Line scan camera

Details Line scan camera Line scan camera

1999-09-30

2001-09-18

Schuberg, Darren (Department: 2872)

Optical waveguides

With optical coupler

Particular coupling structure

C385S119000, C385S120000, C356S121000, C250S227110

Reexamination Certificate

active

06292608

ABSTRACT:

This invention relates to a line scan apparatus for scanning an object line by line to build up a digitised image. In particular, the object may be a substrate containing electronic components. Furthermore, the apparatus may form part of an automated optical inspection system for detecting common manufacturing process faults which occur during circuit board assembly.
BACKGROUND OF THE INVENTION
A typical line scan camera comprises a series of discrete optical sensors arranged in a line to form a linear sensor array. An optical arrangement, also forming part of the line scan camera, allows the linear array of optical sensors to form a one dimensional image of a line portion of the object. By scanning the camera relative to the object, so that the line portion moves across the object, a two dimensional image of the object is constructed.
One familiar application of a line scan camera is in the field of scanners. A flat bed scanner, for example, scans a sheet of paper and converts print on the sheet into a digitised image for transmitting to a computer. In this particular application, the object distance is specified by the structure of the scanner since the paper is assumed to be substantially planar and positioned on the “lat bed”. Known flat bed scanners take advantage of this feature and employ optical arrangements consisting of a linear array of self focussing (SELFOC™) lenses which relay the content of the paper at a known distance onto a linear array of image sensors. These self focussing lenses are each formed from a short section of optical fibre which are designed to operate at a relatively short, fixed working distance from the object. The self focussing lenses have the advantage of being relatively cheap to manufacture.
This invention relates to the application of a line scan camera in the field of circuit board inspection systems. The aim of these systems is to produce a two dimensional image of the inspected circuit board as viewed directly from above the circuit board, i.e., a plan view. This plan view is then examined to establish if the circuit board has been subject to any manufacturing process faults. Common faults include:
Misaligned parts
Missing components
Excess solder and solder bridges
Rotated and reversed polarity parts
Wrong component size
Component leads that are lifted or bent
A first requirement of these systems is the ability to image three dimensional components mounted on the circuit board surface. As such, the optical arrangement used in these systems is characterised by having a relatively large depth of field. This allows objects raised from the substrate surface and objects on the substrate surface to be imaged in focus during a single scanning operation. A second requirement of these systems is the ability to produce a spatially accurate image of the circuit board to enable repeatable inspection results. As such, the optical arrangement used in these systems is also optimised to minimise imaging errors resulting from geometrical distortions such as lens field curvature, perspective distortion and parallax errors.
FIG. 1
shows a basic layout of an optical arrangement for a circuit board inspection system which is designed to have a relatively large scanning area together with a relatively large depth of field. The apparatus
10
comprises a conventional imaging lens
11
, an aperture
12
, and an image sensor array
13
. The apparatus
10
scans across the substrate or circuit board
15
in the direction of arrow B, imaging a moving line segment A—A. A drawback with this arrangement is that at the ends of the line segment A—A, the lines of sight, represented by the rays R in
FIG. 1
, are not perpendicular to the substrate surface. This results in parallax errors.
FIG. 2
illustrates in enlarged detail such a parallax error occurring in the end region of the line segment A—A. The ray labelled R is a line of sight to the optical arrangement
10
, and is incident at an angle &agr;to the plane of the substrate surface. Because the line of sight is riot perpendicular to the substrate surface, the relatively tall component
22
causes the hashed region S to be in shadow. Consequently, a portion of a neighbouring component
24
is obscured so that imaging information of this portion cannot be captured by the inspection system.
The inspection system addresses the problem of parallax by using telecentric lenses in place of the conventional imaging lens
11
and by limiting the scanning area. Unfortunately, this increases the cost of the system and slows down scanning time of the complete circuit board which is undesirable.
SUMMARY OF THE INVENTION
The present invention provides a line scanning apparatus with improved telecentricity whilst maintaining sufficient depth of field. This invention allows objects at a range of distances, such as different heights of components on a circuit board, to remain focussed during a scanning operation. The present invention may also provide an optical arrangement that reduces the object to generate an image which may be sensed by an array of optical sensors.
In accordance with the present invention there is provided a line scan apparatus which comprises a linear array of objective lenses which pair up with an associated linear array of optical sensors. The apparatus further comprises a bundle of tapering optical fibres which optically couple the pairs of objective lenses and optical sensors. Each fibre is arranged such that it optically couples at a wide end to one of the objective lens, and at a narrow end to one of the optical sensor. In operation, the linear array of objective lenses are scanned across the object, with the linear array of optical sensors capturing a one dimensional image line by line to build up a two dimensional image of the object.
Alternatively, in accordance with the present invention there is provided a line scan apparatus comprising a plurality of pixel detecting elements, each pixel detecting element including an objective lens, an optical sensor, and a tapering optical fibre arranged such that a wide end optically couples with the objective lens and a narrow end optically couples with the optical sensor. These pixel detecting elements are arranged such that the optical lenses are disposed in a linear array so that together the pixel detecting elements form a line detecting element which may be scanned across the object.
The tapering fibres have the advantageous effect of redistributing each element in the optical sensor array directly above the patch of the object (pixel) to be viewed. Thus, each element of the optical sensor array is aligned perpendicularly with a lens and a patch of the object to be imaged. In this way, geometrical distortion such as field curvature and perspective distortion can be minimised.
The tapering fibres also have the advantageous effect of enlarging the sensing area of each element in the optical sensor. The lens associated with this enlarged area provides a depth of field so that the apparatus may operate at a range of working distances. Accordingly, the apparatus is able to image three dimensional objects on a surface such as components on a circuit board substrate.
Preferably, the line scan apparatus includes a mask disposed adjacent the linear array of lenses. This mask defines a plurality of light transmitting regions aligned with each of the lenses. Suitably, the line scan apparatus includes a second mask disposed adjacent the linear array of lenses on the opposite side of the lens array from the first mask.
The extent and scope of the present invention is defined in the appended Claims to which reference should now be made.


REFERENCES:
patent: 3918814 (1975-11-01), Weiser
patent: 5493391 (1996-02-01), Neal et al.
patent: 6064067 (2000-05-01), Zhao et al.

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