PCB 제조에서 스크린 인쇄를 이용한 도전성 페이스트의 비아 접속에 관한 연구

Abstract

As electronic products become smaller and lighter with an increasing number of functions, the demand for high density printed circuit boards (PCBs) is becoming stronger. There is a trend in electronic systems toward miniaturization and higher functionality, driving the demand for greater interconnects density at the IC, package, and mother board levels. Thus, the importance of PCB, as a fundamental component of these electronic devices, has been increased to meet the trends of high speed, high performance electronic parts. The major challenge for reducing the size of a printed circuit board with good signal routing capability can be addressed by building up multi-layers and connecting them using high density interconnection technologies. Conventional plated-through-hole boards are insufficient to meet this demand. Therefore PCB makers are adopting more advanced fine circuit technology, adding more layers and using more complex structures. A via-hole technology, which involves drilling and copper electro- plating processes, has been widely employed for modern multi-layer PCB designs as a high density interconnection technique. However, the drilling has become a bottleneck process since it is a serial process wherein each hole is drilled sequentially by a laser or a computer nu- merical control machine. For the inter-layer connection, conventional through-holes have been made to drill one hole by one hole. Until recently, the traditional manufacturing process that uses drilling process and copper electroplating has been a workhorse of a modern PCB industry in that it enabled higher wiring density and lower cost. As function of Mobile device increases, Vias number of PCB to connect each electrical component was increased. Usually, 1,500,000 vias are required averagely in 3-generation mobile phone but average 2,500,000 vias is required according as progress to 3.5 generation. As a result, drilling process became the most neck process. To solve this problem, PCB makers have developed new inter- connection method that uses the screen printing technique and cond- uctive silver paste. A conductive bump interconnection technology has received considerable attention as a powerful method to address this hurdle since both the drilling and copper electroplating processes can be omitted. The purpose of this study is bump formation for interconnection with conductive paste by the method of screen printing. Method that connect layer-to-layer which use silver paste bump, carry out several times printing with conductive Ag paste and metal mask for target height and bump shape. Several times printing is being productivity decline of printing process and manufacture lead-time increase. In this study, drew new paste design that can prove printing reproducibility about bump formation. New paste applied three metal powders instead of two metal powders of general conductivity paste and changed paste properties of matter which control each powder, binder, solvent's content. And, the bumps printing were achieved to the best process at printing speed of 80㎜/sec, cylinder pressure of 5㎜, printing angle of 70degree, squeegee hardness of 80degree, off-contact of 5㎜ and gap of screen mask and substrate of 1.5㎜ with newly designed paste. PCB with paste bumps have bad electric conductivity than copper plated via. Therefore, evaluated the electrical characteristics for paste bump. The results were as follows. First, the contact resistance between Ag paste bump and conducting Cu line were obtained by subtracting the Cu line and bump resistances from the measured total resistance. It was found that the contact resistance drastically changed with the variation of Ag paste bump size and the contact resistance had the largest influence on total resistance. The resistance of a circuit in PCB can be estimated from this kind of relationship as the bump size and fabrication technique vary. Second, the paste bump was evaluated frequency characteristic for various bump size 100, 130, 150, 180, 200㎛. The result could know that Ag paste bump is available in frequency band up to 3GHz. The last evaluation achieved migration stability from Ag paste bump. Series of experiments were performed to measure the existence/non- existence of Ag in the insulating prepreg region. It is used electron probe micro analyzer (EPMA) to improve the detecting resolution of Ag content and achieved Ag distribution function as a function of distance from the edge of the Ag paste bump. As a result, Ag paste bump and Cu filled via measured about 4㎛ and 5.2㎛ , respectively. Thus the both paste bump and Cu plating via was stable in over 6㎛ from migration. In this study, studied the via interconnection by conductivity paste using screen printing technique, is shortening of manufacturing process and is eco-friendly manufacturing process.