Introduction to the typical manufacturing process of flexible circuit boards

Understanding the manufacturing process is very important for designing flex-rigid boards or flexible circuit boards. At the same time, it also tells you the data provided to successfully manufacture the designed board. If you haven’t read Parts 1 and 2 of this series of blogs, read them here and here, and then continue below.

Manufacturing documents for flexible circuit boards or flex-rigid boards

Understanding the manufacturing process is very important for designing flex-rigid boards or flexible circuit boards. At the same time, it also tells you the data provided to successfully manufacture the designed board. If you haven’t read Parts 1 and 2 of this series of blogs, read them here and here, and then continue below.

Manufacturing documents

Let’s talk about manufacturing files, they are very important. We tell manufacturers what we want through manufacturing documents, but they are also a significant factor in misunderstandings or mistakes that can lead to costly delays. Luckily, there are some standards we can refer to to ensure we have no trouble communicating with the manufacturers, especially IPC-2223B (which I also refer to in my blog post).

This boils down to the following golden rules:

Make sure your manufacturer has the capability to manufacture the rigid-flex board of your design.

Make sure they work with you from the moment they build the stack so that the design can accommodate their production process.

Use IPC-2223 as a design reference and make sure the manufacturer uses the same or related IPC standard so you use the same terminology as they do.

Involve them early in the design process.

output dataset

After visiting some local board makers capable of making flex-rigid boards, we found that many designers still send gerber files to their board makers. However, the preferred version should be ODB++ v7.0 or higher, as it adds specific layer types to its work matrix that make GenFlex® and similar CAM tools clearly identifiable. As shown in Table 1, a subset of the data is included.

Table 1: Subset of ODB++ Layer Types for GenFlex (V7.0 and Later)

(Source: ODB++ V7.0 specification)

Introduction to the typical manufacturing process of flexible circuit boards

If we use Gerber or earlier ODB++ files, we have a lot of trouble. That is, fabricators need to separate out the shear paths and die-cut patterns for rigid and flex circuit sections. In fact, we need to use the mechanical layer film to show the avoidance requirements ON the rigid board, and which parts of the flex circuit area will be exposed; showing how the cover layer can be used to strengthen the pads of the components mounted on the flex circuit.

In addition, special attention should be paid to the drilled hole pair and the through hole plating layer pair, because the drilling from the rigid board to the reverse side of the flex board requires re-drilling, which increases cost and reduces yield.

As a designer, the real question is, how to define these areas, layers and stacks?

Use a table to define a cascading stack

The most important document provided to the manufacturer is without a doubt the cascading stack. In order to make a rigid-flex board, it is also necessary to provide different stacks in different fields, and to clearly identify them. An easy way to do this is to make a copy of the outline of the board on the mechanical layer, and identify the areas where different stacks exist, and place the corresponding stack table next to it. Figure 1 below is an example.

Introduction to the typical manufacturing process of flexible circuit boards

Figure 1: A stack diagram showing the fill pattern of the soft and hard circuit areas.

In this example, I used different stack area matching fill patterns to indicate which stacks are contained in the flexible or rigid sections. It can be seen that the “insulation layer 1” here uses FR-4 because it is a reinforcement plate.

This creates new problems, we also need a 2D space to define where to bend or fold, where components and other important objects can cross the boundary between rigid and flexible. I will elaborate on this point later.

Communicating PCB Design Intent

We all know that pictures are worth a thousand words. If we could generate a 3D image showing both flexible and rigid areas, it would help manufacturers get a clearer picture of our intent. Many people currently use MCAD software to achieve this view, and they import the STEP file of the PCB design into the MCAD software. Figure 2 is an example of a reference to this concept.

Introduction to the typical manufacturing process of flexible circuit boards

Figure 2: Bending mechanics model showing design intent.

The additional benefit is that it can help us check the mutual interference between flex and flex and between flex and rigid, avoiding huge mistakes.

component placement

As you can see from the picture above, the flex-rigid board means that components can be placed in 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. Therefore, we need the ability to place components in the middle layer.

Fortunately, AlTIum Designer can place pads on any layer, so this is possible. In addition, silk Screen printing can be printed on flexible circuits. This is not a problem as the overlay material adheres well to screen printing inks. The key is to choose enough contrasting ink colors to penetrate the overlay material. The clarity of the silkscreen is somewhat affected by the fact that it has to pass through the cover film and the fine spacing between them. Again, it is necessary to negotiate with the manufacturer to find a feasible and economical approach.

Note: If we have planned an area on the PCB board for connecting the flex board and place components on these areas, then this is a reasonable area to place embedded components. We need to generate a very clear set of files showing the position of the cutouts and the stacking structure. This creates limitations due to manufacturing methods, such as re-drilling or multiple pressing. Therefore, it is important to communicate your intentions accurately and minimize individual openings. It is best to avoid crossing openings on both sides of the board.

Annotation: Defining Flex Cutouts

Notice in Figure 1, why there are no right angle bends, but what is the minimum radius of each bend? IPC recommends a radius greater than 1.5mm (approximately 60 mils), which greatly reduces the possibility of tearing the flex circuit around the corners. By the same token, both ends of the grooves and slits in the flex circuit should be placed with 3mm (?) diameter or larger holes to prevent tearing. See the example below for details.

Introduction to the typical manufacturing process of flexible circuit boards

Figure 3: Grooves, slits and interior corners should have tear-resistant openings or arcs with a minimum radius of 1.5mm between tangent lines.