Special attention to flexible circuit wiring

Problems such as layer stack design, device layout, and cutting are obvious, but there are many material weaknesses in flexible circuits.

From adhesives with relatively high z-axis expansion coefficients, to low-viscosity PI substrate copper coatings, to copper hardening and fatigue. The do’s and don’ts stated below will focus on adding:

Maintain the flexibility of the flexible board

It is obvious that the flexibility of the flexible circuit must be determined in advance according to the needs, but it must be emphasized again.

If the flexible circuit part is only intended to be folded during the assembly process, and then installed in a fixed position, for example, installed in a handheld ultrasound device, then we are choosing the number of signal layers and the copper type (RA or ED), There will be a lot of freedom.

On the other hand, if the flexible circuit part is to be continuously moved, bent or rotated, then the number of layers should be reduced, and no glue material should be selected.

We can use the IPC-2223B formula (Formula 1 for single-sided, formula 2 for double-sided, etc.) to determine the minimum allowable bending radius according to the allowable deformation of copper and other material characteristics.

The formula for this example is for a single-sided flexible board.
We choose EB according to the actual conditions of use, 16% for seldom bending applications, 10% for flexible installation applications, and 0.3% for dynamic flexible designs

Dynamic refers to the continuous bending and rotation of the product during use, such as the connection of the TFT panel in a mobile DVD player.

Don’t bend around the corner

Usually we recommend keeping the copper traces of the flexible circuit bent in the vertical direction.

But sometimes it can’t be done, so please reduce the bending amplitude and frequency as much as possible. You can also use conical bending according to the mechanical design requirements.

Use arc routing

As shown in Figure 1 above, it is best to avoid the use of abrupt right-angle or rigid 45° angle routing, but use the arc-angle routing mode. This can reduce the stress of the copper skin during the bending process.

Don’t suddenly change the width of the trace

When the trace is connected to the pad, especially the flexible circuit terminal arranged (as shown in the figure below), it will form a weak point of force, and the copper skin will easily age over time.

Unless a reinforcing plate is used or the application process will not bend, it is recommended to use the following gradually narrowing wiring method

(Hint: Teardrop treatment for pads and vias in flexible circuit boards!)

Figure 2: Sudden changes in the trace width or connection to the pad will cause a weak focus point.

Use polygon

Sometimes, it is necessary to place a power supply or ground plane on a flexible board.

If you don’t mind the significant reduction in flexibility and the possibility of wrinkling the copper skin, you can choose to use solid copper.

Generally speaking, in order to maintain a high degree of flexibility, it is best to use shaded polygonal copper.

When I mentioned this, I also thought that traditional shadow polygons would have extra copper reinforcement in the directions of 0°, 90°, and 45°.

The more optimized mode is the hexagonal method.

This problem can be solved by using the negative film layer and the hexagonal pad of the array, and the shadow polygon can be created more quickly by using the copy and paste method.

Figure 3: The use of hexagonal copper can evenly balance the stress at three angles

Provide reinforcement for pads

Due to the use of a low-viscosity adhesive (compared to FR-4), the copper on the flexible circuit is more easily detached from the polyimide substrate. Therefore, it is particularly important to provide reinforcement to the exposed copper skin.

The plated through holes provide proper anchoring for the two flexible layers, so the use of through holes is a very good reinforcement method.

Because of this (providing the extension of the z-axis), many manufacturers recommend adding a 1.5 mil deep coated through hole to the rigid-flex board and flexible circuit.

Surface mount pads and non-plated through-hole pads have no reinforcement measures themselves, so additional reinforcement is needed to prevent separation.

Figure 4: The pad reinforcement method of the flexible circuit, coating, increase anchoring, and reduce the opening of the cover film

Referring to Figure 4, the second option is suitable for a rubberized cover layer, and the third option is suitable for a non-adhesive cover layer.

The use of adhesive protective film will cause “glue overflow” phenomenon, so the gap between the pad and the opening must be large enough to ensure excellent flux formation.

The SMT component pad is the most fragile, especially the flexible circuit will bend under the rigid pin and pad of the component.

Figures 5 and 6 show how to use a cover layer to reinforce the pad at both ends of the pad.

To do this, the pads on the flexible board must be larger than the pads on the typical rigid board.

Looking at the comparison in Figure 6, the SMD pads of the components are mounted on the flexible board. This will significantly reduce the mounting density of flexible circuit components, but compared with rigid circuits, the density of flexible circuits cannot be too high.

5: The cover film opening of the SOW package shows its reinforcement at both ends of each solder pad

Figure 6: Adjust the size of the pad and the opening of the cover layer.

The above is a typical 0603 package form, and the following is a package form modified to use the cover layer to reinforce.

Maintain double-sided flexibility

For dynamic double-sided flexible circuits, try to avoid placing traces in the same direction, but stagger them (Figure 7) to make the copper traces evenly distributed (Figure 8).

Figure 7: Adjacent layer copper wiring is not recommended.

