Metal working – Means to assemble or disassemble – With signal – scale – illuminator – or optical viewer
Patent
1995-10-12
1997-12-30
Vo, Peter
Metal working
Means to assemble or disassemble
With signal, scale, illuminator, or optical viewer
29741, 29833, B23P 1904, H05K 1304, H05K 1308
Patent
active
057016615
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
With the enhancement of component density on the printed circuit board and the increasing number of connections 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 active 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, in 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.
An example of such a system is known from the IBM Technical Disclosure Bulletin, Vol. 34, No. 10B, Mar. 1992, New York U.S., pages 4-6; "Precision superposition component placement tool for end-point sensing".
SUMMARY OF THE INVENTION
An essential feature of the invention is the simultaneity of observation, free from parallax, with a large 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 one or more high resolution cameras, or in combination with a camera) of both the object (the foot print of the leads of the multileaded 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 semitransparent 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 two systems for mutually positioning.
According to a first embodiment the centre of the field of view is selected to be observed at an angle of .pi./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 semitransparent 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 .pi./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 combination 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) of the component. In tha
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IBM Technical Disclosure Bulletin, vol. 34, No. 10 B, Mar. 1992, pp. 4-6.
Gordon David P.
Vo Peter
LandOfFree
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