Thermoplastic circuit boards for user interfaces

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This article deals with thermoplastic printed circuit boards, an alternative technology for all-in-one circuit carriers that are used especially for seamless user interfaces. For the production, different layers of thermoplastic films – each with a special function – are laminated to a flat film composite and then further processed to the user interface. At plastic electronic, thermoplastic circuit boards are developed and produced customer-specifically under the brand name multiskin circuit carriers.

Thermoplastic plastics and their processing

Thermoplastics are plastics that, when heated above a certain temperature, soften and deform, so that they remain in this form after they have cooled down again.

  • Take injection molding, for example: A solid plastic granulate is melted, pressed into a mold, cooled there and then removed.
  • Example thermoforming: A flat film is fixed at the edges, heated, pressed into a mold with compressed air or drawn into a mold with vacuum, cooled there and removed.

Thermoplastics are used for many different surfaces. Be it for the housings of appliances, in the interior and exterior of vehicles or for furniture surfaces. Thermoplastic films are also used everywhere. They provide a great look and the necessary protection against environmental influences. The combination of injection molding with thermoplastic films, known as Film Insert Molding (FIM), combines the outstanding surface properties of the film with the mechanical stability of injection molding in one component.

All these interesting properties are used for the production of an all-in-one circuit carrier that integrates seamlessly into user interfaces. Thereby, thermoplastic foils as basic substrates are bonded to a flat foil laminate, which can then be 3D formed and further processed in injection molding.

In the following, materials and production of the individual components of the thermoplastic circuit board are described layer by layer, as well as the further processing of the flat film composite to the desired user interface.

Conductor tracks on foil

In a first step, conductor tracks are created on a thermoplastic film. Laminated copper foils, vapor-deposited copper layers or printed silver conductive pastes are used for the conductor tracks. Films made of polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), in some cases also polycarbonate (PC), are suitable as substrates.

The criteria for the selection of the material for the conductor tracks include the conductivity or the specific surface resistance, the current carrying capacity and the 3D deformability of the conductor tracks as well as possible structure sizes and costs.

Bulk copper conductor tracks

The copper foils used generally have a thickness of 18 µm. These are laminated over their entire surface onto a mostly 50 µm thick PET or PEN film and structured in the desired layout by photolithography and etching. The smallest structure sizes are 325 µm for the track width and 175 µm for the track spacing. Chemical tin is mostly used as a surface treatment. The conductor tracks produced in this way have a conductivity and current carrying capacity comparable to conventional printed circuit boards with a copper thickness of 18 µm. The copper conductor tracks are practically not stretchable, so that a meander design of the tracks is necessary when manufacturing 3D-shaped circuit carriers.

Evaporated copper conductor tracks

A further possibility for producing conductor tracks on film is the vapor deposition of a thin copper layer on a PET film that is typically a maximum of 50 μm thick. First, a mask is printed directly on the PET film in the form of the negative layout of the conductor track structure. Thin copper layers up to a maximum total thickness of 1 µm are then evaporated in several passes. Then the mask printed in the first step is removed. What remains are the conductor tracks, which are overprinted with conductive carbon lacquer in contact areas and with a non-conductive protective lacquer in all other areas. The smallest structure sizes are 250 µm for the track width and 250 µm for the track spacing. The electrical conductivity of the vapor-deposited copper conductor tracks is almost 55% of the conductor tracks made of copper foil, the current carrying capacity is low due to the very thin copper layer. Evaporated copper conductor tracks can be stretched very well within wide limits and are very inexpensive to manufacture in large quantities.

Conductor tracks made from screen printed silver pastes

Currently mainly conductive silver pastes are used for the screen printing of conductor tracks. Printing is also done on polyester films, but also e.g. on PC foils. Although the specific conductivity of the printed silver conductor tracks, similar to that of the vapor-deposited copper layers, is only 55% of the conductor tracks made of solid copper, the conductivity can be adapted to the respective requirements via the thickness of the conductor tracks. Due to the wider choice of thermoplastic films and the good adjustability of resistance or conductivity over the layer thickness of the printed material, this method for producing conductor tracks can be used particularly flexibly and can meet a wide range of requirements. Some conductive silver pastes are also suitable for 3D deformation of the printed conductor tracks. However, the costs are relatively high compared to the other two methods described.

Thermoplastic circuit boards: special features

Thermoplastic printed circuit boards are usually manufactured with one or a maximum of two wiring levels. The wiring levels can be connected via SMT components such as connectors or resistors. It is also possible to create bridges on one wiring level. In most cases, the via technology known from the standard circuit board is not used for thermoplastic circuit boards for quality reasons.

For technical lighting reasons it is necessary to cover the thermoplastic circuit board with a white solder resist layer. To enable further processing as a thermoplastic film composite, a white reflector film is used for this purpose.

Thermoplastic circuit boards: SMT assembly

The placement of the electronic components on the multiskin circuit carrier is done by conventional pick and place. The correct handling of the flexible thermoplastic circuit boards is particularly important. For large quantities and relatively simple assembly, e.g. with LEDs or with RFID chips, the assembly can be done in a roll-to-roll process. For smaller quantities or more complicated assembly with different components, e.g. with microcontrollers, the handling of the thermoplastic circuit board is carried out with a rigid carrier.

Either reflow soldering with a low-temperature solder or – if necessary – conductive gluing is used. Since the expansion during heating and shrinkage during cooling must be taken into account with thermoplastic printed circuit boards, the lowest possible temperature load (≤ 160 ° C) must be ensured in both processes.

Thermoplastic circuit boards: light management

By using side LEDs with a height of <1mm and a combination of different thermoplastic foils and plates for light management, high-quality lighting elements with high light density and homogeneous illumination without over-shining are generated.

The following materials are used:

  • transparent light guides made of PMMA or PC plates with a thickness of ≥ 1mm
  • white-opaque light separators made of PMMA or PC panels with a thickness of ≥ 1mm
  • Diffuser films made of PET or PC for light scattering
  • highly reflective white PET films as reflectors
  • 100% light-tight blackout films made of PET to cover optical hotspots

All-in-one for beautiful, smart user interfaces

The structure thus produced is laminated behind a suitable surface. This surface can be 3D-shaped or can only be brought into the desired 3D shape after lamination by suitable processes such as High-Pressure Forming (HPF). Further processing of the multiskin circuit carriers using the Film Insert Molding (FIM) process is also required in some applications.

Thermoplastic circuit boards: layer stack

Conclusion

The described technology makes it possible to implement assembled circuit boards with light management and aesthetically appealing surfaces in a thin material composite (embedding technology). The advantages are obvious: Thermoplastic circuit boards for user interfaces are light, thin, can be shaped using 3D thermoforming and can be processed by injection molding. Thermoplastic circuit boards can therefore meet the most important electrical, optical and mechanical requirements for user interfaces very well.

The combination of visually appealing and chemically and mechanically robust surface design on the one hand and electrical functionality and safety on the other hand holds very great market opportunities for the thermoplastic circuit board. From a technical point of view, this fact is also a special challenge for interdisciplinary development and cooperation.

Thermoplastic circuit boards: manufacturing process

The production of thermoplastic printed circuit boards is based on well-known and well-proven production steps. However, it should not be overlooked that, due to the materials used, minor adjustments to the usual production are always necessary. Organizing the supply chain – spanning electronics and plastics – also requires a certain willingness to rethink. The result is a relatively short process chain that can also be mapped regionally.

Thermoplastic circuit boards for user interfaces