Figure 8: Staggered adjacent layers of copper traces are preferred.

Application and Examples of Printed Circuit Board

The more you understand the rigid-flex board technology, the more surprising you will find more stunning applications. Here are a few concise skills and application concepts I have learned.

Dynamic flexibility

Designing the flexible part in your product is generally based on the following two principles:

First, build a compact and efficient equipment;

Second, the circuit dynamically integrates the mechanical structure.

Of course, the role of the flexible circuit can also be selected based on these two principles.

Now, let us take a look at a few flexible circuit examples that can inspire your design inspiration:

Frame structure

This is a very typical application of dynamic flexible circuits, which can be mounted on a 3D printer or on the mechanical head of a CNC machine tool.

Usually, it will be installed along the X direction and the tool head will move along the z axis. This shows two axial movements, and the frame structure itself will also move along the Y axis.

The total length of the flexible circuit is the length of the moving head to the extreme end, plus the length of the bend and bend.

The corner part is used to connect behind the mechanical head that moves on the Z axis, and it shuttles back and forth along the frame structure together with the mechanical head. The terminal of the flexible circuit must have enough bending length.

For this type of application, it is best to use a single layer of cold rolled annealed copper and make the larger the bending radius, the better, this design can extend the life of the product.

Pasting the flexible circuit and the stainless steel strip together can also extend the life of the product.

Figure 1: Initial flexible circuit design.

Manufacturing considerations: Jigsaw

The above example brings up a manufacturing and cost issue very well.

If a right-angled L-shaped flexible circuit is used according to theory, then we can manufacture 6 identical flexible circuits on one panel, and we waste nearly 50% of the panel space. If we solder components on it, we have to pay additional processing costs and time.

The jigsaw diagram of this flexible circuit is shown in Figure 2.

Jigsaw of flexible circuit of control frame.

Another advantage of using flexibility is that if we use the right material and can ensure correct installation, then we can design a crease with a very small radius. This is a good alternative to the previous application, but it should be used in a specific environment.

Figure 3 shows another design that uses a 45° crease instead of the 90° bend in the previous design.

In this case, this crease is suitable because this part of the flexible circuit will be fixed on a large rigid mechanical body, so there will be no excessive loss. This solution will significantly reduce costs, and the pick-and-place processing technology is also simplified.

However, you may think of this: due to the crease, the components need to be placed on the opposite side of the end layer welding assembly.

Figure 4 shows the crease plan on the same panel, and the panel output has doubled!

4: The same panel size-the flexible design of the crease curve scheme doubles the output of each panel!

Planning a cascading structure

Relative to the rigid-flex board, the purely flexible electrical layer stack structure must be simple. However, we still need to place anchor points on the panel. Most flexible circuit designs will require a reinforcement board to be placed in the installed components or terminal area.

Figure 5 shows the layer stack structure of the flexible board used in the above-mentioned rack structure. The part of the reinforced board is a “rigid” stack, which is fixed and displayed in 3D in the PCB editor.

Figure 5: The definition of the stack structure of the flexible board used in the frame structure example.

Figure 6: PCB frame with rotatable flexible design.

Rotating equipment

Take a look at Figure 6. In the PCB editor, we used a horizontal work guide, which helps to design an accurate board profile based on the bending circumference of the flexible circuit part.

At the same time, we can plan and display the crease position of the flexible circuit in the board-level planning mode of the PCB editor, and accurately simulate the bending degree of the flexible circuit board in the 3D mode.

Figures 7 and 8 show the 3D model view of this design.

Figure 7: 3D view of the rotary step control board level.

The long “arm” can make the motor and its control board rotate more than 360°.

Figure 8: Fully folded view of the assembled, including the 3D stepper motor.

In Figure 8, I marked the moving arrow and the fixed end of the flexible circuit to give you an intuitive concept.

This layout design makes it easier to achieve 360° rotation.

This is a hypothetical example. The object is a stepper motor. This design is very suitable for rotating sensors.

Fixed flexible circuit applications

Planar magnetic components (transformers and inductors)

There are more and more applications of using flexible boards or rigid-flex boards as planar magnetic components. The backlight inverter board of the LCD TV has a neat row of step-up DC-DC voltage stabilizing circuits, and a transformer with a flexible circuit as winding is used in the voltage stabilizing circuit.

The winding is composed of a rotating flexible circuit, as shown in Figure 9.

9: Uncrimped four-winding inductor.

Using flexible circuits as planar magnetic components has obvious advantages. Very thin polyimide films can achieve very high safety isolation.

Moreover, the polyimide film maintains stable performance at very high temperatures, which makes it suitable for hot enamel potting processes.

From a loss point of view, the use of etched copper wires certainly requires wider traces, but because of the thinness of the skin, eddy current losses can be reduced very easily.

More refined entrance and exit design can overlap them, and this exit corresponds to the next entrance.

