From Cluttered Wires to Sleek PCB Design

According to Rush PCB Inc, there are infinite number of ways to connect PCBs or printed circuit boards to other boards, electronic devices or power sources. A designer has the freedom to choose from various connectors, cable assemblies, wires, etc. However, their choice of the correct interface is mainly governed by the given application. This often involves factors like environmental, voltage ratings, current-carrying capacity, size constraints, speed/performance, bend radii, and others. Then they must find the right balance between quality, cost, and capability.

Designers often have to balance mechanical considerations and electrical requirements while designing custom cables. They must provide a reliable cable assembly that is appropriate to the application while being cost-effective and manufacturable. The designer must consider some factors while reviewing the basics of a typical cable assembly:

• Carries power from one end to the other
• Carries digital data from one end to the other
• Carries analog signals from one end to the other
• Carries high current over one or more wires
• Requires insulation to protect the wires

In addition, the designer must also consider the following for the wires/cables:

• Voltage rating necessary
• Current capacity necessary
• Length of the cable
• Conductor count
• Termination method (connector, plug, etc.)
• Mechanical constraints
• Performance and speed constraints
• Environmental requirements
• Application specific safety, reliability, compactness etc.

Interconnecting Boards

It is possible to convert many existing designs consisting of multiple boards interconnected with wires to a rigid-flex design. Typically, this may involve a redesign of the rigid PCBs to incorporate the flexible circuit. There are two main advantages to be gained from such a conversion:

• Design Improvements
  • Reliability
  • Functionality
  • Ease of Assembly
  • Reduction in package size
• Cost Reduction

For some applications, cost reduction is the major achievement in converting to a rigid-flex circuit. Typically, these are designs with a large number of interconnections between the rigid boards, which give the assembly a cumbersome and complicated wiring harness arrangement. Most of these harnesses require manual assembly and attachment to the board is either by soldering or through connectors. In either case, it involves a significant amount of manual assembly.

Why Convert to a Rigid-Flex Design?

Converting to a rigid-flex design can eliminate most of the manual assembly, making it a simple and quick plug-in arrangement. A simple comparison of the cost of rigid-flex design versus that of the rigid board, connectors, and cable harness is enough to make a conversion decision. In most cases, although the material cost alone will favor the wired solution, addition of labor costs and time will tilt the case towards favoring the rigid-flex solution. The conversion will lighten even the bill of materials.

Converting for Design Improvements

Converting to a rigid-flex design to gain design improvements is more involved. The designer now has many more design elements they need to review and address apart from the cost benefit alone. In most cases, the designer starts with a wired design to prove the concept. Once they have achieved this, they may have other design elements they must resolve and a wired design may no longer be optimum. For instance, the designer may find:

• The enclosure may not be large enough to fit the wires and cables
• Wires and cables are not meeting EMI/EMC or controlled impedance requirements
• The additional mass of wires and cables is adversely affecting reliability
• Wires and cables are adversely affecting shock and vibration requirements

A rigid-flex design solution addresses the above issues easily. For instance:

• With a flex circuit, the volume occupied is only 10% of the volume of cables and wires.
• Flex circuits have much tighter bend capabilities as compared to cables and wires, allowing them to fit into tight spaces.
• It is possible to shield flex circuits to meet EMI/EMC and impedance requirements.
• Flex circuits improve reliability by eliminating manual operations, and by reducing the potential failure points.
• The reduction in mass by over 80% improves shock and vibration requirements tremendously.

Changing over to a rigid-flex design may require starting a new design phase with greater focus on the options available.

Flex-Based Options

For instance, depending on the application and the cost constraints, designers may have to choose one of the three options:

• Separate rigid boards linked with flex circuits
• Flex circuits with stiffeners
• Rigid-Flex arrangement

The designer must consider a number of variables to select the best option:

• The existing rigid board design and their layer count
• Types of connectors necessary for the interconnections
• Shape and size of the PCBs and that of the enclosure
• Number of interconnections
• Performance requirements such as current capacity, EMI/EMC shielding, impedance, voltage etc.

Each of the above solutions will have its advantages and disadvantages. Fortunately, flex circuit technology offers a wide range of configurations with which designers can meet most of their requirements.

Separate Rigid Boards Linked with Flex Circuits

Many designers prefer to use this solution because of its cost-effectiveness. The design effort required is the least for the rigid boards, which may remain unchanged. The only change is in designing the flex circuits to connect to the rigid boards. The designer has the freedom to choose the type of connector for interconnecting the flex circuit to the rigid board. For instance, if there is limit to the vertical spacing above the board, the flex circuit may use a ZIF connector, or even require soldering to the board.

ZIF connectors are commonly used by designers for connecting flex circuits to boards. The ZIF connector itself requires soldering on the rigid board, and the flex circuit has only to be pushed into the connector for the connection.

Flex Circuits with Stiffeners

Designers prefer to use only flex circuits with stiffeners where the rigid boards have very few components on them. With a low number of components it is easier to replace the rigid boards with flex circuits and at the same time, extend the flex circuits to make interconnections.

Rigid-Flex Arrangement

Designers use the rigid-flex arrangement where they fully integrate the flex circuit, laminating it within the rigid structure. The arrangement is unique as it requires no additional connectors. Although this is the most expensive option of the three, it offers the most in terms of capabilities.

As no connectors are necessary, the rigid part of the PCB can be smaller. Integration of the flex circuits allows for higher flex layer counts, resulting in a higher number of interconnects between the rigid areas. Most often, designers resort to the rigid-flex arrangement when their wired proof of concept design does not meet the space requirements of the enclosure.

Conclusion

At Rush PCB Inc, we frequently face questions from our customers asking for the possibility of converting an existing design to one flex-based. Most customers do not have the knowledge if they can make the conversion or how to proceed. They are looking for a potentially better solution to either address the various challenges they are facing with their current wired design, or to improve their design. In most applications, a flex-based solution brings substantial benefits in terms of cost, reliability, and performance.