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STEP 4: Printing
December 31, 1969 |Estimated reading time: 7 minutes
This article examines new tools and techniques that are emerging in the printing market to meet requirements for enhanced accuracy and repeatability.
Screen printing process engineers and equipment operators face tough challenges to implement commercial processes that support fine-pitch components, such as chip-scale packages (CSPs). As the industry comes to terms with the behavior of lead-free solder pastes and surface treatments, new printing tools and methods have been developed. For example, nickel stencils are gaining acceptance for providing the optimal paste release for a lead-free screen printing process. The lower coefficient of friction of nickel promotes paste release as the stencil and board are separated. On the other hand, the dimensional stability of laser-cutting equipment has been shown to be satisfactory for accurate printing of lead-free pastes onto closely-spaced pads for microBGAs and CSPs. A laser-cut nickel stencil has been shown to deliver the optimal process accuracy, repeatability, cost, and turnaround time.
Established design rules for stencils used with tin/lead pastes call for apertures to be cut 0.1-mm smaller than outer-pad dimensions. Lead-free paste experiments have shown that increasing pad dimensions to match pad sizes or, if possible, to over-print the pad, allows placed components to self-align during reflow. A higher volume of lead-free paste is needed for self-alignment, compared to the behavior of tin/lead pastes, because lead-free solder alloys display higher surface tension and poorer wetting performance. It also can be helpful to increase the pad size, if possible. This may be feasible for passive components down to 0201s, as well as some larger-pitch QFPs. On the other hand, this may not be practical for BGAs or CSP ICs that display interconnect pitches below 0.5 mm.
Upon introduction of lead-free pastes, optimum print results were achieved using higher excursion speeds and lower print pressures. Experiments also revealed that separation speed was a significant variable, which is interesting because traditional tin/lead pastes have been insensitive to these parameter variations. Lead-free materials show notable improvements as formulations mature, and printing behavior begins to resemble that of tin/lead. Nevertheless, lead-free materials still are intrinsically hindered by weaker wetting.
Figure 1. Singulated substrates are raised into a coplanar position and aligned for individually for printing. This creates a virtual panel that allows multiple substrates to be processed simultaneously.
Another approach to compensating for reduced self-alignment capabilities of lead-free pastes is to increase accuracy and repeatability in printer mechanisms. Advanced screen printers can meet requirements for wafer-level processes, such as flip-chip wafer bumping. Typically, this calls for process-alignment accuracy within ±12.5 µm at six-sigma repeatability (2.0 Cpk). High-rigidity chassis technologies have played an important role in achieving these high standards, and the same technologies are applied in the latest platforms for surface mount assembly. This has successfully reduced the number of individual components required to build the chassis, as well as optimizing materials, cross-sections, and dimensions to produce an extremely rigid and lightweight unit.
Tooling Challenges
The board must be held securely to maintain accruate alignment of the board and stencil throughout the process. The greatest challenge with modern, high-density assemblies is to position tooling pins to support the board from underneath. In the past, this has been a time-consuming process that has added to product setup time; the main challenge is identifying suitable sites to locate tooling pins on the underside of the board. This task has become more difficult as component densities have eliminated suitable real-estate on the PCB against which the tooling pins can bear. A custom tooling plate offers a solution, and direct CAD-to-CNC programs have reduced the turnaround time for the supplier to generate custom tooling for a new product. However, because a small design change on the board will usually requires a new tooling plate, this technique typically is less suitable for the prototype and low-volume production tasks that dominate manufacturing activities in North America and Europe.
Reconfigurable arrays of tooling pins that conform automatically to the underside of an assembly offer an economical and easy-to-use solution. Conforming tooling arrays provide at least 18 tooling pins-per-row, with ESD-safe tips. They can be configured quickly at product changeover without requiring an engineer to generate a program or template. After the first board is run, the tooling array is put into a suitable “configure” mode that drives the pins in direct contact with the board to complete the configuration. Configurable tooling arrays provide support for the stencil outside of the printable area. Supporting the stencil and the board in this way enhances paste-volume repeatability by assisting the gasket seal between the board and the stencil.
