What Is a Thin-Film Circuit Board?

Thin-film technology is on the rise as a method to produce circuit boards that have higher performance and dynamic capability than their rigid or thick-film circuit counterparts.

The latest products to use this technology in their production, such as foldable smartphones, smartwatches, and OLED televisions, make use of flexible circuits because they can form any shape. Also, the ultra-thin coatings on thin-film circuit boards allow for the production of higher-density circuits and smaller, lighter packaging.

Thin-Film Circuit Boards

 

Thin-film circuits, also called flexible printed circuit boards (PCBs), house electronic components within layers of conductive and insulating materials. The conductive layer features a pattern of geometric shapes providing connections between the electronic elements without bulky wires.

Flexible PCBs use the same parts as rigid PCBs, but their design allows for dynamic movement, smaller electrical packaging, and adaptable design. The thin-film technology accomplishes this by using a flexible rather than a rigid substrate material of conventional printed circuit boards.

Typically copper, the conductive material may be aluminium, silver ink, indium tin oxide, or carbon. Substrate materials, which vary in their design and application, include quartz, glass, ceramic, and organic materials. [1] Common non-conductive substrates include:

  • polyimides
  • PET (polyethylene terephthalate)
  • PEEK (polyether ether ketone)
  • FR-4

Flexible PCBs may contain a single layer of copper, but functionality increases with additional layers. Multi-layer PCBs form connections using through-holes, allowing for several operational coatings. Another style of PCB known as “rigid-flex” PCB incorporates flexible and inflexible circuits to create a hybrid design solution.

The thickness of the copper or metal layer can range from 0.5 oz to 13 oz; the insulating layer is between 0.17 mm and 7.0 mm thick. [2] Manufacturers use adhesives or vapour deposition to affix the metal to the substrate.

Thin-film technology makes it possible to achieve the precisely desired thickness on the substrate. The combination of thickness and materials depends on the PCB’s function.

Since you can bend, fold, or shape flexible PCBs however you wish, design opportunities are nearly limitless.

Thin Films in PCB Manufacturing

 

Manufacturers use semiconductor and microsystem technology to produce a circuit board of ceramic or organic materials. A common technique for depositing the conductive metal onto the substrate material is physical vapour deposition (PVD) or sputtering.

During the process, the energetic positive ions bombard the target material under low pressure, ejecting particles from the metal. The sputtered particles travel across the chamber to the ceramic substrate’s surface, producing a thin film coating. Types of sputtering processes include DC, AC, and magnetron sputtering.

Inside the vacuum chamber, a noble gas like argon becomes ionised in the electrical field produced between the anode and cathode. The decomposing gas produces positive ions and electrons. When the positive ions receive the free electrons and release their energy near the target, the force sputters particles from the target.

Manufacturers can form high-resolution, thin-film circuit patterns additively by printing the patterns directly onto the substrate using masking or subtractively, by depositing a thin film onto the substrates and removing selective areas through technologies like laser patterning. [4]

Laser Patterning for Thin-Film Circuits

 

The conductive coatings on PCBs allow electricity to flow between the components when using substrates such as quartz glass, polymers, or silicon. The laser patterning manufacturing process accurately removes sections of the conductive layer to create electrical circuits.

Compared to photolithography, laser patterning allows more flexibility in customising for specific applications and reduces the need for consumable materials during the circuit board production process. Preventing disruption to the substrate is a challenge in laser patterning, but using methods like front and back processing can reduce the risk.

Applications of Thin-Film Technology

 

Flexible, thin-film PCBs have a similar function to rigid PCBs, but you can bend and fold them to any desired shape. This characteristic allows designers to use PCBs in devices that need to move or fold during use. In the early days of flexible PCB technologies, applications were primarily in the aerospace and military industries. [3]

These days, PCBs use thin-film technology in a wide variety of fields, including computers, automotive, and industrial. Most communication devices we use today have adopted thin-film technology into flexible PCBs, such as smartphones, tablets, and smartwatches. The flexibility of thin-film PCBs allows for dynamic movement in flip phones and wearable devices, including those common in biomedicine.

Flexible circuits can significantly reduce space, weight, and errors in wiring. While thin-film PCBs may have higher costs for design and fabrication, manufacturers can save money on assembly and testing time.

Additionally, flexible circuit layers allow for better heat dissipation, giving thin-film PCBs a better temperature range for use in different environments. The resistance to movement and vibrations also makes flexible PCBs suitable for transportation applications in cars, rockets, and satellites.

A disadvantage of flexible PCBs is that they are difficult to repair or modify, partly because you must remove the protective film and replace it afterwards, which isn’t an easy task. The circuits can break or receive damage if someone handles or stores them incorrectly.

Also, compared to rigid circuit boards and thick-film substrates, thin-film technology and its PCBs entail higher costs due to the highly specialised design and manufacturing processes.

Final Thoughts

 

Using thin film for circuit boards is a growing technology in the PCB industry, surpassing thick-film and rigid PCBs for many modern applications, including wearable devices, smart technologies, satellites, and industrial machines.

At Korvus Technology, we design, manufacture, and install deposition systems with high levels of customisation and control. Browse our blog for more information on thin-film technology.

References

 

[1] National Research Council. 2005. Linkages: Manufacturing Trends in Electronic Interconnection Technology. Washington, DC: The National Academies Press, 77. https://doi.org/10.17226/11515

[2] Perdigones, Francisco, and José M. Quero. 2022. “Printed Circuit Boards: The Layers’ Functions for Electronic and Biomedical Engineering.” Micromachines 13, no. 3: 460. https://doi.org/10.3390/mi13030460

[3] Bosshart, Walter C.. 1983. Printed Circuit Boards: Design and Technology. Noida, India: Tata McGraw-Hill Education, 353.[4] Halbo, Lief and Ohlckers, Per. 1995. Electronics Components, Packaging and Production. Oslo: Strandberg & Nilsen Grafisk.

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