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Speed Enhancements for Screen Printing
December 31, 1969 |Estimated reading time: 6 minutes
Upgrading capital equipment to accelerate assembly processes is a cost-down technique, particularly in the EMS sector. For manufacturers evaluating an individual upgrade, it is important to be sure it will deliver the required speed improvements on the floor.
By Dave Foggie
The search for speed enhancements moves unceasingly along the SMT line. There is always a weak link. Screen printing throughput, for example, may be higher than the remainder of the line, requiring boards to be racked and stacked, or the print process to be operated at lower-than-maximum speed. If capacity further downstream is increased, screen printing may become the next target for speed-up to keep downstream processes fed with freshly printed boards to populate, reflow, and inspect.
Focusing on screen printing and the range of enhancements and upgrades available for printing equipment and processes, manufacturers can choose from a broad selection to fulfill evolving enterprise goals. The benefits of some, such as conforming tooling, changes to stencil cleaning routines or techniques, and post-print inspection, vary depending on parameters such as changeover rate, paste type, board complexity, or board size. Other speed-up techniques look to perform more operations inside the machine in parallel, rather than sequentially.
Upgrades that focus on reducing core cycle time - those actions that are always constant, regardless of the process - have a more deterministic impact on overall process cycle time. The elements that make up the core cycle time of a print process include transporting the board in and out of the machine, and aligning the stencil with board fiducials.
Process-related Speed Gains
Upgrading tooling, for example, by using a tooling array that conforms to the underside of the board automatically, can reduce configuration and setup time. However, the benefits will be most noticeable in high-mix manufacturing. These typically provide an array of independently floating pins that lock into position automatically (Figure 1). Some systems enclose a gel that supports the board by flowing around underside-mounted components. Conforming tooling saves time at setup and changeover. However, compared to a custom tooling plate, these arrays deliver no measurable benefit in terms of cycle time.
Figure 1. New tooling technologies conform to the underside of the board automatically, with zero programming and minimal user input.
Paperless cleaning is another upgrade offered by screen printing vendors to reduce cycle time. Such systems reduce the time it takes to clean the underside of the stencil compared to traditional paper cleaners. These typically are cassette-based cleaners that also can be replenished quicker than a paper roll.
Another way to reduce the contribution of cleaning time to overall cycle time is to perform underscreen cleaning while the printed board is being inspected. Processes that require more frequent stencil cleaning will benefit the most from quicker cleaning or cleaning in parallel with other processes. Any reduction in cleaning time is amortized across the number of boards printed between cleaning cycles. Calculating the likely return on investment (ROI) in a faster or more optimal cleaning technology is process-dependent.
The duration of any inspection routine also depends heavily on board design and desired inspection coverage. Complex circuit features, areas of high-density interconnect, and large board dimensions increase inspection challenges. Upgrading the inspection system to increase the camera field of view, enhance image-processing algorithms, or increase the processing performance may allow inspection coverage and tact-time targets to be met. However, dramatic speed increases can be achieved by moving away from performing 2-D inspection on each board at line-beat rates. Inspection gathers large amounts of quantitative print data that, while valuable during activities such as new product introductions (NPIs) or continuous improvements, are redundant in high-speed series production. Instead, simplifying image analysis to a straightforward comparison against preset threshold values to provide a simple pass/fail indication is replacing traditional inspection routines. This is a high-speed verification procedure that can allow a higher proportion of print deposits to be verified as acceptable while increasing the overall line-beat rate. Systems in operation today allow process engineers to adjust the number and size of inspection sites, displayed by the software in seconds, to optimize coverage and cycle time and ensure specific circuit features are covered within the line-beat rate’s overall time limit. However, a reduction in print cycle time depends on user settings and certain product characteristics, such as overall dimensions and the presence of complex features. High-speed verification delivers the maximum benefits in high-volume, low-mix manufacturing.
Reducing Core Cycle Time
All upgrades discussed deliver more or less ROI depending on process characteristics or production volume and mix. Manufacturers seeking to increase screen printing throughput in a high-mix environment must prioritize reductions in tooling setup time over high-speed stencil cleaning, for example. Techniques that reduce the core cycle time of a screen printing machine, on the other hand, deliver consistent time savings in each cycle, regardless of the process or manufacturing scenario.
The factors that make up core cycle time include transporting the board in and out of the machine and aligning the stencil with board fiducials. During these periods, the printer cannot add value to the board by imaging paste onto it. Therefore, this time is regarded as dead-time. The duration of each of these stages is constant for a given machine configuration, and does not depend on process variables. Measures to accelerate board transport and fiducial alignment yield deterministic improvements in overall cycle time.
Pipelining boards in and out of the machine is another well-established method to reduce core cycle time. The typical configuration is to have a three-stage pipeline: one completed board leaving the machine, one being printed, and a new board entering the machine.
Another important component of core cycle time is fiducial alignment, which must be completed prior to printing. Rapid, accurate capture of fiducials is vital to minimize this duration, and depends on feature-recognition software, as well as cameras, optics, and lighting. Upgrading these aspects of the machine vision system to improve lighting, camera field of view, and image recognition algorithms delivers a cycle time saving each time a board is aligned.
The board-transport mechanism also is receiving attention as new motion-control techniques enable faster board movement to further reduce core cycle time. Traditionally, boards were transported in and out of the machine while resting on a belt driven by a simple DC motor. Friction holds the board on the belt, meaning there is a natural limit to the maximum possible acceleration that can be applied. As a result, boards move relatively slowly inside a typical screen printing machine, adding to the core cycle time considerably. Replacing this traditional belt-type board transport with a modern solution can eliminate speed restrictions by allowing boards to move more quickly. A rapid-transport conveyor system provides an active board-snugging mechanism driven by a stepper motor, which replaces the friction retention of a traditional belt-type transport. This snugging mechanism can adjust to grip boards of any dimension automatically up to the maximum size allowed by the machine. Once snugged, a high-speed servo-motor transport moves the board (Figure 2).
Figure 2. With active board snugging and fast servo-motor transport, core cycle times have been reduced to four seconds.
Using similar mechanisms to transport the board in and out of the machine, this servo-driven, rapid-transport conveyor can reduce core cycle times of automated screen printers to about four seconds. Because this is core cycle time, it is constant and repeatable. This approach delivers a significant speed-up that can be used when analyzing overall print cycle times.
Conclusion
Modern electronic manufacturers expect to invest frequently in machine upgrades to accelerate SMT assembly processes such as screen printing, but they must be sure of the greatest possible improvements.
Upgrades that reduce core cycle time for screen printing deliver a known speed-up that is independent of other processes or manufacturing variables. However, manufacturers may seek to improve specific aspects of a process if one of these has been identified as a bottleneck.
Overall, a range of upgrades that deliver meaningful improvements to each aspect of the process are essential to enable manufacturers to achieve appropriate speed enhancements - meeting current and future line-beat-rate targets cost effectively.
Dave Foggie, product manager, DEK, may be contacted at 44 1305 760 760; e-mail: dfoggie@dek.com.