| The electronic packaging demands to use lead-free material because of the pressure from environmental pollution, and the development of the replacements of Sn-Pb solder become the hot spot in the field of electronics manufacturing. The lead-free solder and the conductive adhesive are both the prominent replacements,but the lead-free solder has the disadvantages such as the high brazing temperature and the high costs. However, the conductive adhesive not only can both meet the require of electrical conductivity and the adhesive property, but also has the advantages like the low curing temperature(80~200o C), the strong capability of fine line printing, the broad scope of applying, the simple craft and so on. But every coin has its both sides, the current conductive adhesive exists the problems like the high electrical resistivity and the bad mechanical property. In this paper the modified micro-copper powders which are coated by nano-silver as the fillers of conductive adhesive; the influence of curing process on the adhesive’s electrical resistivity and the shearing strength ware also explored both in theory and in experiment; the connectivity of conductive adhesive include electrical resistivity and mechanical property was investigated by connecting copper pad; at last,the influence of nano filler on conductive mechanism was analyzed.The chemical plating method was employed to modify the micro-copper(300 mesh).The solution [Ag(I)-DM]+ was used as the silver resource, and the Na BH was used as reducing agent to get the copper coated by nano-silver particles. The epoxy resin E51 was worked as matrix resin. MTHPA was the curing agent. Silane coupling agent KH-550 could improve the interface of resin and metal. What’s more, the mass ratio of epoxy resin, MTHPA and KH-550 was 100:80:1.The conductive adhesive with different volume fraction was studied by DSC, then the data was used to calculate the curing parameters. Results indicated that, the conductive adhesive with the 3.17 vol. % had the largest output of heat, and it was the eigenvalue of the volume fraction. What’s more, the 3.17 vol. % adhesive was tested by DSC, then the extrapolation method was used to advise the influence of heating rate to receive one group of curing temperature: the initial curing temperature was150°C, the peak curing temperature was 170°C, and the terminal curing temperature was 200 °C. At last, the curing time of different curing temperature was calculated by means of n series reaction model, and the unknown parameter in the model was got by Kissinger and Grane equation. So when the curing temperature was 150°C, 170°C and 200 °C, the curing time was 120 min, 100 min and 60 min respectively.The influences of the three curing parameters: fillers’ volume fraction, curing time and curing temperature, on the conductive adhesive’ s property was studied. Resultsproved that percolation phenomena existed between the fillers’ volume and the electrical resistivity, and the percolation threshold was 23.5vol. %; the electrical resistivity was higher when the curing time was longer, because of the curing reaction was much more fuller. But if the curing time was too long, the resin matrix would be aging to bring adverse effect to resistivity; when the temperature range was100~200°C, the temperature was higher, the resistivity was lower, that means the high temperature could provide more energy to the curing reaction. Therefore, the tests result was the conductive adhesive’s electrical resistivity could be reach as low as 6.386×10-6Ω·cm when the filler’s volume was 23.5vol. % at the curing temperature of 200°C for 60 min. And it exactly accorded with the theoretical calculation.The conductive adhesive was used to connected oxygen-free copper pad, it turned out that the electrical resistivity was lower at the side of conductive adhesive block. In addition, the shearing strength of copper pads connected by conductive adhesive was researched in three respects: the fillers’ volume fraction, curing time, curing temperature.Conclusions showed that there ware three kinds of influence factors: the cohesion strength of conductive adhesive comparing to the bonding strength between adhesive and copper pad, the amount of bubbles in adhesive, the pull-out phenomenon of fillers.Of witch, the larger fillers’ volume fraction was, the bonding strength between adhesive and copper pad was better; the longer the curing time was, the cohesion strength of conductive adhesive was stronger and the bonding strength between adhesive and copper pad was weaker; the bubbles was influenced by the fillers’ volume fraction and the curing time, so the larger the fraction was and the longer the time was, the more the numbers of bubbles were, leading to the shearing strength decreased; when the curing temperature was higher enough, the pull-out phenomenon of fillers could be observed,and there would be a sudden drop in the shearing strength. And the electrical resistivity was taken into account. The best curing parameters were gotten: the filler’ volume fraction was 23.5vol. % at the curing temperature of 170°C for 100 min.At last, the conductive mechanism was explored and the result showed that the conductive adhesive with the fillers of copper coated with nano silver had the outstanding lied in following reasons: the dendritic structure of the fillers increased the chance of the fillers to contact each other, then the constriction resistance was decreased.The nano silver that coated on the copper powders could not only increased the resistance to oxidation, but also decreased the tunneling resistance because of the high interface energy. |
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