Foldable substrates for wearable electronics

Abstract

The personal electronics market is experiencing rapid growth where smaller electronics carrying more functionality is required. Increasingly, small electronic devices are expected to be incorporated into personal accessories and clothing to make them wearable. This raises great challenges to the substrate technology and related assembly technologies in electronic packaging for high density, small feature size, and high performance. The requirement on foldable substrate, especially, is intensive for smaller device foot print and system on flex (SOF), where the chip size is reduced and its pin count is increased. Thus, the formation of highly reliable foldable flex has become an important factor in increasing the functionality of future devices. In this paper, a 2 metal layer flexible substrate was assembled with different active and passive components to form a multichip module (MCM). This is achieved by building 30um fine pitch circuitry upon a thin, flexible substrate. The flex have sufficient area to carry all the components and able to fold in an S-shape, which gives its characteristics to reduce the overall device footprint, thus enhancing the portability and functionality, making it suitable for wearable applications. Several base film substrates have been assessed by means of restoring force test which revealed that lower base film thickness is beneficial for folding with minimum spring back. Also, LCP material is identified to be a good candidate for foldable substrates especially for good electrical reliability requirement. To minimize package size, assembly was carried out on both sides of the flex including Cu pillar reflow, die attach, wire bond and SMTs. Cu pillar flip chip of low die thickness (approximately 500um) is made achievable reliably by applying OSP coating on the bond pads before multiple reflow soldering. The assembled flex is then folded several times into a septum to form a completed module with the body size of 6mm × 4mm × 2.6mm. Such compact modules are demonstrated to function reliably in temperature cycling, high temperature storage and temperature and humidity storage tests. Accelerated high temperature storage test indicate that Cu from the substrate are gradually consumed to form IMCs with the Sn in the solder joint at increasing storage time. It is estimated that the module could survive at least 5 reflow cycles without demonstrating detrimental degradation. As a result, the foldable flex with its unique assembly process is an attractive solution to a wide range applications that satisfy demands for small, multi-functional applications at potentially low cost such as that for wearable electronics.