Characterisation of adhesion between flexible circuit foils and thermoplastic injection moulding compounds • 29 Oct 2019

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By Yibo Su, Brightlands Materials Center

Integrating flexible electronics in injection moulding processes offers a cost effective way of manufacturing flexible electronic products used in various applications, e.g. automotive industry, medical devices and home appliances. The structural integrity of such injection moulded flexible electronic component is realized by either encapsulating the flexible electronics into the injection moulded polymer or utilizing adhesion between the (polymeric) flexible circuits and thermoplastic injection moulding compounds.

Example of encapsulation design

Example of Adhesion Design

The encapsulated flexible electronics can exhibit a well-performed initial structural integrity with a wide range of base foil-engineering thermoplastic compounds combinations. However, inappropriate choices of materials and processing conditions, without optimizing adhesion, can result in undesired separation between base foil and thermoplastic compounds in long-term use, which is inevitable due to the significant difference in coefficient of thermal expansion (CTE) between foil material and thermoplastic compound. The separation can either result in wrinkles and bumps of flexible foil exhibiting deteriorated aesthetic appearance, or influence the joining between electronics and base foil/connection resulting in damaged functionality of the electronics. In addition, if integration is achieved by adhesion between flexible circuits and thermoplastic over-moulding polymer, encapsulation design is not necessary. A significant degree of freedom in product design can therefore be realized.

Since adhesion is of prime importance for the structural integrity of the product, it is necessary to characterize its underlying mechanisms. At Brightlands Materials Center, the adhesion test methodology (ESIS TC4 Peel Protocol [1]) is employed to characterize adhesion between foils and thermoplastic compounds with different foil thickness and foil type. An ISO 8510 standard 90° peeling test is firstly performed to measure the peel force, and subsequently the ISO 527 standard tensile test is conducted on flexible foil only in order to calculate the plastic deformation energy during the standard peeling test. The true adhesion energy is then obtained by subtracting the plastic deformation energy term from peeling force term as shown in the following equation:

Where GA is the true adhesion energy, F is the peeling force, b is the width of peeling specimen and GP is the calculated plastic energy term based on the methodology proposed in ESIS TC4 Peel protocol.

A scientific paper on the evaluated adhesion result and underlying adhesion mechanism between different foil and thermoplastic compounds will be submitted by Brightlands Materials Center to the “International Journal of Adhesion and Adhesives”.

                               ISO 8510 Peeling specimen

                         Example of ISO 527 Tensile test result.