Modern circuit boards follow specific flexible PCB design rules during design and fabrication.
Without such rules, you will develop circuit boards with defects and errors.
As a result, such circuit boards won’t last long in electronic devices or won’t work at all.
Therefore, you must follow the design rules to avoid such and create effective circuit boards.
For this reason, we present the must-knows and design rules that govern flex PCB design. Let’s get rolling.
Table of Contents
- What’s A Flex PCB
- Flexible PCB Design Rules and Standards
- Must-Knows While Designing Flex PCBs
What’s A Flex PCB
If you have spent most of your time dealing with rigid circuit boards, flex PCB design might be unfamiliar territory.
From their name, these are circuit boards developed from materials that can twist and bend freely without breaking or damaging.
Designers love these boards because they are lightweight and thin enough to fit into tight spaces.
As a result, you can use them to create wearables and other compact devices.
Manufacturers design different flexible circuit board types to meet customer needs.
Single-sided flex PCB: These are the lightest and thinnest and have only a single copper conductive layer.
Double-sided flex PCB: Unlike the single-sided option above, this one has two copper layers on either side with vias or plated through holes linking them.
Multi-layer flex PCB: This circuit board has more than two layers. In some cases, the layers could go to as high as ten.
Rigid-flex boards: This is a special group that involves combining rigid and flexible materials. Therefore, they have high performance and limited failures.
High-density interconnects (HDI) are flexible circuits with more technical construction, layout, and design solutions.
Therefore, they contain highly dense circuitry with precise microvias and features.
Flexible PCB Design Rules and Standards
(A modern electronics research development facility)
All circuit board manufacturers follow specific rules and standards to design flexible circuit boards.
These standards govern the PCB development process, including design, fabrication, inspection, and shipping.
Ignoring these standards and rules might translate into developing non-functional products.
So what are these standards, and what do they represent?
IPC-2221: They establish standards for circuit board aspects like thermal management, quality assurance, material selection, DFT, schematic, DFM, and DFA. In other words, it’s the generic standard for all flexible PCB designs.
IPC-2221 standards will help you understand the interconnection structures and know how to place components on a circuit board. However, you need to use these standards alongside the others highlighted below.
We also refer to these as flexible circuit board sectional design standards.
Additionally, they establish flexible design specifications, interconnecting structures, and how to mount components on the PCB.
These standards focus on specific areas of capacitance, material selection, and impedance control.
Furthermore, they offer information on using single or dual ZIF connectors, unsupported edge connectors, and dielectric thickness adjustments.
You must follow these standards to guide you on using adhesives that bond the PCB layers together.
Specifically, the standards provide information on the weaknesses, strengths, and processes of creating a flexible circuit board.
These standards guide you on the performance and qualification of your flexible circuit board.
The latest version is the IPC-6013E which specifies the test and inspection of circuit boards, including impedance and thermal testing.
Moreover, it offers the quality assurance provisions like quality conformance tests and acceptance guidelines.
Flex PCB Design Rules: IPC-600
These standards work for all PCB types, including flexible, rigid, and rigid-flex PCBs.
Therefore, they specify the acceptability criteria for these circuit boards by visual interpretations.
The latest version of this standard is the IPC-600 K.
Flex PCB Design Rules: IPC/EIA J-STD001
These standards provide the required standards for soldered electrical assemblies.
Must-Knows While Designing Flex PCBs
(Male and female engineers working in a factory)
Now that we understand the standards that govern flex PCB design, it’s time to go through the must-knows for a reliable PCB assembly.
Understand The Flex PCB Bendability
How often will the circuit board flex, and to what extent do you intend to flex it?
Generally, how often you flex your circuit board determines whether it is dynamic or static.
Therefore, a static circuit board is bend-to-install and can flex up to 100 times throughout the use period.
However, a dynamic circuit board must be robust and should allow bending for limitless times.
The board’s thickness will affect its bendability and is affected by the following:
- The number of copper layers
- How thick is the adhesive
- The selection of the dielectric material
- The weight of the copper base
Know The PCB Materials
The IPC standards define the materials you should use. Interestingly, flexible circuit boards use materials with better properties than rigid boards.
Some of the flexible material properties to consider are:
- Thermal expansion coefficient
- Fire retarding
- Glass transition temperature
- Moisture absorption
- Dielectric constant
Moreover, the flexible circuit board materials include:
- A conductive copper layer
- Flexible copper-lad laminate
- Coverlay and dielectric insulator
- Adhesives to keep the layer bonds strong
- Surface finishes
Monitor the Flex Board Routing and Layout
(Flexible technology concept)
The routing and layout of your circuit board can either break or make a PCB.
For example, consider whether your components need a stiffener or not.
Some of the routing and layout considerations include:
- Use a large bend radius rather than sharp angles
- Route conductors perpendicular to the bend
- Use tear guards to reinforce materials and eliminate tears
- Use curved traces to lower stress
- Abide by a minimum drill-to-copper distance of 8 mil
- Plated through holes, vias, and other special copper features must be 50 to 60 mil away from the PCB transition regions
- Never employ discontinuities like holes, vias, and cut-outs in bend regions
Know Factors That Determine Cost
You should focus on developing a cost-effective flex board without compromising quality.
The factors that determine cost are:
- The drill hole sizes
- Via features and stiffener types
- Board size and material type
- Copper foil thickness and circuit construction type
- Plating and surface finish requirements
- Dimensional tolerances
- The circuit board grade
Choosing Competent Manufacturers
(Automated robotic electronic circuit board assembly)
The industry is full of both competent and quack manufacturers.
Therefore, you should choose a reliable manufacturer, even if it takes requesting referrals from friends.
Also, you can check for online reviews.
Generally, a competent manufacturer follows design rules and standards.
Moreover, they have relevant registrations and certifications.
Furthermore, they should be there for you throughout the design and fabrication process and still offer a higher quality-to-cost ratio.
Lastly, you can only get a good circuit board if competent and professional technicians do the work.
And the manufacturer should guarantee product testing after production.
Flex PCB Design Rules: What are flex PCB limitations?
Flexible circuit boards have the following limitations:
- First, they have a high initial cost
- It’s hard to modify or repair the circuit board
- The assembly process requires more care, without which the board could damage
- Since the technology is new, few manufacturers are equipped well to produce flexible circuit boards
Flex PCB Design Rules: Why are PCB design rules essential?
First, design rules provide a uniform standard for developing circuit boards.
This means that different people from different parts of the world could design and fabricate flexible circuit boards that you can use to repair your watch effectively.
Moreover, design rules limit manufacturing errors, thus lowering production costs.
Flex PCB Design Rules: Is it easy to design a flex PCB?
It depends! Everything will appear effortless if you have the relevant flex PCB design skills and experience.
However, those without design or electronic experience will struggle.
Fortunately, you can take time to learn and acquire design skills.
Every competent flexible circuit board manufacturer and designer understands there are specific design rules to follow when assembling PCBs.
Ignoring these rules will mean developing a flexible board with some errors.
If you have understood the above standards and must-knows, you’re already halfway into a successful PCB design.
And if there are areas that need clarification, contact us without hesitation.