Qiong Fan

Formulation and Characterization of Electrically Conductive Adhesives for Electronic Packaging

A series of high-temperature stable electrically conductive adhesives (ECAs) was developed. Their matrix resin contained a functional epoxy, a reactive diluent, a silane coupling agent, and a curing agent. These ECAs included uni-modal ECAs with micro silver flakes only, bi-modal ECAs with micro silver flakes and micro silver spheres, tri-modal ECAs with micro silver flakes, micro silver spheres, and nano silver spheres. They all were made up of 15 wt% of matrix resin and 85 wt% of total silver fillers. All had reasonable viscosity, low bulk resistivity, and high thermal conductivity. Their bulk resistivity in the in-plane direction was as low as 5 × 10−5 Ω · cm, and different silver fillers had different effects on this. Their thermal conductivity in the vertical direction, from 5 to 7 W · (mK)−1, was usually lower than that in the in-plane direction from 15 to 30 W · (mK)−1. These ECAs could be cured at 130°C or any higher temperature than this with different curing time, and they all had high temperature stability. Their pyrolysis temperature was above 350°C.

Using a functional epoxy, micron silver flakes, nano silver spheres, and treated single-wall carbon nanotubes to prepare high performance electrically conductive adhesives

In this study, a matrix resin containing a functional epoxy, a reactive diluent, a silane-coupling agent, and a curing agent was used to fabricate three modal electrically conductive adhesives (ECAs) with micron silver flakes, nano silver spheres, and treated single-wall carbon nanotubes (CNT). Results showed that too many micron silver flakes reduced the bulk resistivity and adhesion strength of uni-modal ECAs (matrix resin and micron silver flakes). As the nano silver spheres increased, the bulk resistivity of bi-modal ECAs (matrix resin, micron silver flakes, and nano silver spheres) firstly decreased, and then increased again. The adhesion strength decreased also. The bulk resistivity and adhesion strength of tri-modal ECAs (matrix resin, micron silver flakes, nano silver spheres, and treated CNT) both were reduced by the treated CNT greatly. These ECAs could be cured at 120°C or any higher temperature than this with different curing time. They all had high temperature stability with a pyrolysis temperature above 350°C and a glass transition temperature around 180°C.

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.

Optimization of stiffness for isotropic conductive adhesives

With the rapid developments of electronic packaging, there is an increasing demand on high performance isotropic conductive adhesives (ICAs). However, the traditional ICAs are brittle, sensitive for crack formation and delamination, which is one of the major drawbacks that limits their use in a wide range of applications. Therefore great efforts have been made to make conductive adhesives more flexible. The present work aims at studying of several chemicals in terms of flexibilizing materials to modify the stiffness modulus of the conductive adhesives. The effect of the flexibilizers has been characterized by different methods, such as Differential Scanning Calorimetry (DSC), Dynamic Mechanical Analysis (DMA), Thermogravimetric Analysis (TGA), etc. Moreover, the electrical resistance, thermal conductivity and viscosity are also measured in various conditions. Experimental results indicate that one of the flexibilizing materials using flexible ester-linkage is particular of interest as it offers low electrical resistance, high thermal performance and low modulus without decreasing glass transition temperature (Tg) and influencing curing and decomposition conditions.

Preparation and Characterization of a Novel Epoxy Molding Compound with Low Storage Modulus at High Temperature and Low Glass-Transition Temperature

Epoxy molding compound (EMC) has been widely used as a main material for encapsulation and protection of semiconductor packages because of its low cost, high moisture resistance, high heat resistance, and good mechanical performance. Due to the extensive application of lead-free solder in place of Sn-Pb, soldering temperature is higher than before; this demands that EMC, which is usually used for lead-free solder, should have extremely low thermal stress and excellent stability at elevated temperatures. In this work, 1,3-propanediol bis(4-aminobenzoate) (PBA) was added to an EMC product to form a novel epoxy molding compound (FEMC). PBA had very limited effect on the process feasibility of EMC, and caused reduction of the storage modulus by 40% to 50% at high temperatures and reduction of the glass-transition temperature by more than 10°C, which are very helpful to reduce thermal stress buildup during high-temperature soldering processes. The increases of the tab pull force of copper- and silver-plated lead frames within EMC due to PBA were up to 58% and 117%, respectively. With increasing PBA content in the EMC, water absorption increased in a linear fashion, so the amount of PBA added to the EMC should be limited, preferably to not more than 1%.

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.