The invention relates to an illumination module for illuminating an object to be examined, which module comprises an exit window for radiation generated in the module, a module axis which extends perpendicularly to the central part of the exit window, and a number of radiation-generating elements which are arranged in at least two illumination rings, which illumination rings have a midpoint, which is situated on the module axis, and are individually switchable, the radiations of the different illumination rings passing through the exit window at different angles with respect to the module axis.
The invention also relates to an optical device for the observation of an object, which device is provided with such an illumination module, and to an apparatus for placing electronic components on a support, which apparatus is provided with at least one such optical device.
At present, said apparatus is often used to automatically, very accurately and very rapidly place electronic components on a support, such as a printed circuit board (PCB). Said apparatus comprises one or more robots, provided with a suction pipette, and an equally large number of placement stations associated with the relevant robots. The suction pipette receives a component to be placed from a component input device, whereafter the associated robot transports the suction pipette and the component to a position on a support which, at that instant, is situated in the placement station. Before the component can be placed on the support, it has to be identified and inspected for irregularities, and it must be checked whether the component is correctly oriented with respect to, for example, a system of coordinates of the robot. To this end, the placement station is provided with an optical observation device for observing the component held by the suction pipette, which observation device is alternatively referred to as vision module. This device comprises a camera by means of which an image of the component is converted into electric signals which are electronically processed. A good observation device should meet the following requirements: the image must be free of distortion, the observation device should also be capable of imaging large components, the aperture angle of the beam with which the component is illuminated must be relatively small, and, for specific applications, the angle of incidence of the illumination beam on the component must be as large as possible. To properly observe the component, it is necessary for this component to be illuminated in the right way. To achieve this, the observation device comprises an illumination module.
Such an illumination module is described in U.S. Pat. No. 4,893,223. The known illumination module comprises a large number of light-emitting diodes, also referred to as LEDs, which are mounted on the inner surface of a concave plate and arranged in rings or angle sectors. Since the components to be placed by means of the placement machine may demonstrate different shapes and different surface conditions, the illumination module should preferably be programmable, i.e. it should be possible for groups of LEDs, such as the LEDs of a ring or the LEDs of a comer sector, to be switched on individually, so that the angle at which the object is illuminated and/or the part of the object that is illuminated can be adjusted. The illumination module described in U.S. Pat. No. 4,893,223 is used to illuminate the free end, which is provided with a solder ball, of the connection pins of a component, which is commonly referred to as surface mounted device (SMD). The observation device wherein this illumination module is employed, makes it possible to inspect only the end of one pin of the SMD. To inspect a subsequent pin, the observation device and the SMD are moved with respect to each other. Thus, the illumination module described in U.S. Pat. No. 4,893,223 has a small illumination field, and the angle between the beam of the LEDs and the normal to the surface of the SMD is relatively small. The beams supplied by the LEDs have a relatively small aperture angle, of the order of 10xc2x0. The illumination module in accordance with U.S. Pat. No. 4,893,223 is difficult to manufacture because all LEDs must be accurately aligned to make sure that they all illuminate the same small area.
It is an object of the invention to provide a programmable illumination module having a wide range of applications, which can be readily manufactured and has minimal dimensions, and which, in addition, can be adjusted for a large illumination field, and which is capable of supplying a very oblique illumination beam. This illumination module is characterized in that all the radiation-generating elements are provided on a flat plate which extends parallel to the central part of the exit window, and the principal rays of all radiation beams emitted by the radiation-generating elements extend parallel to the module axis.
The fact that this module is capable of supplying a very oblique illumination beam means that selected radiation, originating, for example, from the outermost illumination ring, includes a very small angle, of for example at least 6xc2x0, with the illuminated surface of the object.
The provision, with great accuracy, of the radiation-generating elements on a flat plate is simpler than the provision, with the same accuracy, of said elements on a concave plate. The flat plate may be embodied so as to be a printed circuit board with electric conductors via which the radiation-generating elements are fed.
A preferred embodiment of the illumination module is further characterized in that opposite each illumination ring there is provided a radiation guide which, in a plane at right angles to the module axis, is ring-shaped in section, the entrance plane of said radiation guide being situated opposite the associated illumination ring, and the shape of the light guide is exit plane situated opposite the entrance plane being adapted to the desired direction of the radiation to be supplied by the associated illumination ring.
