The application of conductive adhesive on SMT


No matter the traditional electronic assembly technique or the advanced surface mounting technology (SMT), the components and integrated circuit are generally connected on the printed circuit board (PCB) by welding process. In the traditional PCB, due to the changeable shape and size of components, wielding process may be regarded as an ideal method. However, regarding the large batch of fine pitch surface mounting technology, the requirements on welding equipment and welding technology have become higher and higher by using welding process, therefore, the production cost will increase accordingly. The superiority of connecting components and printed board by using conductive adhesive has obviously revealed. On the other side, as people have paid much attention to the environment pollution caused by soldering paste that contains lead solder and the damage to ozone sphere caused by CFCs cleaning agent, the research and application of conductive adhesive assembly will become more and more important.

Conductive filler material

Conductive filler material generally includes gold powder, silver powder, copper powder, nickel powder, silver-coated copper powder, silver-coated particles and so on; among them, gold powder is used very little due to its high cost, copper powder and nickel powder are also not used very much due to their poor environment resistance performance, however, silver powder is used a lot.

Conductive adhesive researches that carried out on SMT

Epoxy conductive adhesive

Conductive adhesive that used in SMT is a little similar to SMD-ADHESIVE in the aspect of curing, which requires rapid curing in a short time under relatively high temperature. The difference is that SMD-ADHESIVE requires low strength (usually about 10.0MPa) as it only has fixed effect, while the structural strength mainly depends on welding. However, conductive adhesive requires high strength, which can only have good reliability when the strength is more than 15.0MPa; meanwhile, as conductive adhesive requires low volume resistance, many conductive filler material should be added, which will reduce its strength a lot. Latent curing agent has following two disadvantages: a. it has long curing time (about 1.5-2 hours); b. as it is generally in the form of solid, it is difficult to disperse evenly in the adhesive. Silver powder is generally used as the conductive filler material, in which the particle size in the range of 250mesh-350mesh will be most appropriate, besides, the electrolytic silver powder that has arborization particles will also be a good choice.

Polyacrylate conductive adhesive

Photocuring and heat curing can be used in polyacrylate conductive adhesive. It will be difficult to cure in the place where no light reaches, but the UV curable speed is very fast, therefore, the combination of photocuring and heat curing method can be used. When using heat curing, the curing temperature should be controlled in the range of 1200 degrees Celsius-1500 degrees Celsius considering the storage period, therefore, monomer that has high boiling point should not be used. In order to achieve good bonding strength, several different monomers should be copolymerized under different proportions. In order to achieve better bonding effect and other combination properties, functional monomer or preplolymerization monomer should be used.

Comparison between conductive adhesive and welding

Compared with welding, conductive adhesive has following disadvantages: it has longer curing time; it has lower electric conductivity; it has lower mechanical strength; the adhesion agent has certain toxicity, which may release stimulating or toxic gases under high temperature and will have certain impact on the operators. Therefore, good ventilation facilities should be set in the operational zone and safety training should be taken on the operators; the technology is not very mature at present. The above disadvantages will be improved with further production practice process and the progress of scientific technology as well as the constantly emerging of new material and new technology. Beside, its superiority will gradually reveal. Conductive adhesive also has many advantages, which are as follows: the technological process is simple, the equipment requirement is not very high and the production cost is very low; as it has good maintainability, the components can be easy to remove after local heating, besides, room temperature curing conductive adhesive can be used when it needs to bond again; the curing temperature is low, which is usually less than 1500 degrees Celsius; no scaling powder is existed and no post-soldering cleaning is needed, thus it can reduce the environment pollution; it has lower toxicity than lead-containing solder; it is convenient for the assembly of double-side mixed loading PCB.