A review of electrical and assembly costs and performance characteristics.
I often get asked to compare a rigid-flex concept with a rigid board and wiring concept. Among the key questions asked are: Is there a cost or a lead-time difference? Can rigid-flex handle performance requirements such as high-speed and low-loss? And will it be flexible enough? Let’s use a typical example where we have done this analysis.
To start, we need to satisfy the cost component. If the project is over budget, it is not going forward. So let’s dig into it. Per square inch, rigid-flex will cost more for a given area than a rigid board. This is due to higher-cost materials, as well as extra processing required for rigid-flex. Additionally, rigid-flex takes up more manufacturing area than a rigid board, as it not only has the board area but all the I/O interconnects. So, at first blush, rigid-flex is more expensive. But we can’t stop there; we need to consider the larger cost picture.
In this example, the board has to connect to four I/O circular connectors, each with 80 pins. When using wire bundles, that means we will have 640 discrete solder joints. You need to consider the labor and overhead costs of soldering that many joints by hand. Contrast that with only 320 joints just at the connector, since the other end is built into the rigid-flex. Additionally, those 320 joints at the connectors can be soldered with a machine reflow process, reducing labor content even more. There is substantial cost savings here.
Also, when using wire bundles, the connector selection will probably require solder cups, rather than straight through-hole pins. Often the solder cup versions cost more than the straight pin option.
Then there is the wire bundle. While discrete wire is relatively cheap, this case required 320 wires, cut, stripped and tinned. Those 320 wires do add up. By the way, will you solder the wires directly to the rigid board or use connectors? If the latter, add another 320 solder joints, and more connectors mean more cost.
There are other soft costs to consider if the considerations above do not already tip the scales in favor of rigid-flex.
Rework reduction: Since rigid-flex is fully electrically tested end-to-end, you have a fully functional and verified backbone for all components being installed. There are no connection problems to troubleshoot. Wire harnesses can be miswired; a rigid-flex cannot. The more wires, and the more complex the wire harness, the greater the probability of miswiring.
BoM reduction: A rigid-flex eliminates wires and connectors, reducing the size of the bill of materials and related logistics costs.
We have determined a rigid-flex will result in lower total cost. Now let’s make sure it can meet performance objectives. Here are the easy ones:
Weight/space: Rigid-flex will consistently reduce overall package weight, sometimes as much as 50%. This is because flex can route signals far more efficiently than insulated wire. Flex can use select conductor widths based on signal requirements, while wire bundles must use wire gauges big enough to meet the solder cup requirements of the connector. This can lead to excessive wire bundle weight. In some applications, rigid-flex can be the key component to meeting overall weight requirements.
The rigid-flex will reduce total volume consumed as well, helping to reduce total package size, or leaving more room for critical components. In the case we are considering, the 320 wires take up a lot of space. In fact, there is insufficient volume for all the wires in the enclosure.
Rigid-flex can help achieve a certain form factor driven by the program or marketing teams. Sometimes, a particular finished product shape is required for the end-application, or to just look cool and edgy. The space savings that rigid-flex brings to the table can help achieve this.
Also, rigid-flex eliminates the real estate needed for an onboard I/O for the wire bundles. This can further reduce board size, or repurpose the real estate for more functionality.
Reliability: With fewer solder joints in the device, the mean time between failure (MTBF) will be much higher with a rigid-flex. Also, the lower mass of the circuit makes it more resistant to acceleration and vibration stresses.
Electrical concerns: Rigid-flex provides electrical performance advantages. First, it allows direct wiring from component to I/O, the shortest and most direct signal route. Also, it can shield any sensitive signals end-to-end, providing the best isolation possible.
For impedance, flex materials have excellent properties, allowing design of impedance signals with minimal reflection points. Rigid-flex can reduce signal transmission skew by eliminating the inherent variables of discrete wiring bundles, typically by a factor of 10. Rigid-flex eliminates variation in wire lengths, shielding variations and solder connection lengths. Another flex advantage is the ability to consistently pair signals, as well as distance from plane to signal. Impedance signals are part of many (most?) rigid-flex designs today. In fact, rigid-flex can be used in very high-speed applications using select low-loss flex materials.
A trusted manufacturer can help design rigid-flex to meet flexibility needs. They can help make the tradeoff decisions of material thicknesses, conductor widths, shielding strategies, as well as bonding or unbonding the flex zones. When put together as a package, the circuit can achieve mechanical and electrical requirements simultaneously.
Rigid-flex will almost always be the better choice, especially as performance requirements escalate.
Nick Koop is director, flex technology at TTM Technologies (ttm.com), vice chairman of the IPC Flexible Circuits Committee and co-chair of the IPC-6013 Qualification and Performance Specification for Flexible Printed Boards Subcommittee; This email address is being protected from spambots. You need JavaScript enabled to view it.. He and co-“Flexpert” Mark Finstad (This email address is being protected from spambots. You need JavaScript enabled to view it.) welcome your suggestions.
