PCB | January 08, 2008

Demands of the automotive industry<br>on flex-rigid circuit boards

Flex-rigid circuit boards are always made up of flexible and rigid areas. The rigid parts, often in multiple layers, are connected to each other by a flexible base material. Thanks to the elimination of a large number of interconnection points (solder joints) potential defect sources are avoided, thus leading to a higher system-related reliability.
Modern vehicles feature the most varied electronic systems, for example:
• in the chassis: Anti-lock Braking System, Electronic Stability Program
• in the passenger compartment: switches, windows, mirrors, seats, lighting
• in the drivetrain: timing control, transmission control, starter
• as driving assistance: ACC, navigation system, active security devices

The following example shows the demands made on a motor control device in terms of temperatures and lifetime:

Source: Bosch

Flex-rigid circuit boards are always made up of flexible and rigid areas. The rigid parts, often in multiple layers,
are connected to each other by a flexible base material. Thanks to the elimination of a large number of interconnection
points (solder joints) potential defect sources are avoided, thus leading to a higher system-related reliability.

Instead of several circuit boards, cables, wires, jumpers and connectors, only one product has to be designed, tested and subsequently purchased and stockpiled. That permits savings, above all on the administrative side, which very quickly pays off.

If a particularly high component insertion density is required because the mounting space is very restricted, a double-sided assembly followed by folding of the circuit board can achieve something similar to a multilayer component insertion. (Fig. 08b)

The technological advantages of flex-rigid circuit boards can be summarized as follows:

• High reliability
• No wiring errors
• Easier components insertion
• Less weight and less volume
• Uniform, reproducible electrical characteristics
• Fewer inspections, simplified troubleshooting and repair
• Consistent, higher quality through machine soldering.

To manufacture flex-rigid circuit boards, several technologies are available that are tailored to suit the different requirements. (Fig. 8)

Common to all solutions is that the flex-rigid circuit boards have at least two layers. The flexible and the rigid parts are always electrically connected by plated-through holes in the rigid part. (Fig. 8b and c)

The Multiflex circuit boards can be equipped with components and soldered using standard processes. They are suitable for the press-fit and the bonding techniques. The flexible part can be bent several times in small radii. Depending on the layer count and the flexible material used, this can even be realised for continuous flexural stressing. (Fig. 09)

Typically, polyimide films are used as flexible base material. PEN or PET is also possible under specific conditions.
Alternatively, for extremely high demands on reliability and load capacity, and for high data transmission rates, LCP material (Liquid Crystalline Polymer) can be used.

The asymmetric configuration is the simplest design of a Multiflex circuit board. Besides the low production expenditure, an advantage of this technology is particularly its availability at an acceptable price. Like all flex-rigid circuit boards, the asymmetric Multiflex is easy to use in the further production chain. (Fig. 10a, 11a)

The symmetric configuration satisfies higher demands on the further processability: thanks to the symmetric layer construction this technique offers higher planarity. Through the rigid material on the external layers the further process control is again simplified, since identical parameters, adjusted to rigid material, can be used for both sides. Even for the reliability in the future application this variant is advantageous: Since the flexible layers are not coated with additional copper, the circuit board has a distinctly higher flexibility. Thanks to the balanced construction and the rigid materials on the external layer, this design shows higher temperature resistance in the solder bath test (shock) and the thermal cycling test (stress). (Fig. 12 )

The automotive industry wisely distinguishes requirements on the basis of the application, not on the basis of the build-up technique; i. e. the same standards apply to rigid, flexible and flex-rigid circuit boards. (Example: Bosch)

(Source: Bosch)

On the part of the circuit board manufacturer, besides precise knowledge of the characteristics of no-flow prepregs, also the appropriate combination of rigid and flexible materials is required. For long-term reliable through-hole platings, not only the material important but also the right process control is important. To quote some examples: optimised drilling parameters for the combination of rigid and flexible materials, and the right cleaning steps before and after the galvanic processes. (Fig. 14)

The sum-total of this experience can, without any major additional expenditure, result in distinctly higher reliability under thermal cycling stress, as shown in the following figure for several variants. (Fig. 15a,15b – after 2500 cycles)

The Semiflex circuit boards are a low-cost variant relying on a flexible material similar to FR-4. The rigid area can be built up with different materials (FR-4, High-Tg FR-4). The great advantage is that Semiflex circuit boards can be manufactured and processed like normal rigid boards. In the meantime, RUWEL are offering this technique with two copper layers in the flexible area.

The production of the flexible area can be illustrated with a few photos:

(Fig. 17a; BU: The rigid material is selectively thinned by a special milling cut in the area where the circuit board is intended to be flexible.)

(Fig. 17b)

A flexure of 180° is possible if a bending radius of 5 mm is maintained. For installation and possible service, the maximum number of bending cycles (typically 10) is sufficient in most cases. Even the maximum length of 16 mm does not represent a major restriction if that can be taken into account already when designing the assembly.

(Fig. 18a, 18c, Fig. 19)
If a longer flexible area (> 16 mm) or smaller bending radii are required, the Semiflex circuit board type II is available.

The production technique is similar to that for Multiflex circuit boards, but the cost-increasing flex-film (polyimide) is not used. Instead, a thin, flexible FR-4 material (typical thicknesses: 50 … 127 µm) is used with possibly modified resin systems for better bendability. Also for this construction variant, only a few bending cycles are permitted (typical value between 10 and 20 cycles). But the bending radius is reduced to 2 mm and the flexible area can have two conductive layers. (Fig. 20, 21)

The Yellowflex® circuit board is based on the idea of doing without the flex-film altogether and using the copper itself as the flexible material. The polymer developed to protect the copper is, in the second generation, also suitable for lead-free soldering. (Fig. 23a, 23b)

The advantages of the polyimide-free construction are obvious: Thanks to the low moisture absorption a specific thermal pre-treatment is no longer necessary. In addition, all follow-on processes are fully compatible with FR-4. Both the flexural strength (Fig. 25a, 25b) and the reliability of this variant have meantime been demonstrated.

The yellowflex technology is particularly appropriate for applications where the flexible area is only subject to semi-dynamic stresses; i. e. where the circuit board is bent only for installation and service. Further advantages over the Semiflex circuit board are the small bending radii and the bending angle which is limited only by the length of the flexible area. (Fig. 27a, 27b, 27c)

There are one or even several technologies available to build up the flex-rigid circuit board for every conceivable application. The possibilities range from the single-layer FR-4-based Semiflex board, the more flexible yellowflex variant, to symmetrical Multiflex boards which offer high layer counts and at the same time meet extreme requirements for reliability.

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