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A right solution for every setting

Fight the confusion: a useful comparison of SMD placement machine concepts
A right solution for every setting

For electronics manufacturers deciding to invest in SMD placement machine capability, cost of production (CoP) and operational flexibility are major considerations. There is a confusing array of SMD machine concepts, each fulfilling these necessities to a greater or lesser extent, but how does the user decide which is just right for a particular production task? With a method for characterization of different types, we can assess the alternatives and highlight their suitability for specific requirements.

Sjef van Gastel, Assembléon

Figure 1 shows generic placement machine architecture. In table 1, machine concepts are identified according to the movement of the PCB and the number of PCBs in the work area. There are considerable similarities between the X-1, Y-1 and Y-2 concepts, with Y-2 simply two Y-1 machines under one roof. In each case, the machine comprises overhead gantry/gantries equipped with placement heads and, in most cases, vision on-the-fly between pick & place locations. Feeder banks or trolleys are located on both sides of the gantries, with the advantage of a virtually unlimited number of feeders. Output is boosted by simultaneous pick & place, and the elimination of additional gantry travel for component alignment. Single-axis movements of PCB and gantry servo-systems enhance accuracy. Concept Y-1 offers the flexibility of pass-through or T-mode integration into the flowline.
Disadvantages of these systems include relatively long gantry strokes which negatively influence output, movement of the PCB during placement imposing acceleration forces on mounted components, and the need for intelligent motorized feeders for implementation of pick correction for small components. Matrix-tray feeders require component shuttle or line-by-line indexing. The machine occupies a relatively large footprint. In the case of Y-2, two-sided operation is required. Time penalties are incurred in PCB run-in/run-out, by the need for collision avoidance when two shuttles are used on a common guiding rail in Y-1, and by the complexity of PCB handling within Y-2.
The turret concept
The turret is the most frequently applied concept for chipshooting, with an output under ideal conditions between 40kcph and 53kcph. Its typically 12 to 24 placement heads are each equipped with three to six nozzles that can be changed on-the-fly. Pick from the moving feeder table is simultaneous with place at the opposite side of the turret, while the PCB is located on an XY-table which takes care of positioning it to the exact placement positions. Operations such as component presence check, pre-rotation, visual inspection, component alignment and final rotation, all take place simultaneously between pick & place. In some cases, a split feeder bank allows one table to offer components to the placement heads, while the other is prepared for fast change over.
However, with turret machines there is considerable difference between theoretical and real output. With output determined by both turret index and PCB XY-movements, the latter is the determining factor in most cases, while PCB exchange time is also relatively long with 3 to 5s. Placement of large components also limits output. Accuracy is limited by the movement of the PCB imposing acceleration forces on mounted components, and on vibrations from the moving feeder table. Tray feeding is impossible, and pick corrections for small components are only probable with intelligent motorized feeders. The machine occupies a long footprint, due to the moving feeder table.
The single-gantry concept
The single-gantry concepts 1 and 2 are the simplest, lowest-cost placement solutions, inappropriate for applications with short cycle times. They use one or two robots, respectively, feature a small footprint, and deliver an output limited to approximately 10,000cph (components/hour). In the first, a gantry-type robot, usually equipped with multiple placement heads, moves in the XY-directions while the placement heads can make phi (angle) and Z-movements (height). In most cases, the placement robot is driven on one side of the X-beam by means of a ball-lead-screw (T-drive) or, for high precision, on both sides of the X-axis by linear servomotors (H-drive). Pick corrections are provided by the robot, and component alignment units are located on both sides of the machine, between the pick & place locations. The PCB transport is located in the center of the machine, with a PCB fixed by either edge clamping or locating pins.
