Mechanical Testing for Stretchable Electronics

Abstract

Stretchable electronics have been a subject of increased research over the past decade [1–3]. Although stretchable electronic devices are a relatively new area for the semiconductor/electronics industries, recent market research indicates the market could be worth more than 900 million dollars by 2023 [4]. At CES (Consumer Electronics Show) in January 2016, two commercial patches were announced which attach to the skin to measure information about the user’s vitals and environmental conditions [5]. One of these measures the sun exposure of the user with a UV sensitive dye — which can communicate with the user’s cell phone to track the user’s sun exposure. Another device is a re-usable flexible patch which measures cardiac activity, muscle activity, galvanic skin response, and user’s motion. These are just two examples of the many devices that will be developed in the coming years for consumer and medical use. This paper investigates mechanical testing methods designed to test the stretching capabilities of potential products across the electronics industry to help quantify and understand the mechanical integrity, response, and the reliability of these devices. Typically, the devices consist of stiff modules connected by stretchable traces [6]. They require electrical and mechanical connectivity between the modules to function. In some cases, these devices will be subject to bi-axial and/or cyclic mechanical strain, especially for wearable applications. The ability to replicate these mechanical strains and understand their effect on the function of the devices is critical to meet performance, process and reliability requirements. There has been a test method proposed recently for harsh / high-rate testing (shock) of stretchable electronics [7]. The focus of the approach presented in the paper aims to simulate expected user conditions in the consumer and medical fields, whereas earlier research was focused on shock testing. In this paper, methods for simulating bi-axial and out-of-plane strains similar to what may occur in a wearable device on the human body are proposed. Electrical and / or optical monitoring (among other methods) can be used to determine cycles to failure depending on expected failure modes. Failure modes can include trace damage in stretchable regions, trace damage in functional component regions, or bulk stretchable material damage, among others. Three different methods of applying mechanical strain are described, including a stretchable air bladder method, membrane test method, and lateral expansion method. This work will describe a prototype of the air bladder method with initial results of the testing for example devices. The system utilizes an expandable bladder to roughly simulate the expansion of muscles in the human body. Besides strain and # of cycles, other variables such as humidity, temperature, ultraviolet exposure, and others can be utilized to determine their effect on the mechanical and electrical reliability of the devices.