The conductive mechanism of conductive adhesive


The conductive mechanism of conductive adhesive lies in the contact between conductive filler. The mutual contact of filler is formed after the curing and drying of adhesive, thus it can be seen that before the curing and drying of adhesives, the adhesive and the conductive filler are existed independently and there is no continuous contact between them, therefore, they are in insulation state. After the curing and drying of adhesive, due to the solvent evaporation and adhesive curing, the conductive filler will mutually connect into chain-like shape and will therefore have electrical conductivity. At this time, if the quantity of adhesive is much more than the conductive filler, the conductive filler will not form chain-like shape even after the curing of adhesive. It can be seen that the conductive filler will either have no electrical conductivity or will have unstable electrical conductivity. On the contrary, if the quantity of conductive filler is significantly more than the adhesive, the physical and chemical stability of film will completely lose and will not be linked strongly by the conductive filler. Therefore, the electrical conductivity will be unstable.

mechanism of conductive adhesiveIn Feb. 2004, new type epoxy resin conductive adhesive was successfully developed in China. This product is similar with SMD-ADHESIVE in the curing aspect, but it has more advantages. The adhesive that used in SMT needs to rapidly cure in a short time under high temperature. While SMD-ADHESIVE requires low strength, usually 10MPa will be available as it only will have fixed effect and the structural strength mainly depends on welding; however, conductive adhesive requires high strength as it can only have good reliability when the strength is more than 15MPa. Meanwhile, as low volume resistance is required, many conductive fillers should be added, which will greatly reduce its strength. Latent curing agent should be used as the curing agent of this product and silver powder is generally used as the conductive filler. In the experiment, researchers used carboxyl-terminatedacrylonitrile-polybutadiene rubber modified epoxy resin conductive adhesive as the matrix, special electrolytic silver powder as the conductive filler and produced several kinds of latent curing agents. After being cured for 10mins at 1500 degrees Celsius, the shear strength of these kinds of curing agents will reach 12Mpa when the volume resistance is controlled in lower than 2.0X10-4Ω.cm. However, as these curing agents are solid, they are difficult to disperse evenly. If they are cured in a shorter time, the strength will be lower. For example, when they are cured for 5mins at 1500 degrees Celsius, the shear strength can only reach 8MPa. Although latent curing agent has some advantages, it also has some disadvantages, which are as follows: a. it has long curing time (about 1.5-2 hours); b. as it is in the form of solid, it is difficult to disperse evenly in the adhesive, which needs to be further improved. Currently, there is nothing wrong with the silver metal powder that is used as the conductive adhesive. Generally, the particle size that ranges between 250-350 mesh will be suitable and the dendritic particles will be most appropriate.