Pulsed Atmospheric Pressure Plasma System Applied to PCBs Surface Treatment

Transcription

1 Pulsed Atmospheric Pressure Plasma System Applied to PCBs Surface Treatment Fuhliang WEN, Jhenyuan LIN Department of Mechanical & Computer-Aided Engineering / Graduate Institute of Automation & Mechatronics, St. John s University, Tamsui, Taipei County 25135, Taiwan and Hungjiun WEN, Kuo-Hwa CHANG Department of Industrial & System Engineering, Chung Yuan Christian University, Chungli, Taoyuan County 32023, Taiwan ABSTRACT Since a pulsed atmospheric pressure plasma system (PAPPS) could be applied to surface modification for products with complex geometry in a lack of vacuum environment, it definitely saves a lot of operation cost. The PAPPS has unique features in lower surface charge accumulation and higher processing repeatability due to pulsed parameter adjustment from clever tuning technique in manufacturing recipes, which consisted of an adapted DC power supply, a pulsed high-voltage controller, and a high-frequency pulsed transformer. Based on 2 kw maximum output power and limited working voltage at 20 kv with a bipolar pulsed output mode, through communication interface, the PC remote control system was built for setting the pulsed parameters and for monitoring the variation of electrical power outputs. This low-temperature plasma jet would be employed as the tool for PCB surface cleaning and activating at an atmospheric chamber. Experimental results have demonstrated that the PAPPS is able to offer a stable plasma jet while a tunable DC power source, pulsed parameters and air pressures. After plasma surface treatment, the hydrophilic feature of the printing circuit board (PCB) has been enhanced that meaning of adherent substance ability on PCB surface is improved for component welding and in yield rate. Keywords: Atmospheric pressure plasma, Pulsed parameter, Surface modification, Plasma jet, PCB treatment plasma supply system (PAPPS) has unique features in lower surface charge accumulation and higher processing repeatability due to pulsed parameter adjustment from clever tuning technique in manufacturing recipes [3]. Therefore, the PAPPS was adopted to remove contaminant and to improve the hydrophilic feature on the PCB surface via plasma ion bombardment. Also, the adhesion of metal coating or surface mounting on PCBs would be enhanced and reduce the usage of organic solution for cleaning. There are two major sections described in this paper. First section, the practical design was performed for one set of a pulsed high-voltage power supply involved a high-frequency pulsed transformer which was adapted a DC power supply and a pulsed power controller. Next, under appropriate dry-pressure air and high-pulsed voltage supply, the atmospheric pressure plasma jet was generated via a plasma jetting nozzle. This low-temperature plasma jet would be employed as the tool of surface treatment for PCBs at an atmospheric chamber. Through measured instruments of contact angles and 3D surface profiles, the surface characteristics on PCBs were detected after surface treatment. 2. CONFIGURATION OF PULSED ATMOSPHERIC Breakdown voltage V b relative to air pressure feature PRESSURE PLASMA SYSTEM 1. INTRODUCTION In electronic industry, usually printing circuit boards (PCBs) have been adopted as the substrate for mounting ICs, transistors, resistors and other electrical components and wiring connections among each part. PCBs have been developed from a single-layer to multi-layer structure where needs excellent adhesive quality for surface mounting. Therefore, the plasma is used to surface modification to improve the surface function and quality or yield rates for products [1, 2]. Since the atmospheric pressure plasma could be applied to surface modification for products with complex geometry in a lack of vacuum environment, it definitely saves a lot of operation cost. The pulsed atmospheric pressure Fig. 1 Expectable breakdown voltage on the cylindrical electrode of the plasma nozzle based on Paschen curve [1].

