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Control the Print Process; Safeguard Productivity
December 31, 1969 |Estimated reading time: 8 minutes
To achieve suitable throughputs and yields, solder paste printing must be tightly controlled, from the machine and materials to process parameters. A single, unified process control system can maintain printing in-spec despite the variety of factors that influence the paste-print assembly step.
By Dick Johnson, DEK Printing Machines
Modern surface mount assembly processes are finely tuned, precision procedures able to place minuscule components such as 0201 passives or ICs in fine-pitch chip-scale packages (CSPs) accurately onto PCBs. The underlying goal is to produce the required number of good boards within a desired timeframe; the process must achieve a commercially viable yield at a suitable throughput rate.
Many variables influence the results that can be achieved for a process such as stencil printing with solder paste. To consistently hit high levels of accuracy and precision, total attention to detail is necessary when setting up aspects such as the machine and its associated hardware, the materials, the process parameters, and all procedures. Any errors or inaccuracies can lead to a significant reduction in the numbers of good boards produced, lowering productivity.
The fishbone diagram of Figure 1 highlights the multiple factors influencing the performance of today's stencil printing processes. Operators are increasingly reliant on automated systems to help monitor and maintain in-spec process. However, multiple, individual process-monitoring systems can become difficult to manage. A natural evolution is to use a single, unified system, taking advantage of recent advances in user-interface technology, to more effectively control processes.
Onus on the Operator
The time when all aspects of machine set up had to be carried out manually probably predates many of today's machine operators. It is now accepted that automatic printers are able to read the required machine settings directly from the product file as it is loaded, and adjust the machine settings automatically before printing begins. Even so, the operator remains responsible for ensuring that other aspects of the process are correct and are delivering the expected results. This begins before printing commences, and includes checks such as verifying that the correct type of solder paste is being used and that the correct stencil is loaded for the job.
As the batch progresses, operators have generally been required to manage the process actively, monitoring solder paste usage to ensure timely replenishment, monitoring post-print inspection results to isolate boards with defects and identify process flaws, etc. Post-print inspection is traditionally applied to boards sampled at regular intervals, and is able to generate alerts enabling the operator to identify and sometimes even preempt print defects such as bridging, blocking, or paste-to-pad misalignment. Automated inspection equipment with cameras looking directly down onto the board can be configured to issue an alarm when a preset threshold is breached. This may require the operator to take emergency remedial action such as manually activating the understencil cleaner or correcting stencil alignment.
Figure 1. Factors governing print-process control.
Complementing this on-the-spot process management, manufacturers taking a longer-term, strategic view of their processes also tend to invest in operator training and equipment-maintenance programs. These types of initiatives allow assemblers to manage, maintain, and optimize large numbers of machines – sometimes installed at several different locations globally – and to benchmark and continuously improve machine performance and process yield.
Transferring Responsibility
To further improve on measures such as training and maintenance programs, which demand a commitment from the equipment owners, printer developers increase the on-board intelligence of their automated, in-line machines. One example is the adoption of fieldbus technology, in particular using the Controller Area Network (CAN) protocol. A fieldbus-based wiring infrastructure enables the machine's central controller to send actuation commands and collect diagnostic information from subsystems such as position-control mechanisms. These types of infrastructures enable individual machines to prompt their owners when routine maintenance, such as realignment of the printer rails, becomes necessary. Advantages include a better-optimized maintenance schedule and reduced emergency downtime, among others.
Similarly, a number of real-time status-monitoring systems have evolved to provide operators with more detailed and timely information describing the performance of the current print process. One example is systems that monitor the height of the paste roll on the stencil, when printing with squeegees, to alert the operator when replenishment is necessary. This allows solder paste to be replenished without interrupting continuous operation of the printer, and avoids insufficient-paste-caused defects on printed boards. Similar paste monitoring capabilities are available for enclosed-head print systems.
Whereas traditional post-print inspection systems collect detailed, quantitative information at regular intervals, faster techniques designed to produce a more straightforward good-board/bad-board indication have made it possible to check 100% of printed boards automatically at the line beat rate. More recently, with increased processor performance embedded in new-generation camera subsystems, these types of systems are also able to detect paste bridging at the line beat rate, or to perform quantitative inspection on selected areas of each board. The systems provide reports to the operator in real-time as the process is running.