This flexible concept allows simultaneous pick & place with nozzle exchange on-the-fly if required, and can accommodate different cameras and multiple pipettes. A large number of feeders can be located on both sides on banks or trolleys, with tray and exotic feeders easily integrated, and with opportunities for gang pick and replenishment during operation boosting output. Stationary feeders and PCB improve accuracy. Disadvantages are a relatively long PCB exchange time (2 to 3s) and the need for two-sided operation.A second single-gantry robot can be added to place components on a second PCB within the machine, with parallel pick operations from different feeder sets and parallel placement operations on different PCBs. Although this configuration gives relatively high productivity per square meter, the number of available component codes is reduced by a factor of two in the case of double placement. Also, PCB transport takes more time because of two-fold handling, and acceleration forces act on mounted components on a board during placement correction. Accuracy is limited by the complexity of pick & place corrections.
The dual-gantry concepts
This time, two gantry robots (concepts 1 and 2) share the same workspace above the PCB. While one robot is picking, the other is placing, almost doubling productivity with just a small penalty for workload balancing and synchronization of robot movements to avoid collisions. H-drive is available for higher accuracy, though vibration induced by the second robot may compromise accuracy. Otherwise, advantages and disadvantages are the same as for single-gantry types. Furthermore, because both robots share base, transport and controller, placement costs are reduced.
Typically for dual-gantry concept 2: the PCB transport system divides into two parts, each capable of moving the PCB in X and Y-directions. On entering the machine, PCBs are clamped and shifted towards the front or rear placement area, reducing the distance between pick & place locations to a minimum. Because both robots have their own operating area, collisions are impossible. This configuration yields outputs of up to 40kcph, sharing advantages with single-gantry types, but with the penalty of a complex and time-consuming PCB transfer system and reduced accuracy due to vibrations induced by the second robot.
For the collect & place concept 1 and 2, each machine consists of two pick & place robots, equipped with a multi-pipette revolver head. While one robot picks components on all available pipette positions sequentially, the other is placing sequentially, with pick & place cycles balanced for optimum output. The relatively long travel times between pick & place locations are divided by the available number of pipettes. A multiple pipette beam on a single or dual-gantry placer can also achieve this effect of time sharing. In fact, most dual-gantry placers with multiple placement heads on the gantry can be considered also as collect & place category machines.
This configuration shows many of the advantages of the dual-gantry concept, with placement costs reduced by the sharing of base, transport and controller by both robots and high output per square meter. However, collision avoidance and workload balancing make this a more complex solution, while T-drive and vibration compromise accuracy. Revolver heads with radial pointing pipettes are vulnerable, and nozzle exchange is only possible on a one-by-one basis. The index movement of the revolver head induces extra acceleration forces on the nozzle tips, reducing the attainable output, while limitations on the vertical stroke of placement heads limits component height to 6mm. Real output is much less than theoretical output due to nozzle ”starvation” as a result of component/nozzle relationship restrictions. The depth of the machines creates problems with accessibility. Concept 2 is a similar machine with two times two pick & place robots, each pairing seen as a separate SMD placer with a PCB in its working area.
Multiple pick & place (MPP) concept
Here, a number of PCBs are lined up in a transport row and are indexed simultaneously over the same index stroke. Placement robots are located over the length of the machine, each equipped with a single-nozzle placement head and able to pick components from a limited number of feeders which are located at the front side of the machine. After picking, each robot moves the component to the placement location, with laser alignment on-the-beam. This concept makes it possible to pick & place components in parallel by application of a number of parallel operating placement robots. The total PCB area is covered in a number of index steps, with no PCB transport time added to the pick & place cycle. Because PCB and feeders are stationary, paralleling robots rather than increasing their speed achieves high output. MPP machines can place with high accuracy.
With output up to 120kcph achievable within a small footprint, feeder replenishment during operation, and fast exchange of defective placement robots for minimum machine downtime, this concept is suited to mass production with lowest CoP. In any case, the concept is better suited for output greater than 35kcph. Investment and in turn production volume, can be increased in small increments, while CoP remains stable as a function of the number of placement robots.
This concept is not without disadvantages. The number of feeders relates to the number of placement robots on the machine, and is therefore not independent of output. The output can be sensitive for component clusters on PCBs. Tray feeding is not possible, and nozzle starvation is possible when there is no nozzle-exchange facility on a robot, although the application of special nozzles covering a wide component range can reduce this effect. The introduction of PCB types is more complex, because there are multiple PCBs in the machine. Calibration is also more complex than in machines with only one PCB in the work area, although the application of intelligent calibration techniques can reduce this effect.
