

Flex PCB and rigid-flex PCB board allow you to design the circuitry to fit the device, instead of building a device to fit the circuit board. They are largely used in the industry of aerospace, medical and military applications thanks to dependable reliability.

Flex circuits offer multiple advantages, and the following will show you some key benefits of using flex and rigid-flex circuit technology.

Component/Connector Assembly1.Flex circuits can accept any component or connector that can be assembled to a rigid PCB design.2.Integrated ZIF contacts provide simple modular interfaces to the system environment.3.Additional options are available such as, ZIF Connectors, crimped contacts, direct solder to PCB, etc.

Cost Savings1.Thin and flexible polyimide film requires a much smaller area, reducing the overall finished assembly packaging size and material requirement costs.2.Reduced assembly costs are also seen as fewer parts are needed for the final assembled product.3.A simplified PCB assembly processes can reduce assembly errors as the flex circuit can only be installed one way.4.Flex circuits can eliminate wire routing errors, and reduce test time, rework, and rejects.

High Density Applications1.Flex material properties work very well in high speed controlled impedance designs, which allow better control of impedances.2.Flexible PCB allow for narrow lines giving way to high density device population. Denser device populations and lighter conductors can be designed into a product, freeing space for additional product features.

High Temperature Applications1.Flex circuits materials (polyimide) dissipate heat at a better rate than other dielectric materials while providing the added benefits of vastly improved flexibility.2.Exposed to extreme temperature applications (up to - 200C to 400C) is ok.3.Expansion and contraction are minimized when using polyimide material.4.Good chemical resistance to oils, acids, gases etc.5.Flex circuits boards offer excellent radiation and UV exposure resistance.

High Density Interconnect Board

High density interconnect board or HDI board is one of the fastest growing technologies in PCB industry. HDI boards contain blind and/or buried vias and often contain microvias of .006 or less in diameter. They have a higher circuitry density than traditional circuit boards.

There are 6 different types of HDI boards, through vias from surface to surface, with buried vias and through vias, two or more HDI layer with through vias, passive substrate with no electrical connection, coreless construction using layer pairs and alternate constructions of coreless constructions using layer pairs.

HDI technology is the leading reason for electronic products transformation like camera, mobile, computer, pad, etc. Products do more, weigh less and are physically smaller. Specialty equipment, mini-components and thinner materials have allowed for electronics to shrink in size while expanding technology, quality and speed.

As consumer demands change, so must technology. By using HDI technology, designers now have the option to place more components on both sides of the raw PCB. Decreased component size and pitch allow for more I/O in smaller geometries. This means faster transmission of signals and a significant reduction in signal loss and crossing delays.

While some consumer products shrink down in size, its quality remains at its best level. Using HDI technology is possible to reduce an 8 layer through-hole PCB to a 4 layer HDI microvia technology packed PCB. The wiring capabilities of a well-designed HDI 4 layer PCB can achieve the same or better functions as that of a standard 8 layer PCB. Although the microvia process increases the cost of the HDI PCB, the proper design and reduction in layer count reduces cost in material square inches and layer count more significantly.

A Flexible Circuit Coverlay

In the flexible printed circuit board manufacturing process, a flex circuit coverlay or coverfilm is used to protect the external circuitry of a flexible circuit board.

A flexible circuit coverlay serves the exact same function as soldermask that is used for rigid printed circuit boards. The difference with the flexible coverlay is the flexibility required for flexible circuit boards and the required components for durability.

The coverlay consists of a solid sheet of polyimide with a layer of flexible adhesive that is then laminated under heat and pressed to the circuit surfaces. The required component feature openings are mechanically created using drilling, routing, or laser cutting.

Typical coverlay thickness is 0.001" polyimide with 0.001" of adhesive. 0.0005" & 0.002" thicknesses are available, but only used as needed to meet specific design requirements. 

Polyimide coverlay and flexible liquid photoimageable (LPI) solder mask are two primary options for encapsulating the external circuit layers of a flex circuit board.

Of the two, polyimide coverlay is the most commonly used and preferred solution throughout the industry. It provides a much more robust and durable solution with very good flexibility and a high dielectric.

Flexible LPI solder mask is essentially the same formula that has the same capabilities and is applied in the same manner as on rigid circuit boards.

Tips for Flexible PCB Board Design

Before designing printed circuit boards, I almost unheard of flexible PCB board. But after several years’ experience, I have concluded sound design practice that is specific to flexible PCB is required to eliminate problems in fabrication and assembly. Here are some of them:

Avoid vias in bend areas-- Via should be avoided in bend as it can cause breakage on the flexible PCB when the stress applied goes beyond a threshold.

Clearance between components and flex area-- It is important to maintain a gap of at least 100mil for components and bend area. When placed on a bend area, the solder on the component’s pin became the weakest point as it is not meant to be bent.

Copper to edge clearance-- Like rigid PCB, you’re bound to have power and ground planes on Flexible PCB. It is important to have a decent clearance between the copper area and the edge of the flexible PCB. Neglecting this important guide could result in the copper shorting at the edges when the PCB is cut.

Ground plane design-- Cross hatch polygon is preferred for ground planes as it increased flexibility and reduce weight.Minimum hole size--Bear in mind that it is harder to drill on a flexible PCB than on a regular rigid PCB. Be sure to keep the hole size at a minimum of 10 mils for your design.

The IPC-2581 File Format

Eminent PCB manufacturers eliminate the problems by adopting design transfer standards that addresses all aspects of the fabrication and assembly process. Two new open standards are available, and these enable efficient and accurate data exchange from the PCB designer to the manufacturing fabricators and assemblers. Ucamco administers one of these standards, theGerber X2, while the IPC Consortium administers the other, the IPC-2581. Both are open standards, free from any proprietary restrictions.

Contributors from a wide range of PCB industry segment initiated, developed, and drove the IPC-2581 standard. The IPC-2581 is a single data format and within a single file, contains all aspects of the PCB design, such as layer stackup, materials, assembly, and test details.

With the IPC-2581 standard, the designer can include details of layer stack and information on materials to ensure proper layer order. The standard is suitable for stackups of complex board design such as related to rigid-flex boards, and is capable of handling special materials. It can also include drill and mill data for blind, buried, and filled via types. It also supports information on back drilling, V-grooves, slots, and cavities. For bare board testing, designers can include the net-list as well.

In addition to a complete set of fabrication data, the IPC-2581 can also hold assembly data. Therefore, it can contain not only the pick-and-place information, but also the information on polarity and rotation of a component, enabling support for both stacked and embedded components.

In addition to assembly drawings, the IPC-2581 standard has the capability to generate the documentation for bill of materials and purchasing. Therefore, the standard can tie up with PLM/ERP system data to create links between design and supply chain facilities. The greatest advantage of the IPC-2581 is one single file containing the entire data related to fabrication and assembly.