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Transport Developments Support Fast Changeovers in High-Volume SMT Production
December 31, 1969 |Estimated reading time: 5 minutes
For a decade, parallel placement has been the foremost technology for placing chips in the ultra-high-volume SMT manufacturing environment.
By Jan van de Ven
With its potential for huge, stable, predictable output, the technology became the industry standard in sectors where lengthy production runs were the norm. New technology has substantially improved the flexibility of this concept. As a result, parallel placement now is a practical solution for high-mix manufacturing throughout the volume range from 30 kcph upwards. A new market sector now can benefit from the lowest cost per placement in the electronics manufacturing industry.
This latest parallel placement platform's many features contribute to enhanced flexibility. One of them is a new board transport system, which significantly improves changeover times, reducing downtime to as little as one minute, even for a 100 kcph line.
The Transport Challenge
Previously on parallel placement systems, the board alignment function has been implemented at board run-in. Board-specific carriers with conical pins were used to ensure precise location of the board at each placement head throughout the machine, introducing a time penalty at changeover. Hence, parallel placement is considered better suited to ultra high-volume, low-mix production.
In the new parallel placement platform, centralized board alignment has given way to a board alignment camera on each placement head that performs a positional check at each index of the board through the machine. Local intelligence automatically applies compensation for deviations, opening up the possibility of redesigning the board transport system. Boards now are indexed through the placement system in the same way, but there is no need for specific tooling. Adjustments for board width and thickness are automated, and no runtime calibration is required.
Transport Technology
A patent is pending for the new carrier-less transport system, in which the board is clamped on one edge in the direction of travel. Figure 1 shows the operation of the system, which comprises a fixed frame and a transport beam extending the length of the machine and driven in a reciprocating movement in the X-direction.
Figure 1. In the new carrier-less transport system, boards are indexed through the placement system, with no need for specific tooling.
Boards are run onto the transport beam and onto support rails on both sides. They are lifted by a Z-hoist, which carries a lower clamping jaw to bring the top edge of the board into contact with a fixed clamping jaw. The clamp remains active while the board is moved to the first placement position, and then the board is released back onto the support rails. The transport beam then is lowered and moves back to its starting position to repeat the sequence.
At each index step, the hoist mechanism moves up to locate the boards in further clamping elements extending in the X-direction along the length of the machine, moving them forward a programmed distance to bring them into the work area of successive robots. The run-in module continues to feed boards into the machine.
A high level of engineering has been applied to help achieve a number of specific design criteria. For example, the width of the clamping strip has been reduced to 3 mm, the optimum to provide reliable clamping without compromising placement speed by increasing placement head travel. The clamping elements are designed with dimensions in the X-direction smaller than the longitudinal dimension of the board, ensuring that during transportation each board will be held by at least two clamping elements, one of which will be operational regardless of possible differences in board thickness. The upper jaw of the clamp also is spring loaded to take different board thicknesses into account.
Figure 2. A configurable Zsafe distance protects components mounted underneath the board during vertical movement of the beam.
A further objective of the design is to produce a transport system suitable for boards populated on the underside. The Z-hoist makes contact with the underside of the board via a narrow blade, which occupies the minimum of board space. To minimize cycle time, the returning transport beam has been designed to move in a negative X-direction simultaneously with movement in the negative Z-direction. However, to allow for components mounted on the underside of the board, these movements require the determination of a safety Z-distance programmable for each board (Figure 2). Similarly, the transport beam is moved in a positive Z-direction to the safety distance when the transport beam nears its initial position.
Many boards will not need support pins, but for large, thin, flexible boards they can be fitted off-line to the lower jaw assembly, again with no run-time penalty.
Accuracy Considerations
The support rails exert no undesired forces on the boards that might move them, ensuring positional accuracy. When a board comes to rest in the work area of each placement head, the board alignment system operates in either distributed or local mode to take into account the different accuracy classes for components.
Distributed mode most closely resembles the status quo in parallel placement. Two, perhaps three, fiducials or artwork features are required for accurate board alignment. If insufficient fiducial/artwork features in the work area of a particular placement head, it will take information from a neighboring robot. The scope of distributed operation is one indexing cycle, and obtainable accuracy will depend on robot-to-robot system calibration and transport-to-robot calibration. These are the only calibrations visible to the operator and carried out in a few minutes. The advantage of distributed mode is that there is no need for fiducial/artwork recognition at each index.
In local mode, each placement head independently determines the position of the board at each index, so the use of this mode relies on the presence of sufficient fiducials or recognizable artwork features within the camera's field of view. While distributed mode may prove accurate enough for many placements in typical manufacturing, the new platform features enhanced accuracy through local mode operation for demanding components.
Distributed mode does not exclude the use of local mode for some components, enabling an accuracy/throughput tradeoff on a per-head basis.
Conclusion
Implementation of different accuracy classes on a single placement platform represents a revolution in SMT. The innovation of per-head board alignment and intelligence is supported by no less than three distinct, proven process technologies. As a result, the platform can place components from 0201s through ICs and advanced packages to odd-form components, with optimization for the highest throughput. A modular structure provides for output scaleability from 30 to 100 kcph. More than 300 part numbers are supported, enabling family setup and further enhancing changeover times.
The new parallel placement system eliminates board-specific carriers, feeder reconfiguration and on-line system re-calibration from the changeover equation, thereby achieving the high changeover speed and contributing to lowest cost per placement in high-volume, high-mix environments.
Jan van de Ven may be contacted at Assembléon; E-mail: info.assembleon@ philips.com.