2 Pressure regulating valve and air filter DC power supply Air-extracting HV cable Substrate Plasma jetting nozzle with cylindrical electrode High-frequency pulsed transformer Pulsed power controller High voltage detector Voltage probe Current probe TDS3054B Oscilloscope GPIB interface card Compressive Air LabVIEW System Fig. 2 Pulsed atmospheric pressure plasma configuration and LabVIEW monitoring system Due to the high breakdown voltage for plasma at one atmospheric pressure, there are intensely collisions among air molecules or ion particles to easily form the electrical arc between electrodes of the plasma jetting nozzle. It is high possibility to damage the electrodes or the treated surface on the substrate. Hence, according to Paschen s law [4, 5], the designate breakdown voltage V b for the pulsed power supply would be calculated as equation (1), V b pd B ( p d ) B Apd ln[ A( p d )] ln[ln( 1 ln ln )] where p is the air pressure, d presents the distance between electrodes, and γ effect displays the average amount in the plasma of secondary electrons released from cathode surface after positive ion struck at each time. In addition, A and B stand for the ionization coefficient of collision, which the effect of constant α depends on the various air feature but it can be obtained from relative experiments. They have the relationship as equation (2) described as, (1) B pa exp (2) E p where E (=V/d) is the electrical field applied on voltage V at the distance d between electrodes. Actually, the breakdown voltage V b is the function of the multiplication of air pressure p and electrode distance d, i.e., V b =f(pd). Based on above statement, we always followed Paschen curve, as shown in Fig. 1, to determine the applied voltage. A pulsed atmospheric pressure plasma system consisted of a pulsed high-voltage power supply, the air piping and supply equipment, and a LabVIEW monitoring platform for electrical powers, as shown in Fig. 2. The pulsed high-voltage power supply provided the proper AC high-voltage field through a DC power supply controlled by pulsed power controller and regulated by a high-frequency pulsed transformer. Its electrical specifications are maximum output power of 2 kw, the operating frequencies within 25 khz, and limited working voltage at 20 kv with a bipolar pulsed output mode. Through GPIB and RS232 communication interface, the PC remote control system was built for setting the pulsed parameters and for monitoring the variation of electrical power outputs via the

3 LabVIEW platform, which also consisted of an oscilloscope, a high voltage differentiate probe, a current probe, and a high voltage detector to record the output voltage onto the plasma nozzle electrode after the high-frequency pulsed transformer. In addition, the air supply was a simple piping system which adopted the pump, piping and vessel for the generation of compressive air, plus some adjustable valves and appropriate air filters. surface treatment, the roughness of the PCB surface has obvious modification, as shown in Fig. 4, where displays the uneven surface. 3. PLASMA EXPERIMENTAL DESIGN FOR PCB SURFACE TREATMENT In recent decade, the pulsed plasma has larger progress in coating technique and surface modification, because of its intensive plasma energy tuned by electrical parameters such as power output, frequency, duty cycle, etc [6]. Also, it has unique feature for the arc-suppressed function. However, there are lots of processing factors involving into the PCB surface modification for adhesive improvement. We adopted that the design of experiment (DOE) method to investigate the correlative model in interdisciplinary knowledge like plasma surface processing. Using the categorized technique for environmental factors and controlled factors, the interactive relationship among these factors could be testified for the better operating parameters. Thus, multiple factor experiments would be identified as the optimal combination for PCB surface modification to enhance hydrophilic or adhesive feature. 3.1 DOE parameter determination To find the influenced factors for PCB surface modification, we performed DOE method to determine their correlation among factors. Basically, the conventional DOE procedure should be fulfilled as: (1) collecting the possible factors, like processing time, air flow rate and category, plasma jetting distance, frequency, duty cyclelectrical power (including voltage and current), and substrate materials; (2) classified these influenced factors based upon their characteristics and effects on experimental results in percentage; (3) determining reactive levels for factors; (4) setting the level range. Through above procedure, we finally decided the parameters, including plasma treatment time, duty cycle, power, and ignition voltage onto jetting nozzle [7, 8], would be major influenced factors. 3.2 Plasma surface experiment and measurement According to pre-determined processing parameters, the plasma jetting stream would perform ion bombardment on a PCB of 40 mm 50 mm for surface modification after solution cleaning with methyl alcohol. Also, all treated pieces would measure the contact angle through pendent drop for surface tension, as well as they were detected by a 3D surface profile measuring instrument. The hydrophilic or hydrophobic feature of the modified PCB surface was identified by contact angles as shown in Fig. 3. After ion bombardment of PAPPS Fig. 3 Measuring contact angle by pendent drop mode: (a) contact angle before plasma surface treatment; (b) contact angle through plasma surface treatment for 12 seconds. Fig. 4 Surface profile after 12 second pulsed ion bombardment onto the PCB surface with duty cycle 35% near the operating frequency of 11.6 khz and air pressure of 0.9 kg/cm RESULTS AND DISCUSSION The pulsed duty cycle is reversely proportional to applied voltage onto the electrodes of the plasma nozzle, as shown in Fig. 5. Since the duty cycle stands for the input energy from the electrical power at each valid period, the larger duty cycle of electrical power input then has more supplied energy to compensate the plasma energy loss when ion bombardment continuously impacts on the PCB surface and causes the ion consumption. In contrast, if less duty cycle needs higher applied voltage onto the electrodes to offer the instant energy stock to sustain the plasma status during the ion bombardment in T on of either 16 μs or 20 μs. However, there are two various slope tendencies where T on of 20 μs has more sensitivity than that of 16 μs pulsed operation parameter. Otherwise, both of them have the better efficiency or called less power consumption within the range of 40%~48% duty cycle, as shown in Fig. 6. Even though, so far, we did not have the obvious proof, perhaps the material properties of the substrate and supplied air categories would be the domain factors which

