Zhichao Yuan

The Effect of Functionalized Silver on Rheological and Electrical Properties of Conductive Adhesives

This research used low molecular surface modifiers, and observed that chemisorptions took place through the formation of a bond between silver surface and an adsorbed molecule, which improved the dispersion of silver flakes in the organic resin. Several different functionalizers, such as thioglycolic acid, silane and di-acid, were used to functionalize the silver surface. Results of shear viscosity, bulk resistivity etc. showed that by using these low molecular organic functionalizers, isotropic conductive adhesives (ICAs) with lower shear viscosity and better electrical conductivity at high silver fillers content were obtained. The adipic acid had the greatest effect on the rheological and electrical property of ICAs, so its weight percentage in silver flakes was also optimized; ICAs displayed the maximum electrical conductivity when there was 0.5 wt% of silver flakes.

A highly conductive bimodal isotropic conductive adhesive and its reliability

In this paper, micro silver flakes and micro spherical particles were incorporated into the matrix resin of isotropic conductive adhesives (ICAs). Their electrical properties were investigated. The total weight ratio of silver fillers was kept at 75 wt% for all samples. When the content of micro spherical particles was 8 wt%, the bulk resistivity of the bimodal ICAs reduced dramatically to as low as 1.26×10-4 Ω.cm and its viscosity was 24,289 cP under 5rpm at 25°. Scanning electronic microscopy (SEM) images of the bimodal ICAs showed silver fillers well distributed in the matrix resin. In addition, the lap shear strength of different metal surfaces, and the bulk resistivity shifts during aging time under85°/85% RH for more than 500 hours were also measured. The results showed that bulk resistivity shifts of bimodal ICAs remained stable after further cured and the bond strength on the copper surface was the greatest among the three metal surfaces tested.

Study into high temperature reliability of isotropic conductive adhesive

With the rapid development of technologies for high density assembly and packaging in electronic industry, isotropic conductive adhesive (ICA) has been paid more and more attention as a potential substitute for solder, due to its advantages of low processing temperature, simple processing conditions and good manufacturability. However, studies into the reliability of ICA are not as abundant as those of solder. As a composite material, the failure feasibility of ICA not only depends not only on the variation in performance of different constituent parts, such as high temperature aging of the polymer, aging due to moisture absorption and oxidization of filler particles, but also on interface changes. Thus, the failure mechanism of ICA seems to be complicated and studies into the reliability of ICA are also necessary. Reliability in humidity and heat has been investigated in previous works, and in this paper high temperature reliability will be studied as a comparison. Some reliability tests and results will be given and some failure mechanisms discussed. Finally, we present some discussion about the further optimization of reliability for follow-up studies.

2D heat dissipation materials for microelectronics cooling applications

The need for faster and smaller, as well as more reliable and efficient consumer electronic products has resulted in microelectronic components that produce progressively more heat. The resultant reliability issues from the increased heat flux are serious and hinder technological development. One solution for microelectronics cooling applications is 2D materials applied as heat spreaders and these include monolayer graphene, graphene based films, and monolayer hexagonal boron nitride and BN based films. In addition, thermal performances of the graphene heat spreader were also studied under different packaging structures, including wire bonding, cooling fins and flip chips. Finally, 2D hexagonal Boron nitride (h-BN) heat spreaders, fabricated by different methods, were characterized by different thermal characterization methods, such as resistance temperature detector (RTD) and Infrared (IR) methods. In conclusion, these new novel 2D materials developed show great potential for microelectronics cooling applications.

