Joint Analysis and Reliability Test of Epoxy-Based Nano Silver Paste Under Different Pressure-Less Sintering Processes

Recent years, the sintered silver paste was introduced and further developed for power electronics packaging due to low processing temperature and high working temperature. The pressure-less sintering technology reduces the stress damage caused by the pressure to the chip, improves reliability, and is widely applied in manufacturing. Currently, most existed studies are focused on alcohol-based sintered silver pastes while resins have been demonstrated to improve the bonding properties of solder joints. Hence, the performance and sintering mechanisms with epoxy-based silver paste need to be further explored. In this work, a methodology for multi-factor investigation is settled on the epoxy-based silver paste to reveal the relationship between the strength and the different influence factors. We firstly analyzed the characteristics of commercialized epoxy-based silver paste samples, including silver content, silver particle size, organic paste composition, sample viscosity, and thermal conductivity. Samples were then prepared for shear tests and microstructure analysis under different pressure-less sintering temperatures, holding time, substrate surface, and chip size. Full factor analysis results were further discussed in detail for correlation. The influence factors were ranked from strong to weak as follows: sintering temperature, substrate surface, chip size, and holding time. Finally, a thermal cycling test was carried out for reliability analysis. Epoxy residues are one of the possible reasons which result in shear strength decreasing exponentially.

Enhanced Field Emission of Single-Wall Carbon Nanotube Cathode Prepared by Screen Printing with a Silver Paste Buffer Layer

A high emission current with relatively low operating voltage is critical for field emission cathodes in vacuum electronic devices (VEDs). This paper studied the field emission performance of single-wall carbon nanotube (SWCNT) cold cathodes prepared by screen printing with a silver paste buffer layer. The buffer layer can both enforce the adhesion between the SWCNTs and substrate, and decrease their contact resistance, so as to increase emission current. Compared with paste mixing CNTs and screen printed cathodes, the buffer layer can avoid excessive wrapping of CNTs in the silver slurry and increase effective emission area to reduce the operating voltage. The experimental results show that the turn-on field of the screen-printed SWCNT cathodes is 0.9 V/μm, which is lower than that of electrophoretic SWCNT cathodes at 2.0 V/μm. Meanwhile, the maximum emission current of the screen-printed SWCNT cathodes reaches 5.55 mA at DC mode and reaches 10.4 mA at pulse mode, which is an order magnitude higher than that of electrophoretic SWCNTs emitters. This study also shows the application insight of small or medium-power VEDs.

Low Sintering Temperature Nano-Silver Pastes with High Bonding Strength by Adding Silver 2-Ethylhexanoate

A silver precursor (silver 2-ethylhexanoate) and silver nanoparticles were synthesized and used to prepare a low sintering temperature nano-silver paste (PM03). We optimized the amount of silver 2-ethylhexanoate added and the sintering temperature to obtain the best performance of the nano-silver paste. The relationship between the microstructures and properties of the paste was studied. The addition of silver 2-ethylhexanoate resulted in less porosity, leading to lower resistivity and higher shear strength. Thermal compression of the paste PM03 at 250 °C with 10 MPa pressure for 30 min was found to be the proper condition for copper-to-copper bonding. The resistivity was (3.50 ± 0.02) × 10−7 Ω∙m, and the shear strength was 57.48 MPa.

Degradation Mechanism of Pressure-Assisted Sintered Silver by Thermal Shock Test

This paper investigates the degradation mechanism of pressure-sintered silver (s-Ag) film for silicon carbide (SiC) chip assembly with a 2-millimeter-thick copper substrate by means of thermal shock test (TST). Two different types of silver paste, nano-sized silver paste (NP) and nano-micron-sized paste (NMP), were used to sinter the silver film at 300 °C under a pressure of 60 MPa. The mean porosity (p) of the NP and MNP s-Ag films was 2.4% and 8%, respectively. The pore shape of the NP s-Ag was almost spherical, whereas the NMP s-Ag had an irregular shape resembling a peanut shell. After performing the TST at temperatures ranging from −40 to 150 °C, the scanning acoustic tomography (SAT) results suggested that delamination occurs from the edge of the assembly, and the delamination of the NMP s-Ag assembly was faster than that of the NM s-Ag assembly. The NMP s-Ag assembly showed a random delamination, indicating that the delamination speed varies from place to place. The difference in fracture mechanism is discussed based on cross-sectional scanning electron microscope (SEM) observation results after TST and plastic strain distribution results estimated by finite element analysis (FEA) considering pore configuration.