BY NEIL MACRAILD, DEK
The blurring lines between SMT assembly and semiconductor packaging have been the topic of an ongoing debate for years. Where does packaging stop and assembly begin? With the proliferation of wafer-level chip scale package (WLCSP) devices and the like, the distinction becomes more obscure because these packages are really miniature systems that require both SMT assembly and semiconductor packaging processes.
Just as the disciplines and processes have merged, so have some of the equipment and materials used to address these applications. Manufacturing systems historically used for SMT processes are delivering the same accuracy and resource efficiencies for advanced packaging processes with more flexibility than traditional packaging equipment. One example of this is printing systems for high-accuracy mass imaging of materials.
Screen-printing application experts have applied process efficiencies and expertise gained in the SMT assembly market to packaging processes, and have delivered some flexible, accurate, and cost-effective solutions to packaging firms. In fact, these processes have almost become mainstream. The flexibility of today’s high-accuracy mass-imaging platforms allows device packagers to achieve repeatable, accurate materials deposition for multiple processes on a single platform.
Though the possibilities are limitless for advanced mass-imaging equipment, applications include backside wafer coating, ball placement, wafer bumping, and encapsulation. Many of the packaging process steps used to produce advanced packages – high-speed die (wafer) bumping, substrate bumping, underfill application, and encapsulation of the package – can be achieved through high-accuracy mass imaging.
Through a print and reflow process that delivers single-stroke, unlimited bump quantity, bumping at the wafer and substrate levels is accomplished. Bump height targets of 80 to 150 µm on pitches of 150 up to 500 µm can be achieved, and process development for finer pitches is underway. When bump size and pitch warrants, balls can be placed at the wafer and substrate levels instead of being bumped. Using this process, solder balls as small as 0.3 mm in diameter can be placed onto substrates or wafers with fine-pitch accuracy, delivering first-pass yields of over 99%. Two side-by-side printers are used for this process, the first of which uses an advanced flux imaging technology to deposit flux at each interconnect site. The second printer, equipped with an enclosed ball transfer head capable of holding up to 50 million solder balls, enables the seating of each ball into the flux. With either method, the parallel processing capabilities delivers fast cycle times that are completely independent of I/O count.
As an alternative to the dispensing or injection-molding processes, high-accuracy mass imaging can also be used to encapsulate the package. Using a stencil-applied overmold technique with liquid encapsulant compounds applied via enclosed print head technology, a smooth-surface, void-free molded package is produced. These are a few examples of applications that can be facilitated by the flexibility, speed, and accuracy of mass imaging.
Historically, the natural development cycle for any application is to first establish a repeatable and accurate process and then build on that to achieve high-speed accuracy and repeatability. This is what has occurred with high-accuracy mass-imaging packaging applications. Not only is the parallel process of mass imaging faster due to the one-stroke deposition of a limitless quantity of solder balls, encapsulation material, or die-attach material, but the equipment itself has also achieved fast cycle times. Board transport and print verification technology enables mass imaging platforms to deliver 4-second cycle times. This comes on board platforms which can be re-purposed for a variety of packaging processes.
Years of research and development has evolved these processes to deliver cost-effective, flexible solutions for advanced packaging applications. But it doesn’t stop there. The continued pitch-size reduction, increased device functionality, and manufacturing challenges posed by changing technology requirements such as lead-free mean that forward-thinking equipment and process specialists must continue to push the envelope and stay ahead of new manufacturing necessities and cost-efficient solutions. It will require far-reaching global support, visionary development teams, high-level partnerships with key players in the supply chain, and process knowledge based in creativity and experience to deliver the necessary tools for flexible, cost-competitive packaging process solutions.
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NEIL MACRAILD, Americas general manager, may be contacted at DEK USA, 2225 Ringwood Ave., San Jose, CA 95131; 408/954-8582; E-mail: [email protected].