As seen in the October issue of Flex007 Magazine. With Anaya Vardya of 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. Rigidflex 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 flexcircuit 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. Rigidflex 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.
Reliability
Without question, the rigid-flex option is considered a high-reliability alternative to the PCB and cable solution. For many years, rigid-flex was predominately a mil/aero solution, but over time has become common in nearly all markets. The connector is an
integral part of the board; there are no solder connections between boards.
With rigid-flex, the reliability is dependent on design, not on the assembly process.
It is easy to arrive at the conclusion that moving to a rigid-flex design does simplify things for designing, purchasing, assembling, inspecting, or even accounting. However, the question is, “Does this simplification justify the cost?”
Each application should be reviewed individually.
Fabricators
Moving ahead with your rigid-flex design, how can your fabricator help?
1. Stiffeners versus rigid-flex
Flex with stiffeners to support component areas is a less expensive alternative to rigidflex and worth the discussion. The primary difference in a simple design is the rigid-flex will have a plated through-hole connecting all the layers, while the FR-4 stiffener is used only for component support. The density of component areas is often the driving factor toward rigidflex.
2. Stackup
Your fabricator can help ensure that you are meeting thickness and impedance requirements for the design. They will also provide guidance on materials that are in stock and materials that may need to be special ordered so that material lead-time can be factored into the project plan. Further, your fabricator can also discuss tradeoffs of various materials, so you can be sure you are designing with the most cost-effective construction.
3. Array design and panel utilization
Typically, panel utilization or the “number up” is the biggest cost driver for flexible circuit designs. As with rigid designs, fabricators price by production panel, with the piece price being the panel price divided by the number of parts per panel. It is important to understand your fabricator’s preferred panel size. Common panels sizes are 12” x 18” and 18” x 24”. Fabricators commonly use the outside one-inch border of the manufacturing panel for coupons and tooling holes. Effectively, when designing, optimizing the usable space of 16” x 22” and 10” x 16” with individual pieces or arrays will result in the lowest cost option.
Rigid-flex often takes on unusual shapes that are not necessarily the standard square or
rectangle we see with rigid boards. Standard panelization software may not consider this.
If the design can be reverse-nested to increase the number of parts per panel, this can significantly impact price and is worth time for review when setting up the array configuration.
4. Dynamically flexing
Clearly communicating areas that your flex circuit will be dynamically flexing will greatly benefit your design. Your fabricator will be able to review the design to ensure you are following best practices. Further, when setting the tooling for manufacturing, they will be able to orient the circuits on the production panel properly. Copper grain structure now becomes critical. The orientation with the grain structure could impact the material utilization and piece price.
5. Blind and buried via structures
It is always recommended to interact with your fabricator when developing blind and
buried via structures with flex and rigid-flex. As you develop these structures, you are adding base copper on various layers. This can impact the smallest lines and spaces possible on those layers.
A Case Study
The following is a case study that illustrates why it is important to work with your PCB
fabricator during the design phase. We once encountered a telecommunication application that had a 50% failure during installation due to cracking of the copper in the flex area. When the customer came to us, we reviewed the stackup and redesigned it by:
• Converting stackup to adhesiveless
materials
• Decreasing flex thickness from 11.8 mils
to 8.4 mils (29% decrease)
The extra thickness was adding rigidity to the flex area and causing cracking.
Summary
There are many things to think about when considering a rigid-flex design to solve a packaging problem. Flexible solutions provide numerous benefits, including space, weight, packaging, reliability, and more. Even with these benefits in mind, it can be difficult to justify the added expense when compared to the traditional approach of a rigid PCB and cable solution. It is easy to only make a comparison at a surface level. Digging deeper into the total cost also includes purchasing, receiving, inspection, and administrative cost. A higher number of purchase orders being generated for
the PCB and cable solution when compared to a single rigid-flex design provides a more holistic view of total cost.
Moving forward with a rigid-flex design, it is highly recommended that you work closely
with your fabricator for stackup, array design and material utilization, dynamic flex requirement, and advanced via structures to ensure that you are not unnecessarily introducing added cost. Your fabricator works with rigidflex designs daily—take advantage of that knowledge!
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