Preventing aging of electrically conductive adhesives on metal substrate using graphene based barrier

The Electronic Conductive Adhesive (ECA) is a promising material as a substitute of traditional tin-lead solder, with many advantages outperforming tin-lead containing solder such as environmentally friendly, requiring much lower processing temperature and a much finer pitch. However, one critical problem related to ECA application is that the contact resistance increases significantly during an aging test, particularly at 85°C/85% relative humidity (RH) when ECA is bonded onto non-noble metal surfaces due to galvanic corrosion. Recently, it has been reported that graphene has an interesting and wonderful property, impermeability; it can prohibit most molecules from going through due to the fact that graphene is one of the most impermeable barrier materials, and it has become a potential candidate for anti-corrosion applications. Some papers studying the application of graphene for anti-corrosion of steel and other metals have already been published. In this paper, graphene film [12] barriers were introduced between the ECA and metal pad to alleviate the galvanic corrosion problem, which can lead to the deterioration of contact resistance, then the aging test @85°C/85% was performed for 500 hours. It was found that the contact resistance increased quickly during the first 200 hours of aging test for samples without graphene film barriers, while the contact resistance for samples with graphene barrier remained stable. Samples with graphene film barriers showed a smaller shift of the contact resistance than those without graphene barrier. The results indicated that the graphene film barrier can be used to improve ECAs reliability, especially @85°C/85% conditions.

The effect of functionalized silver on properties of conductive adhesives

This research used low molecular surface modifiers and observed that chemisorptions took place through the formation of a bond between silver surface and an adsorbed molecule, which improved the dispersion of silver flakes in the organic resin. Results of shear viscosity, bulk resistivity etc. showed that by using these low molecular organic functionalizers, isotropic conductive adhesives (ICAs) with lower shear viscosity and better electrical conductivity at high silver fillers content were obtained. Different processing methods and different matrixes were compared.

Study into the application of single-wall carbon nanotubes in isotropic conductive adhesives

Isotropic conductive adhesives (ICAs) with stable contact resistance and high thermal conductivity are highly desirable. In this paper, a new ICA filled with silver fillers and single-wall carbon nanotubes (SCNTs) was developed. SCNTs were acidified in a mixture of 98% H2S04 and 68% HN03 with a volume rate of 3:1 at 50 °C for 3 h. Carboxyl and hydroxyl groups were produced on SCNTs, and the purity of SCNTs also greatly improved. To prepare the ICAs, acidified SCNTs were firstly ultrasonically dispersed in acetone, then added into the matrix resin, and finally mixed with silver fillers. The bulk resistivity of the fabricated ICAs filled with silver fillers and acidified SCNTs was larger, their lap shear strength lower, their contact resistance stability improved, and their moisture absorption remained almost unchanged. In the curing process, the released heat was greatly reduced and transferred to the surroundings for good thermal conductivity of SCNTs, but the curing temperature was not affected. Therefore, acidified SCNTs can be used in ICAs, and in general, will create a good effect on ICAs.

Development and characterisation of nanofiber films with high adhesion

With the development of thermal management, thermal interface material (TIM) plays a more and more important role in electronic packaging. This paper reports the study on fabrication of a nanofiber films used for nano-thermal interface material (nano-TIM) with the adhesive function. The nano-TIM with high thermal conductivity and low thermal resistivity has been fabricated by electrospinning process. In the present work, hotmelt was added into the electrospinning solution to improve the adhesion properties of the film. The morphology of the film was observed by Scanning Electrical Microscope (SEM). The nanofiber films have a nano-scale structure with hotmelt randomly attached into the fiber matrix. Shear tests were conducted to measure the bonding strength of nanofiber films. The results show that the nanofiber films reached a 6.52 MPa in terms of average shear strength, more than 2 times better than the case without the hotmelt addition.

Influence of substrate on electrical conductivity of isotropic conductive adhesive

Isotropic conductive adhesive (ICA) is widely used with different kinds of substrates in electronics packaging applications. Therefore it is necessary to understand the influence of electrical conductivity of ICA from substrate. In this work, we investigated the electrical resistivity of ICA on quartz, PCB and glass substrate. The experimental data showed that the in-plane electrical conductivity of ICA on PCB is almost twice that of the glass substrate, while the conductivity of ICA on quartz is also significantly greater than that of glass, under the same curing temperature and with the same bond line thickness (BLT) of ICA. This paper later concludes that thermal conductive adhesive (TCA) on substrate with higher thermal expansion coefficient (CTE) is likely to give better performance. Finally, Finite Element Modeling (FEM) and analysis shows that this phenomenon could be universal to ICA and TCA.