A New Manufacturing Process for High Volume Production of Ceramic Column Grid Array Modules

Transcription

1 A New Manufacturing Process for High Volume Production of Ceramic Column Grid Array Modules Louis Achard and Isabel DeSousa MLC B/A Engineering IBM Canada 23, Airport Boulevard Bromont, Quebec Canada J2L 1A3 Phone: /6601 Fax: /7171 s: and Abstract Ceramic column grid array modules have been gaining popularity for electronic packaging of high performance asics with over 625 I/Os. To support this growing demand, a new assembly line had to be developed, providing not only high capacity, but also competitive costs and a level of quality suitable for high-end applications. This paper describes how the various aspects of the new assembly line, based on a new column attach process (CLASP), were developed and optimized. More specifically, the paper will describe the use of multi-up processing, a combination of customized tools and standard surface mount equipment, the integration of a semi-aqueous flux cleaning process, automated column planarization, and finally the development of automated column inspection technology. The concepts and processes were developed for both 1.27mm and 1.00mm pitch products on ceramic carriers up to 2600 I/Os. Key words: Ceramic Column Grid Array, Automation, Flip Chip Carriers, and Surface Mount Technology. 87 mils. The columns can be molded onto the ceramic (cast process) or soldered to the I/O pads with eutectic filets or ternary alloy (Pb/Sn/Pd). Figure 1 shows the column array of the final module assembly. 1. Introduction Ceramic Column Grid Array (CCGA) packages have proven to be very reliable high I/O count modules, with up to 2600 I/Os on a 52.5 mm ceramic. These packages offer several advantages, namely, high-density interconnection, compatibility with SMT assembly, superior thermal and electrical performance. CCGAs are offered in a variety of dimensions covering the JEDEC specifications in 1.27 mm and 1 mm pitch. CCGA packages are interconnected to circuit boards through an array of 10/90 solder columns with standard lengths of 50 or Figure mm pitch CCGA array. International Microelectronics And Packaging Society 451

2 Intl. Journal of Microcircuits and Electronic Packaging Until recently, manufacturing of these components had been a manual and relatively low volume process (cast process). This process requires a high temperature reflow (above 300 C), and therefore needs to happen before other bond and assembly (B/A) operations. It requires electrical testing to be performed with probing on the columns. This is not desirable from a cost/quality standpoint, since columns are exposed to damage during subsequent operations, and testing on columns is more expensive than testing on substrate I/O pads. Cast column replacement is also an expensive and low yield process. Finally, this type of process has problems typically associated with manual operations, such as ergonomic issues, high cost, and low capacity. developments of this process, the metallurgy of the column structure, its performance and reliability, as well as the card rework process are described in detail in References 1,2. The structure of the column and joint can be seen in Figure 2. The main features of the clasp joint are as follows, The formation of Pd-Sn intermetallics, with a melting point of 280 C, ensuring that most columns will remain on the module after card rework. Possibility of column / joint removal by adapting a conventional cbga rework process. This is an advantage to card assemblers who can have mis-processed or damaged parts reworked with the same column structure. 2. Objectives When IBM examined possible options to implement a high capacity manufacturing operation, the following objectives had to be achieved, namely; Competitive cost, Minimum column handling / damage, Possibility to perform electrical testing before attaching columns, Maximum throughput, Minimum technological risk, Flexibility to run different form factors with minimum setup costs, Environmentally friendly, Quick and easy module re-columning, Compatibility with second level processes. Figure 2. Cast and clasp column structures. The process flow through the assembly line can be seen in Figure 3. Although a standard paste process was chosen for column attach with the ternary Sn/Pb/Pd alloy, considerable work was required to develop a method to apply paste between the column and the substrate. 3. Strategy One of the main conditions to achieve these goals was to implement a low temperature process. This was required to bring column attach at the very end of the module B/A process, allowing electrical testing to be carried out before column attach, and minimizing handling of the finished modules before shipping to customers. The low temperature process would also allow easy removal of the columns by re-melting the solder joint in a conventional rework process. The main challenge for this low temperature process, however, was to provide a column solder joint which could allow removal of a module from a card without leaving many columns on the card site, unlike a conventional eutectic tin-lead solder joint. This led to the development of a new process called CLASP (Column Last Solder Attach Process), based on a Pd doped eutectic solder, to attach columns to the ceramic substrate. The 452 Figure 3. CLASP process flow. International Microelectronics And Packaging Society

