Electronic packaging for functional electronic textiles

An electronic textile (e-textile) is a textile with integrated electronic functionality. It can be used in many areas, for example, clothing, medical, furniture and aerospace applications. The combination of electronics with textiles requires the use of flexible circuit technology, with electronic components mounted on polymer substrates with conductive tracks, to ensure the textile retains as much as possible their normal physical characteristics and feel. E-textiles in wearable applications are subject to human motion activities and as such the integrated electronic components can be vulnerable to different kinds of stresses such as bending. These forces can potentially shear, pull off or damage the components despite the electronic packaging methods used to protect them. Similarly, temperature changes in the environment can induce thermal expansion stresses in the electronic packaging or components which may result in failure. Therefore the need to develop a new reliable electronic packaging method for e-textiles to mitigate these stresses becomes increasingly important. This thesis presents research into a new reliable packaging technique capable of protecting components against twisting, bending or shear stresses. The use of this packaging technique has been evaluated with the ultra-thin die mounted onto thin flexible polymer film strip which contains conductive tracks for electrical interconnections and power supply for electronics. This electronic strip can be subsequently formed into yarns or woven into a textile. The review of electronic packaging techniques for forming electronic connects between the die and substrate such as flip chip bonding, anisotropic adhesives bonding and wire bonding are also included in this thesis. Finite element analysis (FEA) of the electronic strip is also presented. FEA simulations are used to evaluate the mechanical performance of different electronic packaging assemblies. An FEA investigation is presented in the materials and component dimensions in order to maximize the reliability of the packaging method. The three-point bending, shear, tensile and thermal expansion modelling have been simulated and, in the case of shear load and bending, results validated against an experimental evaluation. The shear and bending experimental results show good agreement with the simulation results and verify the simulated optimal thickness of the adhesive layer. Three under-fill adhesives (EP30AO, EP37-3FLF and Epo-Tek 301 2fl), five highly flexible adhesives (MK055, Nu355, Loctite 4860, Loctite 480 and Loctite 4902) and three substrates (Kapton, Mylar and PEEK) have been evaluated and the optimal thickness of each is found. The Kapton substrate, together with the EP37-3FLF adhesive, was identified as the best materials combination, with the optimum under-fill and substrate thickness identified as 0.05 mm. A novel method for packaging electronics using a thermally deformed Kapton was introduced. The design process for the jig that was used to deform the Kapton and the minimum temperature (360 °C) and time (60 Sec) needed to deform the Kapton has been investigated. This is also the first demonstrated method for reliably incorporating electronic circuits in a textile and that can withstand up to 45, 150,000 and 1470 cycles of machine washing, 180 degree twist test and 90 degree bending test respectively. The new ultra-thin silicon chip (0.025 mm thickness) fabrication method has also been introduced in this thesis to increase the flexibility of the electronic packaging method for functional electronic textiles.