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Plasma metallization turns a non-conductive plastic or paper into a conductive printed circuit board that can be equipped with LEDs.
Plasma metallization turns a non-conductive plastic or paper into a conductive printed circuit board that can be equipped with LEDs.
( Source: Lumitronix)

metalized materials Plastic or Paper as flexible circuit board for LEDs

| Author/ Editor: Hendrik Härter / Florian Richert

Plasma metallization turns plastics or even paper into electrically conductive and solderable boards. These can then be equipped with LEDs, among other things. More than ten years of research and development work have gone into this project.

Plasma metallization is the process by which a wide variety of materials are turned into electrically conductive circuit boards. In this special type of metallization, a patented plasma spray head is used to spray a conductive metal, usually copper, in powder form under high atmospheric pressure onto the base material coated with silver paste. At the same time, the copper is melted by a very hot plasma jet of 10,000 to 50,000 °C and combines with the silver substrate. This metallization process makes the base material conductive and allows it to be soldered in the further course of production and finally equipped with electronic components. This process was developed by the LED company Lumitronix together with its technology partners.

Roots from medicine

This technology is based on more than 10 years of research and development work. The pioneering method has its origins in the medical environment. Implants or prostheses have been sprayed with bone powder for a long time so that they grow together better with the body's tissue and are not regarded as foreign bodies and possibly rejected. The knowledge was transferred to electronics and plasma metallization created a process that will now be used in series production. In contrast to the standard material polyimide, which forms the basis for the majority of all flexible circuit boards available on the market, the developed process of plasma metallization results in many new and inexpensive base materials for circuit boards.

Solderable paper FPCs, the FPC stands for Flexible Printed Circuit, are a novelty so far and offer a multitude of application possibilities. They are cost-effective, open to diffusion and are therefore suitable for large-area applications such as wallpaper or advertising surfaces. PET can also be used as a base material for FPCs. It is cheaper than polyimide, has very good strength, high load-bearing capacity and low weight. Coated with a thin aluminum layer, which is made solderable by plasma metallization, the PET-FPCs can be equipped with components. PET is also transparent and can, therefore, be applied to glass surfaces, for example.

Illuminated glass facades of high-rise buildings, windows or glass doors are conceivable. Furthermore, aluminum FPCs with copper coating can achieve both cost savings and weight reduction compared to pure copper FPCs. Aluminum with a thickness of 100 µm also gives the FPCs a high current carrying capacity and can be used indoors and outdoors. Since the surface is not easily solderable, it has not been used for flexible printed circuit boards up to now.

Metallization with plasma and its advantages

Compared to the standard process used for printed FPCs, in which the base material printed with non-solderable conductors, such as aluminum, is coated with a conductive adhesive with a high silver content, the method of plasma metallization is much more cost-effective. High silver content used in the conventional process leads to considerable financial expenditure and is also costly. Furthermore, it is not so reliable, since conductivity ultimately depends on temperature and humidity.

The high current of more than 10 A, preferably 50 A, used in the irradiation process and the low voltage of preferably 50 V prevent the complete breaking of the molecular chains of the coating material. Exclusion of oxygen prevents direct oxidation of the coating material within the plasma beam. High energies can, therefore, be introduced into the coating material to start the chemical reaction, making the new method particularly suitable for continuous, industrial coating processes with high coating speeds.

Series processing of the flexible blanks will take place on a new production line at Lumitronix. The flexible strip park will be able to assemble the basic blanks using the reel-to-reel process. State-of-the-art machines and high processing speeds will enable production on a large industrial scale. What is special about the flex line is that all possible flex materials can be processed there. Starting with the standard flex material polyimide, wallpaper or paper up to very cheap plastic (PET).

Once the flexible printed circuit boards have been assembled, the boards are soldered in an infrared oven and then electrically and optically tested by a system.
Once the flexible printed circuit boards have been assembled, the boards are soldered in an infrared oven and then electrically and optically tested by a system.
(Source: Lumitronix)

Production lines permit serial assembly of base material in roll form. These rolls are clamped at the beginning of the line and always kept taut by clamp holders, also known as hitch feeders. Thus the further course of the coating with solder paste as well as the assembly with LEDs and other components can run smoothly. After assembly, the flexible boards are soldered in an infrared oven and then electrically and optically tested by a system. The flexible boards can then be cut into individual lengths or different patterns by oscillating knives. Using a special printer, it is also possible to print the flexible printed circuit boards in color and thus individualize them.

The advantages of flexible circuit boards

FPCs have several advantages over rigid boards. Thanks to their flexible base material, they can be used individually and, due to their low weight, offer luminaire manufacturers in particular more scope in terms of design. Also, completely different dimensions can be achieved in terms of length. Compared with rigid PCBs made of FR4 or aluminum, flex PCBs are cheaper and can be subjected to high dynamic and mechanical loads without damage. The processing of the FPCs in the form of rolls with a length of 50 to 100 m also results in lower storage and transport costs. Furthermore, large manufacturers benefit from this form of the printed circuit board as they can process it immediately and automatically on their production line.

Due to the flexible base material, the circuit boards can be used individually and, due to their low weight, offer more scope to luminaire manufacturers in particular.
Due to the flexible base material, the circuit boards can be used individually and, due to their low weight, offer more scope to luminaire manufacturers in particular.
(Source: Lumitronix)

Principally all conceivable base materials may become conductive and solderable. Illuminated wallpapers, as an example, are only one possibility. Metalized paper can be used primarily in advertising technology in the form of postcards, stationery, posters or packaging and can be equipped with light-emitting diodes and other electronic components. Large-area special applications can also be realized. With metalized paper or PET film equipped with LEDs, huge backlit posters or banners can be produced for exhibition stands that can be easily disposed of after use. A conductive surface can even be applied to functional textiles such as curtains, drapes, and clothing and then individually fitted with LEDs and other electronic components.

Plasma metalized base materials can be used for a wide variety of applications. Another possibility is rapid prototyping, in which conductive tracks of silver color are printed on paper with an inkjet printer, then plasma-metalized and then assembled. The printing process is much simpler and less complicated than the conventional etching process, which is very time-consuming and cost-intensive, especially in small runs.

This article was first published in German by Elektronik Praxis.