Development of a Three-dimensional Integrated Solder Ball Bumping & Bonding Method for MEMS Devices Lei Yang\ Wei Liu 1, 2, Chunqing Wang 1, Yanhong Tian 1 'State Key Lab of Advanced Welding and Joining, School of Materials and Engineering 2 Mechanical Engineering Post-Doctoral Mobile Research Station, Harbin Institute of Technology, Harbin 150001, P.R. China Email:
[email protected]. 86-451-86418359 Abstract Compared with integrated circuits, MEMS devices are more sophisticated and diverse. The complex non-planar micro components of MEMS devices raise the urgent needs for three-dimensional (3-D) packaging. To fulfill the specific needs in 3-D MEMS packaging, the basic concepts of a new 3-D integrated solder ball bumping & bonding method for MEMS devices was proposed. A prototype platform was developed according to the concepts with pads detection, micro solder ball manipulation, fluxless laser reflowing and bumps repairing processes. In the prototype platform, an improved center detection algorithm with the combination of the randomized Hough transform and the least squares method was integrated. The detecting tests indicated that the precision of the detecting algorithm can reach 0.21Ilm, and its computing time was less than 10ms.With further experiments verification, solder bumping and microsensors packaging experiments were carried out. 1 Introduction Micro-electromechanical system (MEMS) devices contain many functional elements, such as microstructures sensors actuators and microelectronics. MEMS devices, ~hich ar~ more sophisticated and diverse compared with integrated circuits (ICs), challenge the existed packaging technology. [1] The complex non-planar micro components ofMEMS devices raise the urgent needs for 3-D packaging. In addition, due to the nonuniform feature of packaging for various MEMS devices, MEMS packaging methods should be more flexible. Currently, packaging technology for MEMS devices is originally developed from the ICs packaging. In conventional solder bumping and bonding process, solder bumps are pre printed or placed by vapor deposition or sputtering. Extra process and extra contaminations are the main concerns. [2] Beside these bumping methods which are more compatible for the planar ICs packaging industry, innovative solder bumping technologies were developed. Continuous and drop-ondemand solder droplet printing techniques were developed for preciously placing fme solder deposits onto a variety of smaller substrates. [3] The gravity-accelerated solder droplets were released by disturbing the solder meniscus at the end of the capillary tube by an annular piezoelectric transducer. Nevertheless, the low precision and limited bump size range confined the application of solder droplet printing on MEMS devices. In addition, Pac Tech proposed the laser solder jetting technique to fulfill the specific requirements in MEMS or optical components. [4] Solder balls were melted by laser and jetted in nitrogen atmosphere via capillary on to the substrate. Solder jetting method has potential to be applied for fluxless 3-D packaging. However, the bumps need to be
secondary reflowed to optimize the bump shapes. Furthermore, the solder droplets jetted by the pressure inside the nozzle will impact and damage the dedicated MEMS components. Therefore, in this paper, micro solder balls were used to avoid the droplet impacting. This Micro Solder Ball Bonding (MSBB) technology which uses micro solder balls for interconnection and bonding may be the promising standard 3-D packaging solution for MEMS devices. Based on micro solder balls, MSBB can be more flexible due to the different solder alloys and sizes. Therefore, MSBB is suitable for different types of MEMS devices packaging. Furthermore, with fluxless laser reflowing, the heat source can be applied to the local area on the solder ball accurately and no extra contaminations are introduced in packaging process. Heating effects can be controlled to reduce the packaging impacts to MEMS devices. More importantly, MSBB can be easily automated with high-precision placing platform for MEMS 3-D packaging and also has advantages for MEMS solder self-assembly or self-alignment processes with controlled solder volumes. [5] In this paper, the basic concepts of a new 3-D integrated solder ball bumping & bonding method for MEMS devices was introduced first. Then, the prototype platform was developed according to the concepts. Last, the method proposed in the paper was demonstrated by the solder bumping and microsensors packaging experiments via the prototype platform. With high flexibility, local heating, fluxless soldering and non droplet impacting features ,the integrated solder ball bumping & bonding method offers a promising standard 3-D packaging solution for MEMS devices. 