3 In order to achieve high throughput and minimize set-up costs, a decision was then made to adopt multi-up processing on universal carriers. These carriers, which come in 1.27 mm and Line Description mm formats, enable the loading of up to 12,000 columns through masks specifically made for every ceramic size. The attachment The complete assembly line was installed in a class 10k clean process is then carried out on up to 16 parts at a time, depending room. Although a much cleaner standard than typically found at on substrate size. card assembly sites, it was felt necessary to ensure the highest The third part of the strategy was to adopt a tool set as close as degree of cleanliness for maximum yield and a sure way to be possible to industry standards, in order to minimize technological risk and tool customization costs. As can be seen in Figure 4, Column loading is achieved by semi-automatic vibratory tools. extendable to the much finer pitches of the packaging roadmaps. most of the assembly line is based on standard SMT tools, with A combination of vibration and proper rocking motions results added customization for column loading, extraction, column shaving, and inspection. minutes. Once loaded, the multi-up carrier is inspected for miss- in loading 5,000 to 12,000 columns in a cycle time of a few ing columns by a vision system. Paste application and carrier positioning are then performed with the multi-up carrier and a standard Pick & Place tool. Sintered ceramics have some level of distortion (as a result of shrinkage); the P&P algorithm ensures that all columns fit within the I/ O pad before reflow. Paste inspection is performed by a laser scanner for criteria of paste volume, position of deposit, and bridging. This is a key process control step, as is well known in the SMT world. The tool was chosen for its ability to inspect 100% of the sites and match the base speed of the line. Paste is then reflowed in a forced convection furnace in N 2 atmosphere. The main benefit of the forced convection technology (versus conventional furnaces) is the ability to change profiles quickly and fit a range of different products in single profiles. These furnaces are also quite insensitive to loading, and do not require loading of dummy carriers for thermal mass stability. Extraction is achieved by pushing the ceramic substrate out of the multi-up carrier by means of extractor pins. The more challenging aspect of this tool is to gang-pick and precisely align ceramics in cleaning tray pockets. To prevent compatibility issues with second level attach, it was decided to avoid no-clean fluxes for column attach. It appeared impossible to guarantee compatibility between a no-clean Figure 4. Assembly line tool set. flux residue and all potential card attach chemistries. The last but perhaps most important feature of the assembly Cleaning of the rosin flux residue is carried out in a semiaqueous process, using a biodegradable hydrocarbon solvent and line is the automated inspection of the modules for column position. CCGAs can be damaged by handling and it is never possible to rely on humans to accurately insert modules in shipping very low levels of residue / ionics on the modules. Its implemen- DI water. This process was chosen for its robustness in ensuring trays without risking column damage. Only automated pick and tation in an in-line cleaner provided the capacity to clean both place tools with proper vision alignment can provide this. A modules and carriers simultaneously, with the convenience of customized vision inspection system was thus integrated into a short process time and large capacity typical of in-line cleaners. P&P platform specifically chosen for its ability to precisely position CCGA modules in JEDEC trays. It was decided that no tachment process and already have an acceptable co-planarity, Although columns are held in place precisely during the at- product would be shipped to a customer without having previously been inspected on one of these tools. tight co-planarity, well within the 150 microns industry stan- shaving the final assembly in a precision die provides extremely dard. It also provides a uniform reflectivity to the tips of the columns, an important aspect for vision systems of the P&P tools used to place CCGAs on cards. The last tool of the assembly line is the automated inspection system. The requirements for this tool were the ability to handle parts precisely from tray to tray and to inspect each column for International Microelectronics And Packaging Society 453

4 Intl. Journal of Microcircuits and Electronic Packaging position with respect to the best fit of the column grid at high speed. Such a vision system, which did not exist on the market, had to be custom built and integrated into the P&P chosen for parts handling. This type of measurement is very sensitive to column tip illumination and parasitic reflections. Finding the true center of the columns was the key challenge for this tool. 5. Process Challenges and Issues Paste processes have certain limitations, which had to be taken into account while implementing and optimizing the CLASP process. Controlling the solder joint volume on a large number of I/Os and optimizing fillet shape were important steps to achieve high reliability module interconnexions. Controlling the voiding level of the ternary alloy paste has also driven optimization work, resulting in unique reflow profiles. The incoming quality of materials also has a significant impact on the defect level of the finished product. Solder segments, in particular, need to meet a quality level in excess of 6 sigma, simply due to the quantity of columns per module. IBM had to work closely with its suppliers to develop this level of quality in the wire forming and cutting processes, which until then did not exist in the industry. Solder column quality was a key in controlling factors such as solder wicking, column surface defects, notches, voids, among other factors. Product specifications for these defects are particularily stringent. For example, any notch or void beyond 0.13mm is considered rejectable. The next challenge was to control the uniformity of solder paste deposits. In this case, the key factors were the precision of the multi-up carriers, a tight control of the dimensions of the solder segments, and the use of automated paste inspection equipment. It was also found that paste rheology had to be controlled within tighter limits than SMT standards. Contamination by fibers or particles was found to be a minor contributor to yield losses; this is attributed to the clean room environment. The choice of multi-up carriers proved to be fine for throughput and productivity, but came at the expense of cost (machining over 10,000 holes), fragility (higher probability of damaging one of the carrier holes), and the difficulty of cleaning. Special magazines and rigorous handling were necessary to compensate for these drawbacks. Carriers have to be handled during some cleaning steps and column loading; the ultimate goal remains to have a completely automated closed loop where carriers would only travel on edge belt materiel handling systems. Another aspect, which has driven a significant design effort, is the column shave operation. Previous techniques to cut lead columns required lubrication, but the integration of this process in a pick and place tool after the flux cleaning process required the design of a dry process. This was achieved with the proper selection of blade material and the optimization of die and knife. 6. Card Attach Considerations Assembly of CCGAs on cards had been a difficult process in the past, and the introduction of the CLASP process was an opportunity to improve on some of these aspects. The first issue was with module alignment on the pick and place tools. Major customers wanted a robust way to align parts from the position of the tips of the columns (as opposed to the body of the ceramic, a method still widely used) especially on the fine pitch (1mm) modules. IBM worked with some of the major placement equipment suppliers to develop an illumination system specifically for CCGAs. These systems allow finding all of the column positions on the module and performing a best fit to the card pads, for optimal yield at card assembly. The performance of these vision systems (as well as those of any placement equipment) was also improved by the more uniform reflectivity of the new dry shaved surface of the column tips. The machine vision inspection of all columns carried out by IBM before shipping ensures that no column irregularity will affect pick and place vision systems. Preserving column integrity before card attach was the next challenge. In the early days, CCGAs were packaged in JEDEC format trays with a pattern of ribs holding the underside of the ceramic between rows of columns. This type of packaging was acceptable for automatic handlers, but manual handling in and out of trays often resulted in bent columns. A new type of CCGA carrier, with holes to protect individual columns was therefore designed to alleviate this problem, as can be seen in Figure 5. This design, however, could not be readily manufactured, since available molding process and materials would not work for such fine features. A collaborative effort between IBM and a molding supplier was undertaken to push the limit of the technology; it succeeded in producing the new tray after over a year of development. With the new tray, the level of bent columns due to handling at card assembly was drastically reduced. Figure 5. CCGA in shipping tray. 454 International Microelectronics And Packaging Society