Such a design will be easier to increase the number of turns of the coil than designing multiple independent windings on a plane, as shown in Figure 10.

Figure 10 18-layer winding composed of 2 layers of flexible boards

Extending this concept further, we can use more flexible layers in the design of the converter, and then overlap them.

In the 2-layer flex circuit transformer design shown in Figure 11, the E18 planar ferrite core passes through the cutout on the PCB. This approach can be extended arbitrarily, and its actual limit depends on the thickness of the final folded board.

As shown in Figure 11, the double-sided flexible board finally constitutes an 18-layer transformer winding.

In the center of each cut, there can be a single-turn inductive winding. The side around the curved track can be rotated by half a turn. Since the side of the magnetic circuit area is actually only half effective, you may not be able to completely cover all the area, but by adding one or two additional half-turn turns, you can Achieve full coverage.

Figure 11: Looking at the flexible circuit transformer from top to bottom.

A single high-current winding is installed at the top, and six low-current windings are installed at the bottom, which can be implemented using the Altium Designer bus wiring tool.

This can cause confusion because you have to record the proper ferrite core winding direction orbit.

Since the entire flexible circuit will be folded vertically, I added an arrow on the first layer of the machine, facing each adjacent winding, and reminding me which copper path needs to be laid.

To explain more clearly, please refer to Figure 12.

Figure 12: Mechanical 1 layer shows the outline of the board and the winding direction arrow.

The flexible parts of the core are installed as shown in the figure below.

Please note that it will be connected to the rigid-flex board, usually the circuit is on a 2-layer rigid printed circuit board, and the flexible part is usually an additional layer required for all core windings.

Of course, whether to use a large-area flexible layer or to add more ground layers to the rigid board design needs to be determined by weighing the cost.

The fully folded transformer is connected to the 3D Femex E18 ferrite core through the cutout.

Multi-layer soft and hard board

How to maintain the flexibility and durability of the multilayer flexible board?

Many military, aerospace, or similar high-density designs require compact layout and reliable assembly in a small space. This makes it difficult to use multilayer flexible circuits between rigid boards.

This is also required in high-speed digital designs, because shielding or plane layers need to be added when the bus circuit passes over the flexible board.

The problem now is: In order to maintain good flexibility, the number of flexible circuit layers must be as small as possible.

The usual structure is a PI substrate, a copper skin layer attached to both sides of the substrate, and a PI protective film.

In a “normal” design, the length of the overlapping flex circuits is the same.

As shown in Figure 14, once the assembly is completed, the bending of the flexible part between the rigid boards will generate a lot of tension.

Figure 14: When multiple flexible circuits overlap and have the same length, there will be stretching on the external flexible circuit and squeezing on the internal flexible circuit.

Please note!! Use glue beads in the contact part of the flexible circuit and the rigid circuit.

Experienced rigid-flex board manufacturers will recommend the use of “bookbinding”.

“Binding” is a feasible method, according to the use of flexible circuit bending radius to determine the length of other flexible circuits and substrates. As shown in Figure 15.

You might say that this method is very expensive and a challenge for design.

Usually a better alternative is to use flexible circuits of the same length and radius, but separate different flexible circuit layers without overlapping each other. See Figure 16 below.

Figure 16: Alternative bookbinding structure.

Ultra-tight bending without losing the number of wiring layers

If you haven’t seen it before, this is definitely a magical thing! I took a few photos of the rigid-flex board and flexible circuit board during the PCB West Expo.

Figure 17 is a small circuit board that uses several S-shaped leaf connections to increase the minimum bend radius between the parts. In this photo, you may not see clearly, the components are installed on the part with the reinforcing plate on the back.

Figure 17: It has multiple copper skin layers and maintains a 180° bend.

This concept can of course be extended to multiple uses.

As shown in Figure 18, this is an ultra-flexible display panel. An LED matrix is ​​arranged on the wider, reinforced board part. The entire assembly process will be more stringent, because it is laminated with a lot of copper skin layers and PI film. Similarly, the use of S-bends allows the entire design to be placed into an arc-shaped enclosure.

Each LED in the matrix is ​​individually controlled, so there are many independent traces in this design.

Figure 18: X-Y S-shaped bending flexible array.

Extending this concept even further, the application of Figure 19 is simply too cool. This is a very compact design. According to the PCB supplier’s introduction, each independent flexible circuit part has 8 layers. Such a flexible circuit itself does not have enough flexibility. But using multiple S-bends can make it fold into the final mechanical casing, and even contain hundreds of high-speed memory and display connections.

Figure 19: 8 flexible plies and 4 additional rigid plies. Notice! ! ! The top layer of the flexible board is a solid copper shield, and there are adhesives on the soft and hard board junctions and part of the edges.

For highly creative designers, more applications will always be extended, as shown in Figure 20!

Figure 20: Who said that the rigid-flex board cannot have creases?

The application prospects of flexible intelligent applications are improving all the way, but various supporting facilities in the process of its industrialization will surely face many challenges.