2 3-D Integrated Solder Ball Bumping & Bonding Method In MEMS 3-D packaging, a series of standard effective packaging methods are the research interests nowadays. In this paper, we proposed a new 3-D integrated solder ball bumping & bonding method. Solder bumping and bonding processes can be accomplished simultaneously in one operational step without additional pre bumping and secondary reflowing. Laser was used as the heating source to perform locally reflowing that can avoid additional damage to the thermal-sensitive MEMS devices. Micro manipulation platform was also employed to place the solder ball precisely. In order to repair the unqualified joints or bumps, additional apparatus for repairing process were incorporated in the methods. The 3-D integrated solder ball bumping & bonding method has four operational steps to perform packaging for MEMS devices. The packaging prototype, as illustrated in Fig. 1, contains high-precision platform, micro-vision system,
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laser system, micro-manipulation nozzle, pneumatic system and controlling soft packages. 1) Firstly, the metal pads were identified by the microvision system. The MEMS devices were mounted on the device holder, then devices were sent to the working area of micro vision system for images scanning and processing. 2) Then, solder balls were picked up and placed on the pads. In detail, MEMS devices were moved to the packaging area depending on the location indentified by the micro-vision system. And, the pick-and-place nozzle picked up the solder balls which were singulated and supplied via nitrogen flow in the container. Then, the solder ball was released on the pads preciously via the higher pressure inside the nozzle. 3) Solder balls were reflowed by the laser projected from the nozzle immediately in the nitrogen atmosphere without flux. Solder ball bumping and bonding process were accomplished at the same time. 4) Finally, the bonded devices were scanned by the micro-vision system again to indentify the unqualified bumps or joints. Then, after the unqualified bumps were removed by the vacuum nozzle, the bumping & bonding processes were performed again.
Fig.1 Schematic diagram ofMEMS 3-D packaging method
3 Prototype Platform In this section, according to the original concepts, the prototype platform (Fig. 2) was constructed.
the micro components, identify the metal pads and perform real-time reflowing observation. Laser system was employed to offer heating source which can be used to reflow the solder balls locally. Micro-manipulation nozzle coordinated with pneumatic system for solder ball picking and releasing process. And, these parts were controlled by the codes integrated in the platform.
3.1 Motion Platform High-precision motion platform is the requirement for MEMS 3-D packaging. Prototype platform constructed in the paper includes five axes. Specifically, Xc-axis and Yc-axis were used to horizontally position the MEMS chips that were mounted on the holder. Yt-axis and Zt-axis were used to adjust the location of the laser focus spot. Besides, Zv-axis can move vertically to assist the process of focusing for micro-vision system. The parameters of the motion platform were shown in table 1. Zv-axis (Suruga Seiki, Japan) was driven by stepping motor with positioning error ±O.5Ilm. The maximum positioning error for other motion axes (Parker- Hannifin, USA) was ±1.3Ilm. Table 1 Parameters of motion platform. Axis
Driving Mode
Stroke
Maximum speed
Positioning accuracy
Xc Yc Yt Zt Zv
servo motor servo motor servo motor servo motor stepping motor
lOOmm 25mm lOOmm 25mm 13mm
O.3m/s O.1m/s O.3m/s O.1m/s O.Olm/s
±1.3Jlm ±O.5Jlm ±1.3Jlm ±O.5Jlm ±O.5Jlm
3.2 Micro-vision System Micro-vision system was constructed to identify the center coordinates of the pads, distinguish the unqualified bumps or joints and real-time reflowing observation. The images of the devices, which were captured by CCD, were processed to conduct the operations of the motion platform and micro manipulation tools automatically. Depending on the different functions, the micro-vision system contains the main system and the auxiliary system, as illustrated in Fig. 3.
Fig. 3 Schematic diagram of micro-vision system Fig. 2 images of prototype packaging platform. The platform contains six parts. High -precision motion platform was used to achieve precious positioning operations. Micro-vision system was included to capture the images of
MEMS devices were clamped on the holder in horizontal orientation. In order to capture the images of the devices, the main micro-vision system was set up vertically. The auxiliary micro-vision system was mounted horizontally to observe bumping & bonding process and assist position calibration.
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The pixel size of CCD is 0.71Jlm xO.71Jlm at an image magnification ratio of 4.5 x. As the eyes of the packaging platform, micro-vision system determines the positioning accuracy of solder balls and laser. For the critical circular metal pads, as shown in Fig. 4, the processing steps include images capturing, threshold segmentation, denoising, images binaryzation, edge detection and center detection of the pads.
