Over-moulding of electronic functionalities on flexible polyimide-copper substrates • 5 Nov 2019

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By Mona Bakr, Frederick Bossuyt, Jan Vanfleteren (imec-cmst)
     Yibo Su – (Brightlands Materials Center)

Imec-cmst and Brightlands Materials Center demonstrate over-moulding of electronic functionalities on flexible polyimide-copper substrates. With the development of flexible printed electronics technologies, different electronics integration approaches are of great demand. The combination of electronics with polymeric structures is a new technology platform as it integrates multiple functionalities into plastic products. This includes the integration of electronic components (e.g. resistors, leds, etc.). Such components were mounted on a flexible printed electronic foil using lead free solder, after which the foil can be adhered with a thermoplastic polymer, using an over-moulding process, which is one of the main technologies developed in project Flexlines.

Adhesion between the electronic foil and the injected polymer is a very important parameter, thus imec-cmst and Brightlands Materials Center have investigated this adhesion. In the study, the base foil (polyimide foil with copper cladding) was over-moulded with different engineering plastics into the form of peel test specimens in order to check the best adhesion performance. Once the best polymer material compatible for PI/Cu foil was known, different test vehicles including passive components were made to check the influence of component underfill and glob-top. After over-moulding all 24 resistors remained functional, resistance was measured using two and four probe methods, underfill material showed better results in resistance values and no clear influence for glob top material was noticed according to our design.

Left: Flexible circuit with 24 resistors; Right: Circuit with over-molded features

The integration of polyimide based flexible circuits was realized by injection over-moulding at Brightlands Materials Center. Commercialised thermoplastic injection moulding compounds were employed with a common injection moulding process with a cycle time less than one minute. The integration is achieved by means of adhesion between flexible circuits and injection moulded parts. Therefore encapsulation design is not necessary and a significant degree of freedom in product design can be realised.

Based on the knowledge from aforementioned study, the feasibility of over-moulding polyimide copper foils with assembled components was demonstrated. Flexible polyimide copper foils were designed and fabricated by imec-cmst, on which electronic components were soldered. The components include a thin FR4 board having an NFC chip mounted on and 5 LEDs which are directly assembled on the foil. The NFC antenna is directly realised in the copper.

The antenna and NFC chip can harvest the energy from e.g. a smartphone in order to drive the LEDs. This is a simple example of wireless energy transfer, that could be used to power circuits and read out sensor values using NFC, without the need of having an integrated battery.

Part of this work was presented at the 22nd Microelectronics and Packaging Conference (EMPC) in Pisa, Italy on September 16–20, 2019.