In the future, SMT assemblers will look to take on back-end packaging tasks, including solder-ball attachment at the package-substrate level. Many screen printing platforms have the underlying accuracy to perform these processes. However, tooling solutions capable of handling and aligning singulated component substrates will be required. These must be compatible with standard SMT tooling beds, which are being used by contract IC-packaging specialists. Singulated substrates presented to the machine in a JEDEC-standard carrier, such as an Auer boat, are raised into a coplanar position and then aligned individually for printing. Aligning the substrates in this way creates a “virtual panel” that allows multiple substrates to be processed simultaneously (Figure 1). The process may be to print solder paste, or may involve the attachment of solder balls to create an interconnect array.
To print fine-pitch features for advanced SMT assembly within a lead-free process, at commercial rates of throughput and yield, manufacturers must also take advantage of enhancements that speed machine setup and operation. For example, frame-mount stencil technologies are gaining acceptance for their ease-of-use and rapid setup on the factory floor. When removed from the frame, the foil has a thickness of about 200 mils, which allows many hundreds of foils to be stored close to the point of use. Ready-made storage solutions also are available with built-in filing capabilities that allow an individual foil to be located and retrieved quickly. With aluminum profiles fitted to each edge of the foil, mounting in the frame is fast and safe. The frame applies the correct tension automatically, in north/south and east/west directions, when a foil is mounted. Unlike a conventional mesh-mounted stencil, there is no loss of tension over time.
Advanced frame-mount systems incorporate foils in a variety of materials, including nickel or acrylic materials for printing with epoxy adhesives for double-sided SMT assembly. Several manufacturing technologies also are supported, including laser-cut and electroformed foils. Electroforming produces ultra-smooth aperture-wall characteristics to achieve consistent paste-release performance on low-area-ratio, high-density applications.
In addition to solutions aimed at reducing setup and changeover times, modern business models require engineers and operators to work quickly and efficiently, but leave little time for training. These demands are at odds with the need to introduce next-generation processes and products quickly. Operators must interpret large amounts of data, turning many modern processes into a complex challenge in terms of implementation and control. The latest user interfaces seek to overcome some of these conflicts through enhanced ergonomics, for example, by describing remaining levels of consumables such as solder paste and cleaning solvents in terms of time until replenishment, or “time to go.” Embedding increased levels of intelligence in the underlying software also helps to streamline machine setup.
Figure 2. Software system that presents options in a graphical, menu-based format for use on the factory floor.
Some systems have drawn on techniques from virtual-reality gaming to present options in a graphical, menu-based form that can be interpreted quickly to enhance decision-making on the factory floor (Figure 2). In practice, these systems allow assemblers to achieve first-time prints for new products using complex components such as large BGAs. When processes are running at production speeds, operators also benefit from real-time information regarding levels of process consumables. These are presented in a clear, graphical format, allowing relatively inexperienced operators to avoid unplanned machine downtime.
Modern demands for knowledge on the shop floor also drive migration from conventional customer-support programs to an electronic support (e-support) model. Aided by user-interface enhancements, cutting-edge platforms are taking advantage of a broadband Internet connection to deliver process support and machine maintenance information over the web. As a logical extension to this multimedia support experience, direct push-button voice communication with the vendor’s helpdesk lets operators pose questions to experienced support staff directly, while continuing to work on the machine. The variety and depth of future e-support services will increase in the future, with progressively faster Internet connection speeds. It is already possible, for example, for help-desk staff to commandeer control of the machine remotely - with authorization from the machine owner - to fix problems, fine-tune machine settings, inspect diagnostic information, and offer many services that would have required an expensive and time-consuming site visit.
Conclusion
The sequence of actions for a surface mount screen printing process has changed very little since the advent of in-line SMT assembly. However, screen printing machines and subsystems have changed considerably as new package technologies, material sets, and commercial pressures have come into effect.
Jeff Schake, senior advanced technology specialist, DEK, may be contacted at (607) 779-4384; e-mail: jschake@dek.com.