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Flexible Printed Circuit Design<3>

How to define the areas, layers and stacks of FPC?


Use tables to define cascading stacks

The most important document provided to the manufacturer is undoubtedly the stacked stack.

In order to make a rigid-flex board, it is necessary to provide different stacks in different fields and clearly identify them.

A simple method is to copy the outline of the board on the mechanical layer, and mark the area where different stacks exist, and put the corresponding stack structure table aside.

Next, we need a 2D space to define where is bent or folded, where components and other important objects can cross the boundary between rigidity and flexibility.


Convey PCB design intent

We all know that a picture is worth a thousand words.

If a 3D image showing flexible and rigid areas can be generated, this will help manufacturers understand our intentions more clearly.

Many people now use MCAD software to realize this view, and they import the STEP file of the PCB design into the MCAD software.

Another advantage is that it can help us check the mutual interference between the flexible board and the flexible board and between the flexible board and the rigid board, and avoid huge mistakes.



Component placement

Rigid-flex board means that components can be placed on the middle layer, not just the top and bottom layers.

This is a bit tricky in PCB design software, because usually components must be placed on the top and bottom layers.

We need the ability to place components on the middle layer. Fortunately, Altium Designer can place pads on any layer, so this is possible.

In addition, the silk screen can be printed on the flexible circuit. This is not a problem now, because the cover layer material can bond well with the screen printing ink. The key is to choose enough ink colors that penetrate the cover material and have a strong contrast. The clarity of the silk screen will be affected a bit, because it has to pass through the cover film and the fine spacing between them.

emphasize again! ! These need to be negotiated with the manufacturer to find a feasible and economical approach.


Remark 1: If we plan a certain area on the PCB board for connecting the flexible board, and place components on these areas, then this is a reasonable area for placing embedded components.

We need to generate a set of very clear files, showing the position of the incision and the layered structure. This will result in limitations due to manufacturing methods, such as re-drilling or multiple pressings.

Therefore, it is very important to accurately convey your intentions and minimize individual openings.

! ! It is best to avoid crossing openings on both sides of the board.


Remark 2: Define FLEX incision

IPC recommends a radius greater than 1.5mm (about 60 mils), which can greatly reduce the possibility of tearing the flexible circuit at the corner.

For the same reason, the two ends of the groove and slit in the flexible circuit should be placed with a hole with a diameter of 3mm (⅛) or larger to prevent tearing.

The grooves, slits and inner corners should have tear prevention openings or there should be an arc with a minimum radius of 1.5mm between the tangent lines.

In order to produce reliable rigid-flex board products, manufacturers and end users of flexible circuits need to consider a lot, including design considerations for copper patterns.


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 type of copper (RA or ED), There will be a lot of freedom.

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


Don't bend around the corner

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

But sometimes it is not possible, so please minimize the bending amplitude and frequency, or use conical bending according to the mechanical design requirements.


Use arc routing

As shown in Figure 1 above, it is best to avoid using abrupt right-angle or rigid 45° angle routing, but use 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 focus point, and the copper skin will easily age over time.


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.


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.


Maintain double-sided flexibility

For dynamic double-sided flexible circuits, try to avoid placing traces in the same direction, but stagger them so that the copper traces are evenly distributed.

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