Machine concept analysis
In order to compare machine concepts, a generic timing model was built using the same parameter values for each. With this timing model, it is possible to compare productivity from different machine concepts, and to study the influence of system parameter variation. The parameters defined are the following:
•Servo movements on pipette – third-order speed profiles with Amax = 20m/s2, Vmax = 1.5m/s and Adot = 1000m/s3
•Servo movements on PCB – third-order speed profiles with Amax = 7m/s2, V7 = 0.75m/s and Adot = 1000m/s3
•Pick time 60ms per component
•Place time 60ms per component
•PCB width 500mm
•Feeder width 16mm
•Number of feeders per machine 100
•PCB transport time 2s
•PCB alignment time 1s
•Number of components to be placed 200
•Parameters to be varied – number of placement heads (pipettes), distance between two adjacent placements, Amax, and Vmax
Results from the timing model
Concept X-1: from figures 2 and 3 it can be concluded that the number of placement heads per shuttle has the most significant influence on productivity. The influence of Amax and Vmax is less significant. The reason for this is that high Vmax is only helpful for the long strokes between the pick & place areas. During picking and placing of components this Vmax will not be reached. A high value for Amax only contributes partially, since allowable values for Amax on the PCB are considerably reduced by the possibility of shifting of mounted components in glue or solder paste respectively on the PCB. Flattening of the output when there is more than eight heads is the result of a reduction of the time-sharing effect. Here, the influence of individual pick & place times becomes dominant.
Concept Y-1: the output here is comparable with the productivity of concept X-1 (figure 2). In the case of a T-mode arrangement, the output of concept Y-1 will be less than this of X-1, due to longer PCB run-in/run-out times.
Turret concept: The theoretically attainable output of a turret is limited by the maximum allowable radial acceleration at the nozzle. The components on the nozzle will experience acceleration force as a vector resulting from both centrifugal force (m*w2*R) and tangential force (m*wdot*R). This resulting force vector acting on the component can be kept in equilibrium only by means of a frictional force between component and nozzle. This frictional force is limited by the suction surface of the nozzle and the coefficient of friction between component and nozzle. In practice, the maximum allowable acceleration between component and nozzle is limited to a nominal 50m/s2. Therefore, the diameter of a turret needs to become smaller when a higher output is required. Another limiting factor is the maximum allowable acceleration force on the mounted components. Here, a maximum value between 7 to 10m/s2 is realistic.
There will be three time-determining cycles in the timing diagram of the turret: turret-index movement acceleration limitations, PCB-movement acceleration limitations, and feeder-index movement limitations, depending on displacement between two feeders needed for successive picks. The longest of these cycles will be the limiting factor for productivity. This is illustrated in figure 4, which shows that a distance of 30mm between two placement positions on a PCB will cause an output reduction of about 50%. Large, heavy components will also reduce the turret output. This is the reason for placing large components at the end of the runs, after all smaller components have been placed.
Single-gantry concept: the graphs for this concept show a similar shape to those for concept X-1, but with considerably lower output. For example, in this case the output flattens out at approximately 9000cph with 10 heads. Dual-gantry concept: productivity of a dual-gantry machine is approximately twice that of a single-gantry placer. In reality, it will be slightly less due to collision avoidance between the two gantries. A typical timing diagram of a dual-gantry system is shown in figure 5. A balance between the movement of the gantries is needed to avoid collision, (Twait), causing a slight output reduction.
Collect & place concepts: in figure 6, a flattening in productivity can again be observed as a function of an increasing number of pipettes on the collector wheel. In reality, a maximum of 12 pipettes is possible.
MPP concept: here, the timing model can be compared with that of the single-gantry with only one head per gantry. Since the picking range is limited to 6 to 10 feeders per robot, the travel distances between pick & place areas will also be limited, with a positive influence on productivity. Time needed for PCB run-in/run-out and alignment do not contribute to the total cycle time, since these actions can be done in parallel with pick & place. Therefore, an output of 5 to 7kph per robot is realistic.
