Designing with Ultra-Thin, Flexible Printed Circuit Boards

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Ultra-thin flex PCBs are superior to rigid boards in just about every metric and allow designers to work with 3D configurations. However, designers must take the mechanical properties of ultra thin flex circuits into account. Even though flex PCBs are designed to be flexible, they can tear easily if they flex beyond their limits during use.

This phenomenon is even more true for ultra-thin flex PCBs. Therefore, it is best to exercise caution when working with these boards. It is recommended to design a mechanical board of the circuit and conduct a range of tests on it before stepping into the design stage. The testing phase will check for the board’s ergonomics in terms of installation and misalignments.

flex circuit manufacturing      

It is important for designers to learn about the different types of flex PCBs available as well as their pros and cons.

Types of Flex PCBs

There are several types of flexible circuits available for different applications, the most popular of these include flex, rigid-flex, and HDI (high density interconnect) flex.

Flex PCBs

Flex PCBs, as the name suggests, are the flexible version of rigid circuit boards. They have unique properties such as increased flexibility, durability, and resistance to vibration. These features increase the reliability and repeatability of the boards. A major advantage of flex PCBs over rigid PCBs is that they can be used in 3D configurations.

Most designers prefer to use Flex PCBs in place of wire harnesses. They are easier to work with, install, and much more durable.

Rigid Flex PCBs

Rigid flex circuit boards combine the rigidity of rigid boards and the flexibility of flex boards. As such, they feature a flex area and a rigid area that are permanently connected to each other. This provides the board with unique properties that neither PCB or Rigid boards possess on their own.

It is common for standard rigid flex installations to have a series of rigid circuits connected by flex PCBs. By blending the flex areas with rigid areas, designers can improve their circuit's design capability.

The rigid areas of the circuit are perfect for use as mounting points of the connectors, components, and chassis, while the flex areas provide vibration protection, dynamic flexing, and flex-to-fit. This allows designs to exercise more creativity when brainstorming solutions to complicated applications.

High Density Interconnect (HDI) Flexible PCBs

HDI circuits are useful in applications that cannot utilize flexible circuits. HDI flex circuits provide superior layout, construction, and design options thanks to the use of micro vias. The smaller form factor, superior functionality, and dense flex circuits are among their most useful properties.

The best part about HDI flexible PCBs is that they use thinner materials, allowing them to be used in smaller applications. Being thin does not take away from their reliability and superior electrical properties.

HDI Stackup

It is important to choose the most appropriate HDI stackup when working with ultra thin flexible printed circuit boards. This is especially important when you are working with larger PCBs with multiple pin count BGAs. In such cases, a typical laminate stackup may not be sufficient for high performance applications.

HDI stackups are useful because they provide several layers while minimizing costs.

HDI boards provide several advantages including:

  • Designers can use micro via patterns with greater efficiency, which allows them to work with more channels for routing and results in fewer layers.
  • Their vias and traces are smaller in size, leading to fewer layer sand routing density
  • HDI stackups are the most practical way of working with multiple large BGAs that have a smaller pitch than 0.8mm.
  • They are cost effective.
  • A good HDI stackup leads to improved power integrity and signals.
  • HDI stackup must meet RoHS standards.
  • Newer materials that provide superior performance at an affordable cost are being manufactured.

Materials for Ultra Thin Flex PCBs

It is recommended to use thin base materials to miniaturize ultra thin PCBs. Making PCBs more compact in the vertical and horizontal not only reduces their footprint but also makes them smaller in the Z direction which can further reduce the thickness of the board while providing designers with more space for components.

The most popular materials for use with ultra thin flex PCBs include:

Thin copper layers: They create a smaller interconnect footprint and lines with smaller resolutions. 

Adhesive: High quality adhesives should be used for bonding components and connecting various parts of the circuit together.

Polyimide: Polyimide layers provide ultra thin flex PCBs with extra protection without making the boards any thicker than they already are.

Basic Testing for Ultra Thin Flex Circuits

Testing is an important step that will ensure the reliability of your ultra thin flexible PCBs. Designers can take several approaches to verify the quality of their finished products and designs, especially standards from the Association Connecting Electronics Industries (IPC). These standards include IPC-6013, IP-FC-234, and IPC-2223.

Here are a few testing guidelines that designers should know.


This standard provides useful guidelines for testing the performance of flexible circuits, including flex and rigid flex PCBs. It was first published in late 2013. The standard replaces a number of previous IPC standards including IPC 6013 from 1998.

The standard specifies a number of test methods for ultra thin flex PCBs, including thermal, impedance, and bend testing. It also includes a number of quality assurance provisions, such as sample test coupons and guidelines for quality conformance tests.


This standard specifies design requirements for flexible and rigid flexible boards, especially when it comes to adhesive materials and rigid flex interfaces. The standard also provides recommendations on flex vias and plated through holes.


This standard provides information related to the use of pressure sensitive adhesives (PSAs) for the assembly of ultra thin flex PCBs. It also provides information related to the types of adhesives available, as well as their proper usage, including any strengths, weaknesses, and limitations that designers should know.


IPC-2221 establishes general standards for flex PCB design, component mounting, and interconnections. It is very similar to IPC 2220 and provides design standards for quality conformance testing.

Need help with ultra thin flexible printed circuit boards? Get in touch with expert consultants at Hemeixin PCB here.

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