The use of these radiation guides enables the optimum illumination direction for a specific application to be achieved. The radiation guide associated with a certain illumination ring also makes sure that the radiation from said ring is not mixed with radiation from other illumination rings. Since the radiation from an illumination ring is confined to the associated radiation guide until it exits via the exit plane of this guide, the beams of the radiation-generating elements can have a relatively large aperture angle, for example in the range from 20xc2x0 to 30xc2x0. In addition, the radiation guide ensures, via total internal reflections, that the radiation beam leaving the radiation guide demonstrates a certain degree of homogeneity. In this regard, the length of the radiation guide and the aperture angle of the LEDs of the associated illumination ring play a part.
To further increase the homogeneity, the illumination module is preferably further characterized in that the exit surface of all radiation guides, with the exception of the outermost radiation guide, is provided with a diffuse body the surface of which facing away from the radiation guides has such a shape that the radiation beam exiting from a radiation guide through this surface has a desired direction.
The radiation guides may be filled with air. Preferably, however, the illumination module is characterized in that the radiation guides, possibly with the exception of the innermost radiation guide, are filled with a transparent synthetic resin.
In accordance with a further characteristic, the synthetic resin is polycarbonate or polymethyl methacrylate. These synthetic resins, which are also known by the abbreviations PC and PMMA, respectively, are highly transparent and of good optical quality.
Preferably, the illumination module is characterized in that scattering particles are integrated in the synthetic resins.
If so, these synthetic resins have a diffuse effect, and the relevant radiation guides can contribute to rendering the radiation leaving the illumination module uniform. The different radiation guides may exhibit different degrees of diffusion.
Under certain conditions it may be attractive to also fill the innermost radiation guide with a synthetic resin.
In accordance with a further characteristic of the illumination module, the outermost radiation guide has a reflective surface for reflecting radiation towards the exit plane of this radiation guide, which reflective surface consists of two parts which enclose an obtuse angle with each other.
Said reflective surface divides the radiation beam, which propagates through the radiation guide and which has a certain intensity distribution, into two sub-beams which are positioned at the location of the object such that the original intensity distribution is lost and a more uniform distribution is obtained. In this manner, also the radiation beam supplied by the outermost radiation guide obtains a sufficiently uniform distribution.
An embodiment of the illumination module which is attractive for the observation device of a component placement machine is characterized in that it comprises three illumination rings.
This illumination module is still compact enough to be fitted in the placement machine, and it also enables good illumination, with sufficient intensity and at different angles, of the components to be achieved.
For the radiation-generating elements use can be made of different elements. For example, use can be made of diode lasers. Diode lasers emit a relatively narrow beam of high intensity and the radiation thereof can be readily oriented in the desired direction. For the radiation-generating elements use can also be made of luminescing diodes, provided the elements do not have to be switched rapidly. A preferred embodiment of the illumination module is however characterized in that the radiation-generating elements are formed by LEDs.
These LEDs are inexpensive and available in different versions as regards radiation intensity, dimension and aperture angle.
Dependent upon the application of the illumination module, one or more illumination rings may comprise diode lasers and the other illumination rings may comprise LEDs. It is alternatively possible that one and the same illumination ring comprises a number of diode lasers as well as a number of LEDs.
The illumination module is preferably further characterized in that at least one of the illumination rings comprises at least two types of LEDs, which emit radiation of different wavelengths, and LEDs of the first type alternating with LEDs of the second type in said illumination ring.
For example, all illumination rings may comprise alternately red, green and blue LEDs which can be switched on in different combinations. As a result, the color of the illumination can be optimally adapted to the color of the object to be examined and/or the background thereof. The contrast of the observation device can thus be improved considerably, also if the camera of the observation device is a monochrome camera. It is alternatively possible to use LEDs of only two different colors, or provide LEDs of different colors in only one illumination ring, or provide only one type of LED in one and the same illumination ring; however, the LEDs of the different illumination rings are of a different kind.
An illumination module which is particularly suited for use in a component placement machine may further be characterized in that, within the central illumination ring, a transparent plate is provided which encloses an acute angle with the module axis.
By means of this plate, it can be precluded that a component which has fallen from the suction pipette situated above the illumination module can fall on the camera situated below the module.
This illumination module may be further characterized in that an optical filter is provided on the plate.