Fig. 4 Images processing steps for the circular metal pads. Processing contains four steps, a) image capturing, b) threshold segmentation, denoising and binaryzation, c) edge detection, and d) center detection. In the micro-vision system, a new images processing algorithm was developed to detect the center of the circular pads. For detecting the circles, Hough transform and least squares methods are the most common detecting algorithms. [6] Hough transform method has good performance in circle detection, but it requires a large amount of computing time. Least squares method has higher accuracy in detecting and needs less computing time, but it depends on the image quality of the edge. In this section, the center detection was accomplished by the combination of the randomized Hough transform (RHT) and the least squares method. 1)The improved detecting method can be divided to coarse detection and refmement process. The improved algorithm has four steps to detect the precious center coordinates of the pads. 2)Firstly, the circle edges of the pads were detected using Canny edge detector. Canny edge detector is one of the most practical and commonly used algorithms for edge detection [7]. Connected sets of edge points extracted by Canny edge detector were shown in Fig. 4c. 3)Then, RHT was used to perform coarse center detection of the pads. Depending on the parameters of the devices, detecting area can be pre modified to reduce the computing time. 4)According to the coarse detected center, a closed region was determined with a slightly larger square outside the pad and a slightly smaller square inside the pad. The sets of edge points inside the closed region were defined as the pad edge points.
5)Finally, the least squares method was used to detect the center coordinates for refinement process. In order to evaluate the computing time and accuracy of the improved center detection algorithm, the experiments were conducted with 10 samples. The image size captured for processing is 800 pixels> 800 pixels. The square boundaries defined in the third steps have 20 pixels spaces. The experiments showed that the computing time of the improved algorithms were 10 ms and maximum error for coordinates detection was 0.3 pixels. In other words, the maximum coordinates detecting error is only 0.21 urn, Therefore, the micro-vision system of the packaging platform can fulfill the high precision requirements in MEMS packaging. 3.3 Solder Ball Micro-Manipulation In packaging platform, pick-and-place nozzle was mounted on the motion platform to manipulate the micro solder balls (diameters between 40Jlm and 300Jlm). Pick-andplace nozzle was mounted coaxially with laser focusing lens. Thus, laser beam can transmit through the glass cap on the nozzle to reflow the solder ball. As illustrated in Fig. 5, solder bumping and bonding process were accomplished by solder ball picking, positioning, releasing and reflowing. The nozzle picked up the solder balls supplied by the feeding apparatus. In the feeding apparatus, solder balls were singulated and supplied via nitrogen flow in the container. After solder ball picking up process, the nozzle was positioned to the center of metal pad, as shown in Fig. 5a. Then, the nozzle was moved to the certain releasing height (Hr ) that was confirmed by the diameters of the solder ball. The solder ball was released by the higher pressure inside the nozzle. The pressure was adjusted by nitrogen gas. Solder ball releasing process was shown in Fig. 5b. Finally, the solder ball was heated by laser in the nitrogen atmosphere. Once the solder ball was heated to the melting point, the solder ball wetted the metal pads to form the bumps or joints without flux, as shown in Fig. 5c.
Fig. 5 Solder bumping and bonding process. The process contains three steps, a) solder ball picking and positioning, b) solder ball releasing, and c) solder reflowing. The pick-and-place nozzle can be replaced easily depending on the solder ball sizes. Diode pumped solid state laser (JOL-R60, JENOPTIK) was used with the wave-length of 1064 nm. Laser was coupled to fiber for transmitting and then was focused by the focusing lens. Evaluating the diameters of the focusing spot, the 1/e2 full width of the Gaussian laser beam was 1061lm. 4 Experiments In this section, bumping and bonding experiments were conducted via the developed 3-D packaging method. Firstly, the solder bumping process was verified. The commercial solder balls manufactured by Senju (Japan) with
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diameters 200llm have the composition of Sn96.5Ag3.0CuO.5 (wt.%). Sold balls were protected in the nitrogen atmosphere to avoid oxidation. The arrays of 3x3 Au/Ni/Cu pads (diameters, 170llm) were used for bumping. Bumping process was carried out sequentially in designed path. Solder bump images captured by CCD of micro-vision system were shown in Fig. 6a. In solder reflowing, laser energy absorbed by the solder balls affects the evolution of the solder shapes. In other words, laser power density and heating time are the two dominant parameters in reflowing. Scanning electron microscope (SEM) images of bumps with different processing parameters were shown in Fig. 6b. Concave spot was caused by the opposite acting forcing of metal vapor with high laser power density. If the heating time was inappropriate long, the substrate will be burned. Besides, offsetting can be formed with less energy and large positioning error. The qualified bumps were reflowed by optimized processing parameters. Processing experiments were conducted to confirm the reflowing parameters. Then the parameters were optimized with shear tests (DAGE 4000). The results shows that solder balls reflowed with 161 kw/cm ' power density in 40 ms have both high shear strength of 167.5 gf and qualified shapes.
