Technical Abstracts
In Case You Missed It
Reliability
“Electrochemical Failures as a Function of Flux Volume under Bottom Terminated Components”

Authors: Mark McMeen

Abstract: The activity of flux residue changes when trapped under a low-profile leadless or bottom-terminated component. There are three factors to consider: 1) Standoff gap: lower-standoff gaps block outgassing channels. Low-standoff gaps change the nature of the flux residue by leaving behind flux activators, solvents, and functional additives that normally would be outgassed from the residue; 2) Narrow pitch: miniaturized components have a decreased distance between conductors of opposite polarity. There is a higher potential to bridge conductors with flux residue; 3) Cubic volume of flux: increased I/O in combination with thermal lugs creates a higher cubic volume of flux left under the bottom termination. High flux volumes can block outgassing channels and bridge conductors.

A QFN test board was designed to study the research hypothesis that electrochemical failure phenomena are related to the volume of flux residues present under bottom-terminated components. The designed test board (QFN-11) has four quadrants with varying mounting pad dimensions and component standoffs, resulting in varying amounts of flux volume. The test boards were evaluated using surface insulation resistance (SIR) methodologies, followed by component cross-sectioning and visual analysis of the areas under the BTC components. An understanding of the relationship between flux volume and BTC pattern design is expected to aid assemblers in mounting pad design to reduce the risk of electrochemical failures. (SMTA International, September 2020; smta.org/page/knowledge-search#search/entry-details/5ff5024a954f42067b3f61a4/)

“An Investigation on Function of Current Type on Solder Joint Degradation in Electronic Packages”

Author: Wenhui Cai, Fei Huang, Kai Liu and Mohammed Alaazim

Abstract: As in real applications, several alternating currents (ACs) may be injected into electronic devices. This study aims to analyze their effects on the lifetime of solder joints and, consequently, shed light on these effects at the design phase. The authors investigated the effects of current waveform shapes on performance and reliability of solder joints. Three common and extensively used current shapes in several simulations and experiments were selected to study their effects on solder joint performance. In the case of a triangle current type, the results demonstrate a severe thermal swing and stress fluctuation in the solder joint, because the critical states lack any relaxation time. In fact, the stress intensification in the solder under application of the triangle current type has been shown to contribute to increasingly brittle intermetallic compounds. An accelerated increase of on-state voltage of power semiconductor was also observed in under application of the triangle current type. (Soldering & Surface Mount Technology, March 2021, emerald.com/insight/content/doi/10.1108/SSMT-06-2020-0025/full/html)

Transient Electronics
“Printable and Recyclable Carbon Electronics Using Crystalline Nanocellulose Dielectrics”

Authors: Nicholas X. Williams, et al.

Abstract: Electronics waste can lead to the accumulation of environmentally and biologically toxic materials and is a growing global concern. Developments in transient electronics – in which devices are designed to disintegrate after use – have focused on increasing the biocompatibility, whereas efforts to develop methods to recapture and reuse materials have focused on conducting materials, while neglecting other electronic materials.

Here, the authors report all-carbon thin-film transistors made using crystalline nanocellulose as a dielectric, carbon nanotubes as a semiconductor, graphene as a conductor and paper as a substrate. A crystalline nanocellulose ink is developed that is compatible with nanotube and graphene inks and can be written onto a paper substrate using room-temperature aerosol jet printing. The addition of mobile sodium ions to the dielectric improves the thin-film transistor on-current (87µA mm−1) and subthreshold swing (132 mV dec−1), and leads to a faster voltage sweep rate (by around 20 times) than without ions. The devices also exhibit stable performance over six months in ambient conditions and can be controllably decomposed, with the graphene and carbon nanotube inks recaptured for recycling (>95% recapture efficiency) and reprinting of new transistors. The authors demonstrate the utility of the thin-film transistors by creating a fully printed, paper-based biosensor for lactate sensing. (Nature Electronics, April 2021, nature.com/articles/s41928-021-00574-0)

This column provides abstracts from recent industry conferences and company white papers. Our goal is to provide an added opportunity for readers to keep abreast of technology and business trends.