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Step 4: Printing
December 31, 1969 |Estimated reading time: 9 minutes
The principle of high-throughput printing is based on analysis and optimization of individual components while considering the overhead operations and materials. When given the task of developing a high-speed surface mount line, cycle time and overall throughput often is based on theoretical times.
By Edward C. Nauss
The printer sets the pulse rate of the SMT line. Often considered the bottleneck due to ever-increasing placement rates, the printer has evolved through experience and development to match or exceed requirements of high-throughput manufacturing. Current cycle times average around 20 seconds per print cycle. Decreasing this number further requires cycle time analysis and optimization that is broken into three parts — base machine, overhead and materials. By separating each section and by examining the individual components of each, the optimum cycle time can be found while maintaining maximum yields.
Operations, Overhead and Materials
When determining a stencil printer to meet the throughput of downstream equipment, most state the cycle time "without print." This number should be used as a guide to what can be achieved when comparing machine to machine. This number is based on the ability of the printer to transport and align the substrate to the stencil, move the substrate to and from the stencil, and transport the substrate downstream. Not indicated in this number are the overhead functions such as print stroke and speed, wiper actions and frequency, automatic dispensing material and frequency, and, if time allows, post-print inspection. The materials presented to the process, which include the quality of the solder paste, stencil and substrate design, directly influence how much these overhead functions affect overall cycle time, which are considered variables in the cycle time equation. When a production engineer independently examines and then combines these three parts, a conclusion can be made as to what cycle times can be achieved realistically.
Basic Machine Requirements
Based on a 12-second cycle time, a machine will produce a theoretical rate of 7,200 boards per day, 36,000 per week and 1,872,000 per year. Based on this production rate, basic design principles are built into the machine to address this day-to-day rate. Dialing up the speed beyond base design limits will have repercussions on long-term reliability and accuracy.
- Frame design — The frame should accommodate the axis movement. Supplemental axes speed increases will cause the frame to flex, resulting in accuracy and reliability loss.
- Solid casting design — Decreases flexure and diminishes the need for repair and calibration.
- Ease of maintenance — Includes easy access to components to minimize repair downtime.
- Access to software — Most transport software has delays installed to handle worst-case scenarios. The machine should have access to these timers to help produce better cycle times.
Machine Operation Speeds
To determine base machine operation speeds, the first step is to ignore the aspect of the overhead operations and only consider the transport, vision and handling system used in the print process. This will be the foundation for the overall speed, which will govern the actual final throughput.
Transport. To determine the required transport system, single-lane applications should not be considered because of the distance required for travel and the inability to print when there are delays downstream. Triple-track systems (Figure 1) preload substrates during the print cycle, minimizing the actual distance to the print position, and are independent of downstream delays. Dual-lane transport systems (Figure 2) give both the throughput of triple track while giving the user maximum flexibility. The machine can be configured to adapt to a single- or dual-lane design. The printer could be configured to produce the product on both lanes, produce both the primary side and secondary side simultaneously, or build two completely different products. Choosing the right transport system is critical, as this will limit overall process throughput. The transport motors must have the capacity to handle the throughput as well as wear aspects on the transport belts.
Figure 1. Triple-track conveyors allow boards to be staged independently on the input track, the print chamber and the output track.
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Figure 2. Dual-lane transports allow maximum throughput and flexibility by having two independent tracks that are synchronized to feed either dual- or single-lane configurations.
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Handling System. Once the substrate is loaded, handling becomes critical. This involves how the substrate is stopped, positioned and supported through the actual print process. Positioning has two options: optical and hard stop. Optical applications are adequate but involve some delays incurred with overtravel and positioning. Hard stops will allow the maximum transport speeds without overtravel and are the most accurate in positioning a substrate to the support structure. The hard stop should be positioned in the camera to minimize camera axis movement, as the next step is to align the substrate to the stencil. For high-throughput printing, a dedicated workholder, whether single-sided or machined for double-sided, works best. Most high-speed lines are designed for dedicated products and are not subject to high changeover. Holding the substrate, the choices are vacuum and clamping. Vacuum is required for substrate thickness of 0.040" and below, but should be avoided because of its holding effect through vias during the substrate's separation from stencil. Side clamping addresses the forces applied to the substrate in the Y direction, in which over-the-top clamping can address any warpage issues.