Fig. 6 Images of the solder bumps bumped by the prototype packaging platform, including a) images captured by the main micro-vision system and b) SEM images of bumps with different laser power and heating time. Then, bonding process was verified by the micro sensors joints bonding experiments. The devices were mounted on the certain holder with 45° rotation. Solder balls with diameters l Ouum have the composition of Sn96.5Ag3.0CuO.5 (wt.%) were used for bonding. Solder balls were reflowed with 100 kw/cm/ power density in 30 ms. SEM view of the joints were shown in Fig. 7.
Fig. 7 SEM images of micro sensors joints.
proposed. The basic concepts of the method can be briefly described as pads detection, micro solder ball manipulation process, fluxless laser reflowing and bumps repairing process. A prototype platform has been developed with high-precision motion platform, micro-vision system, laser system, micromanipulation nozzle, pneumatic system and controlling soft packages. An improved center detection algorithm was integrated in the micro-vision system with the combination of the RHT and the least squares method. The detecting tests indicates that the precision of the detecting algorithm can reach 0.21Ilm, and its computing time is less than 10ms.With further experiments verification, 3x3 arrays of Au/Ni/Cu pads were bumped by the solder balls with diameters 200llm via the prototype platform with the optimized processing parameters. Then, the method was demonstrated and applied to package the microsensors. Results of these studies should further test the packaging flexibility for various MEMS devices.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 51005058), National High-tech R&D Program (863 Program) of China (Grant No. 2007AA04Z314) and Natural Scientific Research Innovation Foundation in Harbin Institute of Technology (HIT. NSRIF. 2009037). References 1. R. Tummala, Fundamentals of Micro systems Packaging, McGraw-Hill (New York, 2001), pp. 543-561. 2. Ho-Young. S, Jae-Woong. Nand Kyung-Wook. P, "Formation of Pb/63Sn Solder Bumps using A Solder Droplet Jetting Method," IEEE Transactions on Electronics Packaging Manufacturing, Vol. 28, No. 3(2005), pp. 274-281. 3. Q.B. Liu, M. Orme, "High Precision Solder Droplet Printing Technology and the State-of-the-Art," Journal of Materials Processing Technology, Vol. 115, No. 3(2001), pp.271-283. 4. E. Zakel, L. Titerle, T. Oppert, et al., "High Speed Laser Solder Jetting Technology for Optoelectronics and MEMS Packaging, " Proceedings of the International Conference on Electronics Packaging, Tokyo, Japan, Apr. 17-19, 2002. 5. R.R.A. Syms, "Surface Tension-Powered Self-Assembly of Microstructures-The State-of-the-Art," Journal of Microelectromechanical Systems, Vol. 12, No. 4(2003), pp.387-417. 6. T.C. Chen, K.L. Chung, "An Efficient Randomized Algorithm for Detecting Circles," Computer Vision and Image Understanding, Vol. 83, No. 2(2001), pp. 172-191. 7. Z. Hocenski, S. Vasilic and V. Hocenski, "Improved Canny Edge Detector in Ceramic Tiles Defect Detection, "32nd Annual Conference on IEEE Industrial Electronics, 2006, pp.3328-3331.
5 Summary With fluxless laser reflowing, micro solder ball bumping & bonding technology offered a promising 3-D packaging solution for MEMS devices. In this paper, a new 3-D integrated solder ball bumping & bonding method was 2011 International Conference on Electronic Packaging Technology & High Density Packaging
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