To successfully design a flexible PCB, it is important for the designers to have a basic understanding of the flex-design requirements. This article will explain some of these essential design guidelines.
Flexible PCBs were initially used within the military industry, but nowadays they are used in nearly every industry.
With that in mind, let’s take a close look at these 10 design requirements:
- The prerequisites of flex design
- Dimensional drawing must define rigid to flex interfaces
- Flex PCB stack-up construction and layer order
- Descriptive notes for the PCB manufacturer
- Flexibility and bend radius
- Dimensional stability
- Need for shielding
- Drill symbol chart
- Dos and don’ts of flex design
The prerequisites of flex design
First and foremost, there are several board elements that must be included in your design. These include:
- Layer count
- Finished board thickness
- PCB materials used
- Surface finish
- Drill size
- Drill to copper
These may seem like obvious requirements, but they occasionally go overlooked. When that happens, the PCB manufacturer will need to follow up for clarification, delaying turnaround time.
Dimensional drawing must define rigid to flex interfaces
The dimensional drawing identifies a number of critical measurements for the PCB design. The dimensional drawing should define the rigid to flex interfaces. It should describe where these two types of materials meet. The typical outline tolerance is +/- 0.010″.
The dimensional drawing of a flexible PCB design provides the following information:
- Stiffeners’ location and dimensions
- Thicknesses of each section of the PCB and the materials to be used
- Static or dynamic flex type during operation of the circuit. A PCB that flexes a maximum of 20 times is called a semi-static flex PCB. A PCB that is regularly flexed and twisted is called a dynamic flex PCB. Printers use dynamic flex PCBs as the circuit flexes throughout the operation of the printer.
- Locations where the board rarely flex and frequently flex.
Flex PCB stack-up construction and layer order
Stack-up differentiates between rigid layers and flexible layers in the PCB. The flexible PCB stack-up drawing will give the thickness of each layer including the copper weights of the conductive layers. It will also indicate the impedance traces and the values of impedances (50 ohms or 100 ohms). Sierra Circuits can assist you in designing your flex stack-up.
Descriptive notes for the PCB manufacturer
The PCB manufacturer expects descriptive notes about the following:
- Class type (class 1, class 2, class 3), wiring type, and installation use requirements
- Flexible copper clad material to be used
- The cover coat material
- Maximum board thickness
- Minimum size of plated through holes
- Electrical test requirements
- Color of coverlay
- Color of silkscreen
- Board markings such as part number, version, and company logo
- Packaging and shipping needs
Flexibility and bend radius
The flexibility of a flex PCB is determined by the bend radius of the flex material used. Bend radius is the minimum angle the flex region can bend. The below illustration helps us to understand the concept of the bend radius.
Knowing the number of times your flex PCB will bend is crucial in your design. If a PCB is bent more times than the design allows for, then the copper will begin to stretch and crack.
A PCB that flexes a maximum of 20 times is called a semi-static flex PCB. A PCB that is regularly flexed and twisted is called a dynamic flex PCB. Printers use dynamic flex PCBs as the circuit flexes throughout the operation of the printer.
The bend radius depends on the number of layers and the type of material used in the flex.
|1 Layer (single-sided)||Flex Thickness x 6|
|2 Layer (double-sided)||Flex Thickness x 12|
|Multi-Layer||Flex Thickness x 24|
Dimensional stability is an extremely important factor that needs to be addressed by designers, particularly in high-density designs. Small dimensional changes can easily occur in the circuitry panel as it undergoes processing. Additionally, these PCBs are exposed to various processes including etching, electroplating, pressures, temperatures, and chemistries. It becomes important for designers to consider the above changes for deciding the part numbers for a new flexible circuit.
Implementing scaling factors
Designers must apply various scaling factors on secondary layers of the PCB board. These dimensional changes occur in the PCB as it is cured during the heat lamination process. Several in-process measurements must be adopted by the designer and fabricators with the help of scaling factors. These scaling factors help the manufacturers to anticipate the loss percentage and scale-up the PCB dimensions as per the designers’ requirement. These calculations are performed via dynamic methods. Implementation of a finalized drilling program in a multilayer circuit helps to improve dimensional stability as well.
Various software-controlled operations and calculations are performed to analyze dimensional shifts. These calculations are performed with the help of optical fiducials. These methods involve measuring the targets that are present on the outside corners of the panel. Proper alignment is done after several illustrations, thus implementing the necessary X, Y, and theta corrections.
Need for shielding
Shielding is another important parameter that needs to be considered for the flex circuits. Various methods are implemented to incorporate shielding. The shielding is for limiting the effects of EMI and ESD. The shielding also helps in keeping the controlled impedance requirements under wraps.
Four types of shielding are typically performed by designers including,
- Copper Clad
- Copper crosshatch
- Silver paste/epoxy
- EMI thermoplastic shielding film
Drill symbol chart
The drill symbol chart indicates all of the finished hole sizes, as well as the hole size tolerance, for your circuit board design. To learn more about PCB drilling read PCB Drilling Explained: The Do’s and the Dont’s. The drill symbol chart summarizes the drill holes’ information present on the board. An example of a drill symbol chart is shown below. The standard finished hole size is +/- 0.003″ but this is never assumed, so this measurement must be stated on your design drawing.
Each of the symbols represents a drill size. The other columns in the chart provide information like hole shape, and hole type (Via or non-plated).
Dos and don’ts of flex design
- Avoid placement of vias in the bend areas of the flex circuit. However, if there is a requirement of placing a via in your flex circuit, identify a region where there is no bend to place a via.
- Always use curved traces instead of traces with corners.
- Avoid abrupt changes in the width of the traces.
- Minimum clearance of 20 mils should be present between the copper annulus and the nearest via.
When we design a flexible PCB it is very crucial for us to provide the vital information(drawing requirements) to the manufacturers as this information helps them to fabricate the flexible PCB as per the designer’s expectation without any time delay.
For an even more in-depth look at drawing requirements, check out our Flex PCB Design Guide.
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