5 Column tilt also became a concern for CCGA customers, especially those using the ceramic to align CCGAs on cards. Column tilt is inherent to the attachment process and does not affect module reliability, as long as it is below 11 degree after card attach. When CLASP was initially introduced, the average column tilt was 4.5 degree. It was recently reduced to 2.5 degree by modifying the shaving tool design. This will contribute to additional placement accuracy and reduced tilt after card attach. The final card attach issue was card rework. As discussed in Reference 1, card rework had to be optimized for the new CLASP structure, using a specific type of hot gas rework tool and following a recommended procedure documented by IBM 3,4. Any module damaged at card assembly, from handling or as a result of card rework, can be returned to IBM for quick re-columning, at a fraction of the cost of a new module. The rework process can accept modules with columns at any level of damage. References 1. S. Ray, M. Interrante, L. Achard, M. Cole, and I. De Sousa, et al., Clasp Ceramic Grid Array Technology for Flip Chip Carriers, Proceedings of Semicon West 99, San Jose, California, pp. F1-F7, July S. Ray, M. Cole, and L. Goldman, et al., Clasp Ceramic Column Grid Array Technology and Line Qualification Report, IBM Internal Report, pg. 3, March C. Milkovich and L. Jimarez, Ceramic Column Grid Array Assembly and Rework, IBM P/N APD-SCC-201.0, pp , March C. Milkovich and J. Nash, 1mm CCGA Assembly and Rework Development & T1 Hardware build, IBM Technical Report, pp. 2-4, April Conclusion This high volume integrated new CCGA manufacturing process has been in operation for over a year now, and has produced nearly 2 million modules. The original objectives were largely met and the project is now in the cost reduction phase of the technology cycle. Both IBM and card assemblers have acquired an expertise that makes CCGAs a well understood and accepted technology for high performance electronic packaging. Expansion of the assembly line has since been undertaken, as forecasts predict a strong growth in demand for CCGA modules in the short and long term. The basic and flexible design of the line will be the starting point for the introduction of lead-free CLASP and future generations of product such as 0.8mm pitch products, and form factors beyond the current maximum size of 52 mm x 52 mm. I/O counts will probably reach over 3000 within the next 3 years. About the authors Louis Achard is currently Staff Engineer at the IBM microelectronics packaging facility in Bromont, Canada. He received a B.Eng. Degree and a M. Eng. Degree from McGill University in Mechanical Engineering. He joined IBM in 1985, and has since then been involved in various aspects of manufacturing technologies for microelectronic packaging and substrate manufacturing. He has been responsible for ceramic column attach technology for the last four years. Isabel De Sousa is also Staff Engineer at IBM in Bromont. She has an Engineering Degree in Physics and M.Sc.A Degree in Materials from Ecole Polytechnique de Montreal. She has worked for IBM Canada for ten years, and is process engineer for the column technology since Acknowledgments The authors wish to acknowledge the contributions of J. Dankelman and S. Dussault during the initial study of the line concept, G. Mercier and Y. Ferland for the design of key tools, M. Martel and M. Côté for the development of the cleaning process, M. Bonneville for developing automated inspection., M. Robert for developing shipping trays, and finally J. Dery for valuable input. International Microelectronics And Packaging Society 455