| With the development of electronic products towards miniaturization, diversification and flexibility, the market competition appeals for the technique to fabricate the circuit boards with high quality, low price and within a short manufacturing period. Especially, the conductive lines with finer linewidths and narrower space distances are required to enhance integration scale of the devices further. With the support of National Natural Science Foundation of China and National "863"Hi-Tech Research & Development Program of China, a method of the conductive line preparation directly written by laser micro cladding electronic pastes technique was presented in this paper firstly around the world. With this novel method, the conductive lines were fabricated directly on different kinds of insulative substrates, such as ceramic, glass and resin etc. to form the circuit boards. At the same time, the concerned fundamental scientific problems on the conductive line fabrication were studied systematically. On the base of above research, equipment for conductive line preparation by laser micro cladding was assembled. Following are the main results on the project: The conductive line preparation directly written by laser micro cladding electronic pastes process was created firstly over the world, namely through the procedures of preparing the electronic paste coating—drying—laser heating the paste coating to form the pattern—cleaning untreated area by organic solvent—sintering the patterns, thus the conductive lines can be fabricated on different substrates. According to above new process, the silver conductive lines with minimum linewidths of 20 μm, 50 μm and 80 μm were fabricated on ceramic, glass and resin substrate respectively by the use of the focused fiber laser to irradiate the conductive paste coating. The values of the linewidths even broke through the minimum values prepared by the current silk (or stencil) screen and thick film print processes. Moreover, compared with current laser direct writing technology, the fabrication efficiency was enhanced with 23 orders of magnitude. The conductive lines had the smooth surface. The adhesive strength between conductive lines and substrates can reach the order of megapascals, while the resistivity of conductive lines can reach the level of 10-5 Ω.cm on resin and glass substrate, and 10-6 Ω.cm on ceramic substrate, which is at the same level of bulk silver. All of these results demonstrated that the resistivity and adhesive strength of conductive lines fabricated by above process could meet the demands for industrial applications. The effects of laser processing parameters on thickness and linewidth were studied systematically, which are both important parameters to characterize the conductive line quality. The experimental results demonstrated that the laser power density and scanning speed have little influence on the thickness of the films but have an obvious effect on the linewidths. Especially, the laser spot sizes played an important role on linewidths. Finer linewidths can be fabricated with smaller spot sizes. General speaking, decreasing the power density and increasing the scanning speed can diminish the widths of the conductive lines. However, there exist certain limits about both power density and scanning speed, beyond which the fabricated conductive lines with a good quality cannot be fabricated. The concerned mechanisms about the effect of the power density and scanning speed on the quality of the conductive lines were discussed systematically. DSC, DTA, TGA, SEM and EDS have been used to characterize the phase change behavior of the electronic pastes during heating, based on which a physical model about the conductive line formation mechanism by laser micro cladding was established. The concrete description is as following: the heat effect of laser irradiation makes organic bonder and part inorganic bonders take chemical reaction, which bind the metal particles to form conductive patterns. Then the un-irradiated areas are cleaned by organic solvent and only the conductive patterns irradiated by laser are left on the substrates. During the subsequent sintering processing, the organic bonders will be vaporized, and in the meantime the inorganic bonders will sink down to the substrate, which is very beneficial for enhancing the adhesive strength between the conductive lines and the substrates. The research results also showed that, owing to the differences of substrates and sequent sintering temperature, the conductive mechanisms of the conductive lines are different from each other on different kinds of substrates: as far as the resin substrate is concerned, the conductive lines were made of a complex network in which the silver particles were adhered each other and to the substrate by the organic bonders, but the adhesive state between the neighboring silver particles is just a simple "mechanical contact". As far as the glass substrate is concerned, the silver particles have been linked more closely and become the quasi-continuous conductive meshy films because of the high temperature diffusing of the silver particles during the period of sintering. For ceramic substrate, the silver conductors have even become continuous films with metallurgical morphology because of the higher sintering temperature. Novel special equipment for laser micro cladding was developed and manufactured on the basis of fundamental research about the conductive line preparation directly written by laser micro cladding electronic pastes. This equipment was composed of two lasers (oneis a 50 W fiber laser and the other is 30 W CO2 laser) and a 3-dimensional worktable. The laser beam was transferred and focused to the surface of workpiece. The scanning speed and directions were controlled by a numeric control system, by which the laser micro cladding process can be accomplished. The numeric control software with intelligent property right by ourselves was developed according to the above-mentioned system. The Open System Architecture of PC + motion control card was used in the control system, which made the system more flexible and transplantable. The control software can not only have the functions of mini-CAD and general motion control, but the end user can convert the general file format generated by software such as AutoCAD, Protel into fabrication files directly. This function improved the fabrication efficiency greatly and enlarged the application area of this system. Moreover, this equipment can be used in the other fields such as laser precision machining, laser micro manufacturing and so on. Summing up above mentioned, the technology and equipment of the conductive line preparation directly written by laser micro cladding electronic pastes have created a new way for conductive line preparation with fast, maskless, short cycle and high efficiency. Under the conditions of new product research and development, small-scale production, this technique will be accepted by the industrial enterprises because of its low cost and high efficiency in comparison with current processes. In addition, the technique can be applied to directly writing thick film chips, fabricating the micro electronic and mechanical system (MEMS) etc.
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