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The key to PCB-assembly optimization, programmable conveyor systems can link computer-integrated manufacturing equipment for maximum yields.
By Jim Moran
Whether an OEM or contract manufac-turer (CM), in setting up and operating a printed circuit board (PCB) assembly line, there are three predominant concerns: on-time delivery, maximum yields and low production costs. How well these goals are met has a direct impact on the essential result: a quality product produced profitably.
Rapid Changeover: An Added ComplicationThe ability of an OEM to cost-effectively produce a quality product is challenged by a number of factors, including rapid movement toward smaller devices, higher board densities and diverse component mixes. For the CM, there is an added complication: the ability to achieve rapid changeovers with minimal downtime from one job to the next.
Such considerations are among those that have led to a concept called computer-integrated manufacturing (CIM). Though the idea itself is not new, many manufacturers are now depending on CIM; the software has been perfected and the need for optimization is paramount. Thus, process equipment, such as stencil printers, liquid dispensers, placement machines, in-line test and inspection stations, and rework systems, is being linked into sophisticated control and information operations a good beginning. What must not be overlooked is how the boards are handled between each process (Figure 1).
Figure 1. A PCB assembly line showing how process equipment can be linked with board-handling systems, including conveyors, label placers, shuttle gates and buffers.
Defining CIMTo understand what board-handling systems conveyors, buffers, inverters, label placers, loaders/unloaders, etc. can do, one must first turn to the role of CIM in PCB assembly. While the composite software can vary (depending on the processing equipment on hand), CIM software generally employs computer networks and databases for enhancing quality, efficiency and productivity by controlling and monitoring the performance of each operation, with the exception of reflow ovens.
In most configurations, computer-aided design (CAD) data are converted into manufacturing information for the processing equipment. Setup time is dramatically reduced while changes are automatically incorporated into the programs, test data and documentation for each machine. However, one of the most important benefits of CIM software is its ability to balance production loads and to sequence product movement through the in-line processes. This is done by taking two factors into account: the performance capabilities of each machine and the component mix of the boards.
In PCB assembly, events occur sequentially, making individual in-line machines interdependent. A board can be conveyed from one unit to the next only if the latter is ready to receive it and if the first machine is ready to pass it on. The "ideal" is for both machines to be ready at the same time. Obviously, because different machines run at different processing speeds, other equipment is often introduced into the line to add capability at bottleneck areas. For example, a fine-pitch placer may slow the line and "degrade" the next-in-line chipshooter's throughput. To solve the problem, another placer, or perhaps a system that handles a different range of components, may be incorporated into the line to mount discrete parts and odd-form devices before the chipshooter takes over. In either case, with CIM, the assignment of components to each machine is based on balancing its output to achieve optimum throughput.
Spotting ProblemsAnother important benefit of CIM software is its ability to track performance and to identify problem areas quickly (e.g., jamming boards or feeders running out of components). When such anomalies occur, the information is relayed to the responsible process engineer or line operator for assessment and resolution. CIM software also collects statistical process control (SPC) data. While SPC reports are routinely and automatically generated for various purposes, the data can also be accessed and sampled during production runs for "on-the-spot" analysis.
CIM provides an "umbrella" type of line management to improve and, ideally, ensure the integration of processing equipment. As efficiently as the machines are programmed and the output balanced for optimum assembly, the PCBs still must be transferred from one station to the next. True optimization remains out of reach without the equipment that links the individual machines.
Taking CIM to the Next LevelA PCB assembly line with the most-efficient, smoothest-running, least-troublesome equipment can still be affected by process problems, such as bottlenecks and interruptions, between stations. A board-handling system that functions at the same level of proficiency as the stencil printer, liquid dispenser, component-placement equipment, reflow oven and inspection system is essential to line efficiency. Conveyors must transport boards at the proper rates, which generally vary from application to application. Buffers must store the boards and release them according to programmed timings and in the proper sequence (i.e., last-in/first-out [LIFO] or first-in/first-out [FIFO]). Inverters should turn the boards on cue for reverse-side assembly, and bar-code labels must be placed on PCBs without affecting line pacing. Unfortunately, in the absence of conveyors and other board-handling systems providing seamless integration of an entire assembly line, the implementation of CIM software to run the processing equipment will be ineffectual.
Figure 2. A buffer in operation in an assembly line. When down-line slowdowns or stoppages occur, the buffer permits up-line processing to continue by storing boards vertically after exiting the reflow oven. PCBs are discharged as "instructed" by CIM software controlling line throughput.
The good news is that board handlers are now designed to operate in CIM environments (Figure 2). Conveyors, label placers, loaders/unloaders, buffers, inverters and shuttle gates now come with CIM ports, enabling the machines to interface with CIM programs and recipes (operating parameters) for complete line integration. The result: more efficient operation and the ability to accommodate engineering and production changes without modifying the software for each machine. Changeovers are accomplished by reprogramming the CIM software rather than servicing individual processing stations. And dimensional adjustments such as rail widths can be completed automatically in new configurations in minutes instead of hours or even days.Finally, CIM-compatible board-handling equipment can generate data and reports for various analyses, including SPC documentation. The immediate benefit: identifying a problem (e.g., a board lodged in a buffer magazine) when and where it occurs. Over time, the new handlers can assess production data to determine where improvements can be made for increased yields and profitability.ConclusionTo meet the challenges of component and product miniaturization while maintaining quality and yields, PCB assemblers are implementing CIM software to manage, integrate and balance the operation of in-line process equipment. However, true optimization and line efficiency require that board-handling systems linking various machines be incorporated into the total CIM solution. Today's CIM-compatible conveyors, buffers, loaders/unloaders and other related equipment now come with ports configured to interface with CIM programs, instructions and recipes. With such a capability, OEMs and CMs can expect improved profitability and a strengthened competitive position.
JIM MORAN is the electrical engineering manager at Crown Simplimatic Inc., P.O. Box 11709, Lynchburg, VA 24506; (804) 385-9181; Fax: (804) 385-7813; E-mail: email@example.com; Web site: www.crown-simplimatic.com.