Total machine productivity is the output per module multiplied by the number of robots installed per machine. This modular set-up enables the user to phase investment, to match the number of placement robots in a machine to the required production capacity. Modular- ity has a positive influence on the CoP of this machine concept. This is illustrated in figure 7. The reason for the lower CoP of the MPP, compared to turret and collect & place concepts is that the investment for MPP consists of two parts. There is a fixed portion consisting of machine base, PCB transport, machine controller and vision module, and a variable portion consisting of placement robots, feeders and nozzles. With the MPP concept it is possible to increase both investment and machine output in small increments, leading to a flat CoP curve as illustrated in figure 7. Other concepts, such as turret and collect & place, can increase the machine productivity in large steps only (whole machine increments), leading to higher CoP (saw-tooth curves in figure 7). It can also be seen that the single and dual-gantry Concepts are preferred when a lower output – approximately 35kcph and below – is required, while turret, collect & place and MPP concepts show better CoP performance above this cross-over point.
An outlook
Machine concepts where the board is not moving during placement are preferred because: potentially better placement accuracy is achievable as no accelerations act on mounted components. It is expected that placement accuracy will continue to become a more decisive factor due to increasing miniaturization in electronics. And there is higher output attainable, since are no limitations due to possible shift of mounted components. Machines matching this requirement are single and dual-gantry, collect & place and MPP concepts.
Machine concepts with a modular structure offer the best match between productivity and investment. Low-cost machines with limited output, for example single and dual-gantry, offer this modularity, but this can lead to a large number of placement machines in line, requiring greater floorspace and more operators. Therefore, the MPP concept delivers lowest CoP for outputs in excess of 35kcph. Modular machines also offer highest availability due to low MTTR.
Machines with single-side feeding and operation at the same side are preferred because they minimize line floorspace and reduce the number of operators. In an optimal concept attention must be paid to optimization of productivity by minimum travel distances, to performing auxiliary actions such as PCB run-in/run-out and vision processing in parallel with placement, thus requiring no additional time. Finally, modular machines offer best upgradeability opportunities for the future.
ZUSAMMENFASSUNG
Keine Frage, im Angebot sind eine Fülle von höchst unterschiedlichen SMD-Bestückautomaten. Die Übersicht über die Vor- und Nachteile für bestimmte Applikationsbereiche ist folglich nicht leicht. Neben den oft recht unterschiedlichen Ausführungen im Detail sind es vor allem die verschiedenen Maschinenkonzepte, die bereits das bevorzugte Anwendungsgebiet vorgeben. Hier geht es darum, die unterschiedlichen Placement-Konzepte zu charakterisieren und damit die für sie speziell geeigneten Einsatzschwerpunkte in der SMT-Baugruppenfertigung zu definieren.
RÉSUMÉ
C’est clair: la gamme comprend une profusion de robots d’équipement SMD extrêmement différents. En conséquence, la vue d’ensemble sur les avantages et les inconvénients pour des secteurs d’application précis n’est pas facile. En dehors des modèles souvent très différents dans le détail, ce sont surtout les nombreux concepts de machines qui fixent à l’avance le domaine d’application privilégié. Il s’agit de caractériser les divers concepts de placement et de définir le noyau des interventions adaptées à vos besoins dans la production de modules SMT.
SOMMARIO
Non c’è dubbio, in offerta sono disponibili un mare di robot di connessione SMD molto svariati. Di conseguenza non è facile ottenere una chiara visione d’insieme di vantaggi e svantaggi in determinati campi d’applicazione. Oltre alle realizzazioni, molto spesso differenti al dettaglio, sono soprattutto i vari concetti di macchine che determinano sin dall’inizio il campo d’applicazione preferito. Qui si tratta di caratterizzare i vari concetti di Placement e con ciò di definire i rispettivi punti fondamentali d’applicazione nella produzione di gruppi costruttivi.
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Titelbild EPP EUROPE Electronics Production and Test 11
Issue
11.2023
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