As a result, this plate may fulfill a second function. The filter may be a color filter by means of which it is precluded that ambient radiation, i.e. radiation not originating from the illumination module, can reach the camera. The filter may also be a grey filter by means of which the intensity of the illumination radiation originating from the component can be adapted to the sensitivity of the camera. This enables the illumination module to be used in combination with different cameras, or to further increase the intensity of the illumination radiation to a level above that of the ambient light so as to reduce the influence of the ambient light on the observation.
Another embodiment of the illumination module is characterized in that, within the central illumination ring, a semi-transparent plate is provided at an angle of, in principle, 45xc2x0 with respect to the module axis, and in that an additional radiation source is provided which is arranged such that radiation from this source is incident on the semi-transparent plate and reflected thereby towards the exit window of the illumination module.
As the rays of the additional radiation source enclose angles in the range from 0xc2x0 to, for example 20xc2x0 with the module axis, the module can now provide a bright-field illumination for the object in addition to the dark-field illumination which is supplied by the radiation-generating elements of the illumination rings. In the case of a bright-field, or normal, illumination, radiation which is specularly reflected by the object is captured by the imaging optics and the camera, and the object is imaged as a bright object. Details of the object which scatter the illumination radiation, or reflect it outside the imaging optics, can now be observed against a bright background. In the case of a dark field, or oblique, illumination, the object reflects the illumination radiation outside the imaging optics. The scattering details of the object reflect the radiation in the imaging optics and the camera and are brightly imaged against a dark background.
The last-mentioned embodiment is preferably further characterized in that the additional radiation source is formed by a number of additional radiation-generating elements arranged along at least one row, in combination with a radiation-conducting plate the entrance face of which is directed towards the additional radiation-generating elements, and the exit face of which is directed towards the semi-transparent plate.
Preferably, the additional radiation-generating elements are arranged in at least one circle segment at one side of the semi-transparent plate.
In this case, the bright radiation field is shaped so as to be in the form of a ring segment the length of which is determined by the position of the midpoint of the semi-circle with respect to the center of the transparent plate.
In order to obtain a more two-dimensional foreground illumination, the illumination module is preferably further characterized in that a second radiation-guiding plate is arranged against the exit face of the first radiation-guiding plate at an angle of, in principle, 90xc2x0 with respect to the first radiation-guiding plate, and in that a reflecting roof prism is situated at the location of the transition from the first radiation-guiding plate to the second radiation-guiding plate, so as to reflect, in two opposite directions in said plate, two halves of radiation exiting from the first radiation-guiding plate.
As said radiation halves are subject to total internal reflection in the second radiation guide, the beam incident on the semi-transparent plate is elongated in a direction transverse to the direction of the row of additional radiation-generating elements, and this beam additionally demonstrates a more uniform intensity distribution. The roof prism is embodied in such a manner, i.e. the angles thereof are chosen to be such that a part of the radiation leaving the first radiation guide is passed by the prism, so that radiation also exits through that, central, part of the exit plane of the second radiation-guiding plate, which is situated at the level of the roof prism.
In accordance with a further characteristic of this illumination module, the roof prism is provided in the second radiation-guiding plate.
Preferably, however, the illumination module is alternatively characterized in that the roof prism forms part of the first radiation-guiding plate.
If so, the prism can be more readily provided and, in addition, the optical properties of the second radiation-guiding plate are not adversely affected.
Preferably, this illumination module is further characterized in that the second radiation-guiding plate takes the form of a cylinder segment whose cylinder axis extends parallel to the module axis.
The outer ray of the second radiation-guiding plate can then be chosen to be such that the upper part of this plate can be slid into the innermost illumination ring, so that, also if the additional radiation-generating elements for the bright-field illumination are employed, the illumination module can remain compact.
The additional, radiation-generating elements can alternatively be formed by at least two kinds of elements which emit radiation of different wavelengths, whereby, in a row, elements of the first kind alternate with elements of the other kind(s). In this case, also the color of the bright-field illumination can be adjusted, which may yield a higher contrast.
The invention also relates to an observation device for observing an object, which observation device is provided with an illumination system for illuminating the object, and with a camera for receiving an image of the object. The observation device in accordance with the invention is characterized in that the illumination system is an illumination module as described hereinabove.
Finally, the invention also relates to a component-placement machine for placing electronic components on a support, which machine is provided with:
a frame;
at least one robot;
a transport system for transporting supports;
a placement head for each robot for placing components on a support, which placement head is secured to an arm of the robot, and
component-positioning means for each robot for positioning a component which is held by the placement head associated with the robot.
This placement machine is characterized in that the observation device is a device as described hereinabove.