Preparation and performance of Ag-coated Cu flakes filled epoxy as electrically conductive adhesives
© Ren et al.; licensee Springer. 2014
Received: 2 April 2014
Accepted: 9 June 2014
Published: 8 September 2014
A low-cost and high performance Ag-coated Cu flakes filled epoxy was prepared as electrically conductive adhesives (ECAs) for light emitting diode (LED) packaging. As-prepared ECAs consisted of bisphenol-F-epoxy resin, micro-sized Ag-coated Cu flakes and other additives. The ECAs with content of 70 wt % Ag-coated Cu flakes pre-modified by silane coupling agent (SCA) exhibited much lower bulk resistivity (8.4*10−3 Ωcm) than that of ECA filled with 70 wt % Ag-coated Cu flakes without SCA modification (1.6*10−1 Ωcm). An appropriate content of diluent (below 20 wt %) was beneficial for the flakes’ regular dispersion in resin, which enhanced the electrical conductivity (10−4 ~ −10−5 Ωcm) and mechanical strength (>14 Mpa) of ECAs.
In recent years, Light Emitting Diode (LED) has rapidly developed to high power and high brightness for illumination due to its small size, low-energy consumption and other advantages –. The electrically conductive adhesive (ECA) is usually used for LED packaging, which requires high electrical conductivity, thermal conductivity and mechanical properties. Electrically conductive adhesives are composed of polymer binder (epoxy resin, phenolic, polyurethane, silicon, etc.), and conductive filler (gold, silver, copper, carbon black, graphene, etc.) –. The polymer binders provide mechanical strength, and the conductive fillers provide electrical and thermal conduction. ECAs require fewer processing steps, lower processing temperature, and they are environmental friendly and energy damping compared to traditional solders ,.
The Ag-filled ECAs are the most mature product in commercial market due to its excellent electrical and thermal conductive property. However, because of the high price of Ag, great attentions and efforts have been made to reduce the costs of ECAs ,. Cu has the most comparable property to Ag but it is easily oxidized and corroded. Ag coated Cu materials have been developed to hinder the Cu oxidation, whilst maintain the excellent electrical and thermal conductivity ,. Generally, high filler loadings are needed to get better electrical and thermal conductive performance. However, too high filler loading will deteriorate the mechanical property. Therefore, it is still a challenge for the preparation of ECAs with a high electrical and thermal conductivity without adversely affecting the mechanical properties.
Herein, we successfully prepared Ag-coated Cu flakes filled ECAs and investigated the effect of adhesive composition on the performance.
Ag-coated Cu flakes were provided by Shenzhen Jinchuanyu Science and Techonlogy Company. Reactive diluent, JD350 and silane coupling agent (SCA), KH570 were provided by Shenzhen Jiadida Chemical Company. Non-reactive diluent, Diisobutyl phthalate (DIBP) were purchased from Sinopharm Chemical Reagent Company. The bisphenol-F-epoxy resin, curing agent, Methylhexahydrophthalic anhydride (MeHHPA) and catalyst, 1-cyanoethyl-2-ethyl-4-methylimidazole (2E4MZ-CN) were purchased from Shell Chemical Company. All chemicals were analytical grade and used as received without further treatment.
Treatment of the Ag-coated Cu flakes
The Ag-coated Cu flakes were added into a mixed solution of ethanol and KH570, then ultrasonic treatment for more than 1 h, and the powder were received by filtration, and then rinsed with ethanol to remove remnants of KH570. The powders were dried in a vacuum oven at room-temperature for 24 h.
Preparation of Ag-coated Cu flakes filled ECAs
The resin, diluent, curing agent, coupling agent, and catalyst were mixed using SpeedMixer (DAC 600 Mixer Range, FlackTek) in a specific ratio (100:50:120:3:7) at a speed of 2500 rpm for 5 min until all ingredients were evenly dispersed. Then micro-sized Ag-coated Cu flakes were added into the viscous polymer matrix and the matrix was stirred in the same rotational procedure as above. Finally, the different formulated adhesives were thermal cured at 150 and 180°C for other properties test.
Characterization and performance measurements
Scanning electron micrographs (SEM) and energy-dispersive X-ray spectroscopic (EDS) were performed on FEI Nova NanoSEM450 instrument.
Where ρ (Ωcm) is the bulk resistivity, R (Ω) is the bulk resistance which measured by a Keithley 2000 multimeter, w (cm), t (cm) and l (cm) are the width, thickness and length of the silver-epoxy resin conductive adhesive stripes respectively. Each ECAs sample was tested at five different positions, and the averages were determined.
Where τ (MPa) is shear strength, P (N) is measured with an electronic universal testing machine at pull rate of 5 mm/min, B (mm) and L (mm) are the binding width adn length respectively. Each ECAs sample was tested five times, and the averages were determined.
Results and discussion
In summary, the electrical conductivity of ECAs was improved remarkably by the filler treated with SCA. A proper amount of non-reactive diluent could reduce the viscosity, and benefit for the flakes’ regular dispersion. The electrical conductivity and mechanical strength was enhanced due to the face-to-face between fillers. The Ag-coated Cu flakes filled ECAs might be applied in LED packaging.
This work was financially supported by the National Natural Science Foundation of China (No.21203236), Guangdong and Shenzhen Innovative Research Team Program (No. 2011D052, KYPT20121228160843692), Shenzhen Electronic Packaging Materials Engineering Laboratory.
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