Study Of Sputtering Metallization Of BaTiO₃-based PTC Ceramic

Barium titanate-based ceramic positive temperature coefficient thermistor is a kind of ferroelectric semiconductor material with lots of excellent electrical properties. It is widely applied in the areas of sensing, temperature compensation, thermal overload products, mobile communication, blue-tooth, LAN, computers, laptop, high definition TV, car radio, automotive electronics and military electronics products. The global annual turnover of PTC has exceeded hundreds of billions, with an annual increase of 27%. Currently, the thermistors are with lower resistance, higher voltage, higher reliability and greener manufacturing technique.The metallization of ceramic is to manufacture the required electrode. Some developed countries as Japan has begun to adopt sputtering technology to metalize. China's PTC yield is world-leading, however the domestic PTC production is not quite satisfying:almost all PTC companies still use the plating-lead screen printing-silver plating process, with the disadvantages of large energy consumption, high cost, polluted process and films that can't resist the melting of high temperature lead-free solder corrosion. Since the silver prices continue to rise, there is little gap of profit for PTC production companies. Reducing cost, solving the pollution and improving the metallization quality has become the priority of China's electric ceramic industry.This paper analyzes the ohmic contact mechanism between the electrode and barium titanate-based PTC ceramic and the influence of sputtering parameters on metalized properties. We adopted the DC sputtering and prepared ten different electrode films, which can all reach the requirements. Besides, we have developed the large-scale production line and realized in many companies. The main research work is as follows:1. We analyze the ohmic contact mechanism between the electrode and barium titanate-based PTC ceramic, study the influence of sputtering parameters on metalized properties and successfully prepared ten different electrode films with DC sputtering. Experimental result and large-scale production application shows that the ohmic contact resistance is less than 1 ohm, the adhesion force is larger than 0.59Mpa and the electrode can withstand 400℃high temperature melting lead-free solder corrosion and stays stable after the aging test performance. This electrode preparation method is with lower cost and suitable for large-scale industrial production, which is first being achieved in China and has reached international advanced level. The research result is published at the 18th International Vacuum Conference and indexed by SCI journal.2. We studied the influence of pre-treatment process, film thickness, film material and sputtering parameters of sputtering power, target-substrate distance and base pressure on film performance. The result shows that(1) thorough cleaning is of vital importance before metallization; (2) 8 cm target-substrate distance,0.5 Pa sputtering pressure,20W/cm2 sputtering power density, sputtering rate of 70nm/min for Ag target,68nm for Al target,60 nm/min for Ni target,50nm/min for Cr target, 40nm/min for Ni-Cu target,33nm/min for Ni-Cr target,15nm/min for Ti target can guarantee desirable quality of metallization and fast filming velocity.3. We developed the multi-layer of "transition layer/barrier layer/welding layer" to metalize the PTC ceramic. This coating structure has shown advantages of strong combination, low contact resistance, good weld ability, low cost and strong resistance to high-temperature solder corrosion. This film structure applied in PTC metallization is of the first time and a national patent has been applied on it.4. We have carried out industrial production on the basis of laboratory experiments and participated in the technique design and equipment debug. The technology has been applied in the largest PTC companies in China-Changshu Linzhi Electronics and Dongguan Ouperlong Electronics. Results show that the costs can be reduced by 60% and power consumption could be saved by 40%, the products have reached international advanced level, creating significant social and economic benefits.