Bond Strength and Failure Mechanisms of Nonconductive Adhesives for Stretchable Electronics

Teemu Salo,Mikko Kanerva,Jukka Vanhala,Aki Halme

doi:10.1109/tcpmt.2020.2980121

Teemu Salo, Mikko Kanerva + Show 2 more

Open Access

https://doi.org/10.1109/tcpmt.2020.2980121

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Abstract

Over the past few years, there has been an increasing demand for techniques that allow the forming of stretchable electronics systems from the combination of rigid printed circuit board (PCB) modules and stretchable substrates. The durability issues between the module and interconnects have been solved by optimizing the module’s geometry. However, the limiting factor is a reliable attachment method of the module on the substrate. The use of nonconductive adhesives (NCAs) for bonding is one of the most potential techniques due to their low costs and ability to form bonds fast and without a high-temperature cure. In this article, we focused on the testing of different stretchable electronics joints from readily available NCAs and different rigid module materials. The joint samples were tested by using a peel test setup. The fracture surface analysis was carried out by applying the Fourier transform infrared spectroscopy (FTIR). Three different classes of failure mechanisms were identified. The best results were achieved with a novel nonstructural adhesive joint. The nonstructural adhesive joints had a good (0, 28 N/mm) average maximum bond strength with the rigid and smooth FR4 substrate, which made the stretchable substrate elongate considerably (85%) during the peeling. The joint samples from structural adhesives, traditionally used in the electronics industry, were suboptimal.

Highlights

T RADITIONAL electronic circuits are powerful yet inherently rigid
We have investigated the applicability of available adhesives with the materials of stretchable electronics to find the optimal combination for the assembly of the stretchable circuit board
The shapes of the peel test curves are considered with the determined failure mechanisms to conclude with the performance of the nonconductive adhesives (NCAs)

Summary

Introduction

The rigidity restricts the formability and deformation during operation, which further limits the usability of rigid circuits in complex applications. The problem is solved with stretchable electronics that can accommodate very high strains and comply with deformations by elongating them [1]. The behavior of the elongation is reliable and provides for compatibility, enabling new implementations of these electronics, such as wearable applications and multisite instrumentation typical of the Internet of Things [2]. Of the many ways to produce stretchable electronics, one way is to attach small intelligent islands on a highly elastic substrate. The islands are electrically connected by stretchable interconnections on the compliant substrate. The interconnections are shaped and optimized per composition so that they intrinsically elongate until they reach very high maximum strains [4]. The islands form an intelligent network with the interconnections, in which functional operations are distributed to several islands

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