Electronic textile (e-textile) technology, in which microelectronics is embedded into fabrics, has spawned an exciting new class of products that can be used in the fashion, medical, military, and other industries. Examples of applications under research are real-time gas detection in wearable systems, electronic mats that melt ice on roofs, and physiological monitoring garments that measure heart rate, respiration, and temperature. To ensure these smart textiles maintain the look and feel of traditional fabrics as much as possible, they typically employ flexible electronic circuits. In real-world use, wearable e-textiles will be subject to motion, bending, and the jarring effects of washing. Such activity will impose shear and bending forces on the embedded electronic circuits, potentially threatening reliability and performance.
The challenge for e-textile packaging engineers is to maximize the reliability of the embedded circuits while keeping the package as thin and flexible as possible. The flexibility of an electronic circuit depends on the materials used for the substrate, components, and interconnections that make up the circuit, as well as the dimensions and configuration of these materials. Many e-textile circuits use adhesives to attach electronic die to substrates or to serve as stress-reducing underfills in flip chip assemblies. It is critically important that the adhesives used in e-textiles offer sufficient flexibility and bond strength in order to ensure reliability under conditions of bending and other movement.
Master Bond EP37-3FLF
Master Bond EP37-3FLF is an exceptionally flexible epoxy compound that forms high strength bonds that stand up well to physical impact and severe thermal cycling and shock, making it ideal for e-textile applications. Because it is flexible and produces a lower exotherm — heat released during the polymerization process — than conventional epoxy systems, EP37-3FLF lessens the stress on sensitive electronic components during cure. Reducing stress during cure is essential for protecting fragile die and other components in ultrathin, flexible electronic packages.
EP37-3FLF bonds well to a variety of substrates, including metals, composites, glass, ceramics, rubber, and many plastics. It offers superior electrical insulation properties, outstanding light transmission, especially in the 350- to 2000-nm range, and is serviceable at temperatures from 4K to 250°F. EP37-3FLF can be cured in 2-3 days at room temperature or in 2-3 hours at 200°F. Optimal properties are achieved by curing overnight at room temperature followed by an additional 1-2 hours at 200°F.
Master Bond EP37-3FLF was selected as one of six adhesives tested in a study of flexible electronic packaging for e-textiles conducted at the University of Southampton.1
The goal of the University of Southampton study was to investigate the influence of material selection and component dimensions on the reliability of an e-textile packaging approach under development. The packaging structure uses flip-chip technology to attach multiple ultrathin die to a flexible plastic substrate. Patterned interconnects and bond pads on the plastic strip serve to link the individual die, with solder used to make the electrical connections between the die and the interconnects and underfill adhesive applied to reduce the stress on each die. The resulting long, thin circuit can be surrounded by packing and covering fibers to form the e-textile.
Key Parameters and Requirements
The key measures of reliability investigated in this study were the shear load and bending stresses of the adhesive and substrate layers of the flexible package. For the purpose of experimentally validating simulation techniques the research group was using to optimize the flex packaging, the researchers developed a simplified version of the packaging, consisting of a single electronic die mounted to a plastic substrate using an adhesive. This electronic die on package (EDOP) served as a test vehicle for assessing the reliability of the packaging method as a function of various materials, dimensions, and configurations of the die, substrate, and adhesive. Experimental results could then be compared to simulated results to determine whether the simulation technique could be used to tweak the design of the more complex package.
Results: Shear load and bending tests
In the first part of the study, the researchers focused on the performance of the adhesive layer of the EDOP package, testing six different adhesives, including Master Bond EP37-3FLF. They conducted shear load and bending experiments on a number of EDOP samples, each of which consisted of a Kapton substrate, a silicon die, and one of the six adhesives under test. Multiple EDOP samples were created for each adhesive tested, varying the adhesive thickness of each sample by controlling the amount of adhesive dispensed.
An experimental apparatus was set up to facilitate the shear load and three-point bending tests. For the shear load tests, a force was applied and transferred through the test apparatus to one end of the silicon die (refer to Figure 1). The force was increased until failure occurred. For the bending tests, the substrate was clamped at either end and a force was applied and transferred to the underside of the substrate (refer to Figure 2). Again, the force was increased until failure occurred. For each test, the force that caused failure was recorded for each adhesive type and thickness.
For both the shear load and bending tests, Master Bond EP37-3FLF exhibited the best performance. The external forces required to cause failure in the EDOP samples that used EP373-3FLF were significantly greater than the forces required to break all the other samples across the entire range of thicknesses. For the EP37-3FLF samples, the optimal thickness — the thickness at which the external force required to break the sample was the highest — was 0.048-0.05 mm.
1Li, Menglong, Tudor, John, Torah, Russel, Beeby, Steven. Stress Analysis and Optimization of a Flip Chip on Flex Electronic Packaging Method for Functional Electronic Textiles, IEEE Transactions on Components, Packaging and Manufacturing Technology, Vol. 8, No. 2, February 2018, pp. 186-194.