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Flex Talk: The Myth About Rigid-Flex Costs

This column was co-written with Anaya Vardya, American Standard Circuits.

Do you cringe when you think of the option of rigid-flex? It is not an uncommon reaction when talking with designers and engineering managers about using rigid-flex to solve a packaging problem. Why? The most frequent answer is, “They are so expensive.” While it is true that a rigid-flex PCB is typically more expensive on the surface when compared to rigid-board solutions with cables and connectors, a lot is being missed with that mindset.

Benefits

First, let’s discuss the many technical benefits associated with rigid-flex solutions. Rigid-flex PCBs can:

1. Serve as a remedy to natural product packaging problems

Flexible circuits are often chosen because they help solve problems related to adding electronics inside the product they serve. They are a true three-dimensional solution that allows electronic components and functional and operation elements (i.e., switches, displays, connectors, etc.) to be placed in optimal locations within the product, assuring ease of use by the consumer. They can be folded and formed around edges to fit the space allowed without breaking the assembly into discrete pieces.

2. Reduce both weight and volume requirements

Flexible circuits are appreciably lighter than their rigid circuit counterparts. Depending on the components used and the exact structure of the assembly and final products, they can save as much as 60% of the weight and space for the end-product compared to a rigid-circuit solution. Additionally, their lower profile can help a designer create a lower profile product than is possible with a nominal 1.5-mm rigid board.

3. Reduce assembly costs

Before the broad use of flexible circuits, assemblies were commonly a collection of different circuits and connections. This situation resulted in the purchasing, kitting, and assembly of many different parts. By using a flex circuit design, the amount of part numbers required for making circuit-related interconnections is reduced to one.

4. Eliminate the potential for human error 

Because flexible circuits are designed as an integrated circuit assembly with all interconnections controlled by the design artwork, the potential for human error in making interconnections is eliminated. This is especially true in the cases where discrete wires are used for interconnection. 

5. Facilitate dynamic flexing

Nearly all flexible circuits are designed to be flexed or folded. In some unusual cases, even thin rigid circuits have been able to serve to a limited degree. However, in the case where dynamic flexing of a circuit is required to meet the objectives of the design, flexible circuits have proven best. Modern disc drives, for example, need the flexible circuit endure anywhere millions of flexural cycles over the life of the product. Other products, such as laptop hinge circuits, may only require thousands of cycles, but it is the dynamic actuation capability enabled by the flex circuit that is key to its operation.

6. Improve thermal management due to being well-suited for high-temperature applications

High temperatures are experienced both in assembly with lead-free solder and in the operation of higher power and frequency digital circuits. Polyimide materials are well-suited to the management of high-heat applications. Not only can they handle the heat, but their thinness also allows them to dissipate heat better than other thicker and less thermally conductive dielectrics.

7. Improve product aesthetics 

While aesthetics may seem like a low-order advantage, people are commonly influenced by visual impressions and frequently make judgments based on those impressions. Flexible circuit materials and structures look impressive both to the seasoned engineer and the layperson. It can make a difference in the decisions made in some applications, especially those where the user gets exposure to the functional elements of the product. 

The increasingly sophisticated electronics being developed are pushing more designs to rigid-flex. Thinking through the benefits listed above, you become convinced that rigid-flex is the right direction for your next project. The next step is convincing your boss or program manager that this concept is the best solution. 

You are now battling that same perception; rigid-flex is more expensive. However, you cannot compare only the cost of the rigid board and cables to the rigid-flex. You need to look more holistically at the total cost of the design. 

Costs

Here are the key factors to consider when comparing the cost of rigid-flex to a PCB and cable solution:

1. Design

Because you are merging multiple boards, only one design is needed with a rigid-flex. With the rigid PCB and cable solution, multiple PCB and cable assembly designs are often required. The cost of generating each design should be included when doing a comparison of both options.

2. Cable and connectors

It is common for someone to compare the cost of the rigid boards with the cost of the rigid-flex and jump to the conclusion that the rigid-flex is too expensive. However, the cost of the cable and connectors should also be considered in this decision. This includes the cost of kitting for assembly, labor, in-process inspection, cable assembly test, final test, PCB tooling and test charges, and the cost of engineering time required for each of the items.

3. Assembly operation

Similar to the concept of the cost of the design, a rigid-flex solution requires only one assembly. The PCB and cable solution can require two, three, or even more individual boards to be assembled. The total cost of assembly should be included in this review.

This includes a similar list to the one in point two, along with multiple set-ups of the assembly equipment, and engineering time required for each assembly operation.

4. Testing

Not only does rigid-flex require one test operation compared to possibly several for individual boards connected by cable, but it also provides the ability to test the full assembly before installation.

5. Order processing

The cost associated with processing orders is often overlooked. Rigid-flex is one unit. Multiple boards, cables, and connectors can require several purchase orders to be placed, monitored, received, inspected, handled, stored, and payment processed. These costs should also be captured in a comparison of both options.

To read the full version of this article which originally appeared in FLEX007 Magazine, click here.