4 need the further research to demonstrate this guess Voltage on the plasma nozzle [kv] Ton=16 us Ton=20 us Drop Contact Angle (degee) Pulsed Duty Cycle [%] Fig. 5 Pulsed duty cycle with respect to the supplied voltage on the plasma jetting nozzle based on the different T on time Plasma Treatment Time (second) Fig. 7 Effect of plasma treatment time on the contact angle with pendent drop mode. Power Consumption [watt] Ton=16 us Ton=20 us Surface Roughness [um] Roughness Peak: Z(high) Average Roughness: Ra Roughness Valley: Z(low) Pulsed Duty Cycle [%] Fig. 6 Pulsed duty cycle with respect to the plasma power consumption based on the different T on time. Figures 7 and 8 illustrates the treatment time of plasma ion bombardment effected on the contact angle and surface roughness. When the longer duration under plasma ion bombardment, the drop contact angle is reduced to less 50 ; that is, the modified surface has more hydrophilic feature. However, at the plasma treatment time between 4 seconds and 10 seconds, there is no obvious improvement in contact angles, except for 12 seconds or above, as shown in Fig. 7. In another point of view, the average roughness curve, Ra, could be Plasma Treatment Time [second] Fig. 8 Effect of plasma treatment time on the surface roughness. changed effectively by plasma treatment time, as shown in Fig. 8. The plasma ion bombardment is positively affected on the PCB s surface roughness. From this figure, we understands that the improved hydrophile might be the difference in roughness peak Z(high) and roughness valley Z(low), but not in R a. 5. CONCLUSIONS

5 Experimental results have demonstrated that the pulsed atmospheric pressure plasma supply system is able to offer a stable plasma stream while a tunable DC power source, pulsed parameters and air pressures. One of optimal parameters includes the operating frequency of 25 khz and 46duty cycle, air pressure in 0.4 kg cm 2, and electrical power of 0.77 kw, resulting in contact angles of drop-shape varied from 65.1 fallen into 34.1 after 12 second plasma surface treatment. Also, the surface average roughness (Ra) was carried from 1.19 μm levitated to 2.17 μm after surface modification in 10 seconds. It is an obvious proof of the PAPPS system useful capability in PCB surface cleaning and activating. After plasma surface treatment, the hydrophilic feature of PCB plates has been enhanced that meaning of adherent substance ability on PCB surface is improved for component welding or mounting. 6. REFERENCES [1] Andreas Sch utze, James Y. Jeong, Steven E. Babayan, Jaeyoung Park, Gary S.Selwyn, and Robert F. Hicks, The Atmospheric-pressure Plasma Jet: A Review and Comparison to Other Plasma Sources, IEEE Transactions on Plasma Science, Vol. 26, No. 6, [2] N St J Braithwaite, Introduction to Gas Discharges, Plasma Sources Science Technology., Vol. 9, 2000, pp [3] E. Panousis, F. Clement, J.F. Loiseau, N. Spyrou, B. Held, J. Larrieu, E. Lecoq, and C. Guimon, Titanium Alloy Surface Treatment using an Atmospheric Plasma Jet in Nitrogen Pulsed Discharge Conditions, Surface and Coatings Technology, Vol. 201, Issue 16-17, 2007, pp [4] Claire Tendero, Christelle Tixier, Pascal Tristant, Jean Desmaison, and Philippe Leprince, Spectrochimica Acta, part B 61, 2006, pp [5] H.E. Wagnera, R. Brandenburga, K.V. Kozlovb, A. Sonnenfeldc, P. Michela, and J.F. Behnke, The Barrier Discharge: Basic Properties and Applications to Surface Treatment, VacuumVol. 71, 2003, pp [6] Gnter Mark, Tutorial: PPST 2003, June 9th-10th, Tokyo Japan, [7] Plasma.html [8] Edward V. Barnat and Toh-Ming Lu, Pulsed and Pulsed Bias Sputtering Principles and Applications, Kluwer Academic Publishers, 2003.