Figure 2 illustrates the results of high-speed post-print inspection performed at the line beat rate by analyzing images streamed at up to 1700 mm2 per second. The system quickly compares detected paste coverage for each pad with preset thresholds and generates easily interpreted color-coded results highlighting pass, fail, or warning indications. The system takes advantage of recent advances in user-interface technology using large, color TFT-LCD displays to present information to operators in a clear and intuitive format.
This technology enables consumables management, aids real-time management of the print process, and simplifies set up to eliminate sources of human error (Figure 3). Features such as step-by-step menu-driven guidance, on-board video tutorials, and error-recovery assistance enable operators to complete complicated set up procedures quickly and confidently, without undergoing the high levels of specialist training traditionally required. This also enables more flexible utilization of staff and skills, which can help manufacturing organizations to improve efficiency and become more responsive to customers.
Proactive Productivity Protection
More recently, tools have emerged to provide more proactive verification of correct process set up. These have further reduced the potential for operator errors that can otherwise lead to high instances of bad boards. Identification technology, such as barcoding, is a major enabler of this capability. During set up, the operator can scan the identities of items such as the stencil, solder paste, and squeegee blades as they are fitted. The machine then compares this data automatically in software with the identities specified in the product file. In the event of a mismatch, the machine can be halted until the mistake is rectified. This effectively eliminates human errors at set up as a cause of stencil-printing defects. It also enables operators to complete set up more quickly by circumventing laborious manual crosschecking.
Figure 2. Post-print inspection with built-in analysis and good-board/bad-board indication.
Moreover, linking each individual board identity with the data from these process-verification activities provides detailed traceability as required by manufacturers serving markets such as medical, automotive, or military. When scanners identify each board at the moment it enters the machine, the data can be time-stamped and linked with the stored verification data and any other information as required, such as a recording of temperature or humidity. This level of traceability provides a complete description of the prevailing process parameters at the time of manufacture. In addition, it is possible to record effective boards and reject counts, thereby providing manufacturers with accurate insights into the performance and management of their process.
Widespread adoption of verification and traceability techniques boosts to stencil-printing yields and throughput. Combined with post-print verification and process monitoring such as paste-roll sensing, these systems deliver effective help for operators to achieve higher standards of productivity more quickly than has typically been possible in the past.
A Unified Toolkit
The shared objective of these various process-monitoring, verification, and traceability systems is always to help maintain and increase line productivity. However, as each new idea has arrived and evolved separately, the operator has consistently been responsible for collecting and interpreting the results from each separate system and for carrying out the correct response. It is a natural next step, now, to unify such diverse tools into a single system. Combining the strengths of these tools helps reduce the operator's workload and eliminate potential sources of error.
Using today's leading-edge machine-vision technology, it is now possible to perform paste-on-pad, bridging, and alignment inspection concurrently with paste deposition, by scanning at speeds up to 36,000 mm2 per second without increasing cycle time. Moreover, any failed units can be identified and isolated – using suitable hardware – automatically and in real time. This can help prevent defective boards from reaching downstream processes such as component placement and reflow.
The ability to accept other data sensed in real-time, such as paste-roll height, will also enable appropriate corrective actions, such as automatic paste dispense, and warnings or reports to the operator for information purposes. This approach can completely relieve the operator of many low-level process-management tasks.
Figure 3. Advanced user interface technology provides a portal for multiple printing management tools.
Today's menu-driven user interfaces, which provide rich graphics and on-board help for operators, are an ideal platform for converged process control tools such as these. Coupled with large color displays capable of supporting multiple concurrent windows, emerging generations of tools can combine data from verification and traceability modules, which automatically prevent printing until detected problems are rectified, with intuitive set up and reporting capabilities.
Conclusion
Operators and developers of capital equipment such as screen printing platforms identify increased automation as the best way to achieve greater process performance, yield, repeatability, and throughput. Many valuable systems have successfully eliminated potential sources of errors at set up and during the process runtime.
Now is the time to unify these systems, taking advantage of advances in user-interface design and enhanced inspection capabilities, to create a concurrent and seamless environment from product set up to high-speed production of large quantities of verified, known-good boards. SMT
Dick Johnson, product group manager, DEK Printing Machines, www.dek.com.
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