With the enhancement of component density on the printed circuit board and the increasing number of connect¬ ions per component, the rejects and cost of rejects will increase if process control remains unaltered. In parallel therewith there is a tendency to place the passive and activ components in separate process steps, first the passive components in a reflow oven/solder bath and, subsequently, the active components with a system as described. With state-of-the-art technology, removing a component does not cause any problem, but the problem arises when the component is to be positioned or placed back respectively, i a controlled manner; the leads of the component are to coincide with minimum clearance with the corresponding pads on the printed circuit board.

Positioning systems to be used if the problems outlined above occur are known, it is true, but always have the drawback of having an extremely complex structure and being rather costly.

In contrast, the present invention provides a system characterized by the features as defined in Claim 1. An essential feature of the invention is the simultaneity of observation, free from parallax, with a larg field of view and a sufficiently high resolution (the human eye in combination with a magnifier and/or a binocular microscope, or in combination with a camera) of both the object (= the foot print of the leads of the multi-leaded component) and the pattern of pads to which the leads are to be fixed.

For considerations of robustness the semi transparent mirror used to this end in the invention is evaporated on a flat transparent substrate which has a finite thickness. The space behind the semi transparent mirror is called the image space; the space in front of the mirror, the observation side and component side, is called the object space. The mirror substrate having a finite thickness leads to a displacement of the image space relative to the object space. The inventive idea is the compensation of this displacement by the introduction in the object space of a compensation plate having identical optical properties. The invention provides several embodiments as claimed in Claims 2 and 3.

According to a first embodiment the centre of the field of view is selected to be observed at an angle of π/2 . Depending on ergonomics and desired field of view, object plane and image plane will form a 20-to-30-degree angle relative to the beam splitter. The semi transparent mirror is sandwiched between substrate and compensation plate. Deviations of the printed circuit board or footprint from their corresponding surfaces have a marginal effect on the misalignment after unison. In the case of high-pitch components, an alternative method of referencing will have to be implemented. According to a second embodiment a supporting plate is chosen which is equidistant to the contact side of the printed circuit board holder, on the one hand, and, on the other hand, to the upper surface of the compensation plate which upper surface likewise functions as the contact area for the foot print of the component leads, all this in combination with the observation of the lead pattern and the print at an angle differing from π/2. If the condition of equidistance is not fulfilled, this will lead to parallax; object and image observed at the same angle no longer coincide after being united (see course of process) .

The choice between said first embodiment and said

second embodiment is also made on the basis of the surface structure of the pad carrying members. If they are not flat, for example, with warped printed circuit boards (in combin¬ ation with high-pitch) , it may happen that the printed circuit board is not located in the theoretical plane of the contact area of the printed board holder, as a result of which the leads do not correspond to the footprint (print) o the component. In that event the first embodiment is strongl favoured. The second embodiment presents favourable possibil ities for high-pitch components on flat substrates.

Side-effects such as the human eye serving as a detector also play an important role in this invention. In the first embodiment the object/image plane is perpendicular to the axis of observation. A binocular microscope can be applied and is probably desired for the observation of high- pitch components. In the second embodiment the object/image plane is at an angle of approximately π/3 to the axis of observation. The focusing ability of the human eye enables the observer to observe sharply every detail within the whol field of view. People having normal and presbyopic eyesight observe magnified versions of the details through a low- magnification magnifier.

The resolution of the human eye, for that matter, is dependent on the spacial frequency and the contrast between the objects to be observed. Ambient lighting will always cause more light to fall on the pad carrying member than on the under side of the component. A dedicated addit¬ ional lighting system provides the necessary contrast betwee the pads, on the one hand, and the footprint of the leads, o the other.

The invention will be further explained with reference to the drawings 1 and 2 in which the two embodiments for the optical system will be shown in a diagrammatic and simplified manner in connection with the positioning and locating arrangement. In these drawings:

Figure 1 gives a diagrammatic representation of a

cross-section of a system 1 according to a first solution. The displacement of the component towards the pad carrying member is effected by a rotation around axis 1. The pad side of the pad carrying member is fixed in plane 2, the imaginary plane of the footprint of component 4.1 lies in plane 4, the semi transparent mirror lies in plane 3.2; the three planes 2, 3.2 and 4 meet in axis 1 while the angles between the planes are mutually equal. The beam splitter assembly 3 comprises the semi transparent mirror 3.2, the mirror substrate 3.1 and the compensation plate 3.3. Observation takes place through magnifier 5, arranged in such a way that the optical axis 6 of the lens means is perpendicular to the centre of the field of view i.e. in the centre line 4.4 of the component fixing means. The component 4.1, the component fixing means 4.2 and, if required, the heating means 4.3, are shown in a diagrammatic way. Depending on ergonomics and desired field of view, the object plane and image plane will form an angle of approximately 30 degrees relative to the beam splitter. By displacing the printed circuit board in the directions of X and Y and rotating the component fixing means, the images of the pads and the connecting leads are arranged to be superimposed.

The beam splitter assembly is then reproducibly removed from the path of the component. The component fixing means holding the component is turned onto the printed circuit board in a rotating manner.

Figure 2 represents a system according to a second solution in which the reference plane 8 is the contact plane of the pad carrying member. Perpendicular thereto is the axis of displacement 10.4 of the component. The beam splitter assembly 9 comprises the semi transparent mirror 9.2, the mirror substrate 9.1 and the compensation plate 9.3 the upper surface of which compensation plate functioning as a contact plane for the footprint of the connecting leads of the component 10.1. Mirror substrate and compensation plate should be adjusted to be perfectly in parallel. The angle of

the beam splitter assembly should be adjustable, so that the beam splitter can be adjusted in parallel with the contact plane of the pad carrying member. The beam splitter assembly is height-adjustable so as to satisfy the condition of equidistance of the contact side of the printed board holder to the semi transparent mirror, on the one hand, and to the upper surface of the compensation plate, on the other. With the selected configuration there is achieved that, once the connecting leads have been aligned to the corresponding pads - once the splitter assembly has been removed in a reproducible manner - and the leads have been displaced vertically, these leads will coincide with the corresponding pads. The magnifier 11 with optical axis 6 is adjusted, so that the plane of focus 11.1 lies within the field of view behind the printed circuit side of the pad carrying member.