1. Field of the Invention
The invention relates to a vacuum pipette for gripping electrical components by suction. The vacuum pipette is configured for mounting in an exchangeable manner onto a shaft of a placement head of an automatic placement machine, and the component being grippable by suction onto an end side of the vacuum pipette, where the end side is remote from the shaft.
2. Description of the Related Art
A vacuum pipette of this type has been disclosed, for example, by International Patent Document WO 98/33369 A. Accordingly, this vacuum pipette has a central vacuum channel which merges with an end-side suction opening which is provided with a filter grid. The latter is intended to prevent small components from getting sucked into the suction channel. The correspondingly narrow passage openings of the filter grid reduce the suction cross section to such a great extent that the suction force no longer suffices to reliably hold larger components.
The components are provided closely spaced apart in belts, for example. There is a risk, particularly in the case of small components, that during collection, components from adjacent pockets will be gripped by suction. In order to eliminate this risk, the suction grid must be kept correspondingly narrow. The pitch spacing between the grid webs must be kept as large as possible so that the cross section is not excessively reduced by the grid webs. But this creates a risk that components having a very small cross section will tip over into one of the passage openings and be sucked through edgewise. Since the cross section of the suction channel is relatively large, the component may be drawn unimpeded into the control and generator system for the vacuum and cause functional disturbances there, in particular through accumulations.
Component miniaturization currently extends down to dimensions of 0.25×0.25×0.5 mm. Correspondingly fine passage openings can no longer be realized by injection molding, so that the sucking-in risk is particularly high here. An erroneous vacuum interrogation can cause the vacuum pipette without component to be contaminated with solder paste of the printed circuit board, resulting in a reduction of the suction cross section of the filter grid, and a vacuum interrogation leads to evaluation errors. The different suction cross sections of the filter grids of different types of pipettes result in very different vacuum threshold values for the detection of the presence of a component, and thus the interrogation reliability is correspondingly impaired.
Furthermore, Japanese patent document JP 10-242694 A discloses a placement head with a shaft having a holder for the vacuum pipette fitted to its free end. The suction channel of the vacuum pipette is formed such that it is continuous over its whole area. Also, a filter device for the suction air is placed onto the shaft end that prevents sucked-in particles from passing into the interior of the placement head. Due to the labyrinthine flow profile, sucked-in particles can accumulate between the shaft and the center sleeve and, with a relatively long use of the vacuum pipette, constrict the flow cross section to such a great extent that the vacuum interrogation is disturbed. The filter device itself is part of the holder and must be exchanged when a specific degree of blockage has been reached.
In a similar manner, according to Japanese patent document JP 2-170600 A, a filter cylinder with an extended filter grid is placed onto a tubular holder of a placement head, the exchangeable vacuum pipette configured for mounting onto the free end of the filter cylinder. The filter cylinder constitutes, in the extension of the holder, a separate intermediate element that requires an additional mounting step with an additional coupling point, reduces the installation accuracy of the vacuum pipette, and increases the structural height. The increased distance between the pipette tip and the holder additionally reduces the stiffness of the system, which, under the influence of the high accelerations of the positioning movements, leads to a reduction of the positioning accuracy.