Vision System. The vision system needs to meet the requirement of ±0.0005" at Six Sigma for repeatability. Glass encoders in both the X- and Y-axis ensure accuracy of the camera position. The ability to recognize fiducial marks regardless of the variations due to lot variation or plating issues is overcome using a combination of direct and diffused lighting for vision recognition. Downtime associated with fiducial errors due to the vision lighting system's inflexibility must be avoided. The camera path and park position also are critical to cycle time. The ability to have a camera park position behind the rear movable track minimizes the travel distance the camera needs to move before and after positioning and alignment. Once the substrate is positioned, the camera path should take the shortest distance to the first and second fiducial, followed by the park position. If the park position is in the upper right hand side of the substrate (Figure 3), fiducial positions should be lower left and upper right of the substrate.
Figure 3. Optimum camera path for hard-stop position, fiducial recognition and camera park position.
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The next step is to observe basic machine operations with a stopwatch. Break down the individual components and assign a cycle time to each. Look for delays or points where improvement can be made. Access to timers in the transport system allows more flexibility in this area. Any cycle time decrease will add up when the process is put together.
Overhead Operation Speeds
Now that base operation speeds have been optimized, the correct machine configuration base speeds should be under 10 seconds. Optimization of overhead operations will include print head, wiper, dispenser and inspection.
Print head. A closed-loop feedback system to ensure consistent pressure through the stroke should be mandatory. The stroke's start and ending positions should be as close to the apertures as possible. Using the squeegee blades as a dam to prevent paste from going into the aperture is a plus. Metal blades are best because they will maintain a consistent edge and print angle. The material used will determine the speed at which the blades travel. Solder pastes have a typical print speed of 1 to 2" per second; however, some are available that allow a range of 4 to 6" per second. As the print speed reaches the envelope, the process window gets smaller. Because paste is under constant flux due to solvent evaporation, this may induce issues over time. As seen in the table, as speed increases, the return in cycle time diminishes and the risk of introducing defects increases.
Wiper. How well the paste, substrate and stencil interact directly influence the wiper frequency and functions. How well the substrate gaskets to the bottom of the stencil surface is critical to wiper frequency. The frequency should be between every 15 to 25 prints. Profiling the wiper can help, whereas different wipe programs can be instituted based on product count. Finally, solvent should be considered if not already part of the process. High-speed pastes tend to leave flux that requires solvent to be removed.
Paste dispenser. For high-throughput applications, a dual 1,200 g cartridge paste dispenser will minimize the time lost to changing empty cartridges. Dispense rates will be based on usage, as well as the amount of paste that rolls outside the print area. The dispenser's objective is to have the correct amount of paste on the stencil at all times. This will need to be done on an ongoing optimization to increase the time between dispenses. Dispense rates will vary on design and consumption, with typical dispense rates 50 to 100 boards for standard SMT and 25 to 50 boards for paste-in-hole applications.
Inspection. For high-throughput printing applications, an in-line automated optical inspection (AOI) system is best because a printer inspection system will affect the overall print cycle time. At an accelerated rate of production, more product can be built, as well a higher number of defects left unchecked. Even if the demand rate is not required, by decreasing cycle time the printer can perform more inspections, which will affect product yield rates directly.
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Material and Overhead Operation Speeds
The printer and the print process are only as good as the material supplied. The stencil, paste and substrate quality and how they mesh together directly affect throughput quality, as well as the effect overhead functions will have on cycle time.
Printed circuit boards (PCB) should adhere to IPC pad design rules while using a bare copper pad with a protective coating. The mask should not exceed the pad height and be distributed between fine-pitch devices (gasketing). Fiducials should be distributed on all four corners (0.060 circles or squares), but different shapes top and bottom to prevent false fiducial recognition if the PCBs are loaded incorrectly.
Stencils can be cut using laser or e-fab techniques. All apertures should have a greater than 1.5 aspect ratio or 0.66 area ratio. When purchasing stencils for high throughput, always have a backup in case of damage and replace when consumed.
Paste must be adapted not only for the printer but also for each segment of the line. Reflow characteristics and the ability to hold components in place during high-speed pick-and-place are the other requirements. For the printer, verifying proper paste mesh size to match aperture design, good rheological properties and the ability to withstand fast squeegee speeds are key.
Conclusion
Quality and quantity print results require the examination and optimization of each segment of the printer. Understanding the limits as well as the intangibles that contribute to cycle time will allow the user to produce a process with realistic cycle times as well as the needed quality. Consider 12-second cycle time, where the loss of one second means the difference of 144,000 boards over a year's time.
Edward Nauss is responsible for applications development and sales engineering at EKRA-America Inc. He can be reached at 34 Saint Martin Dr., Marlborough, MA 01752.