Automatic optical inspection systems enable the efficient and cost effective monitoring of printed circuit boards during manufacture. Large circuit board formats and high throughput common in today's low cost manufacturing environment suggest the use of optical line scan systems. Good imaging over large fields is crucial for the operation of such line scan systems.
The use of linear sensor arrays such as contact image sensors (CIS) is known in the technically different fields of flatbed scanners and photocopiers. Here the surface being inspected is flat such as a piece of paper or photograph, not a printed circuit board, which has components of varying heights. Furthermore, current CIS imaging systems are designed for working at very close distances from the target surface. This is in contrast to the needs of an automated optical inspection system where the surface under test must be in the order of 30 mm-40 mm from the imaging lens assembly in order to allow for the height of components placed on the surface.
Currently, line scan systems include a camera lens that images the work surface of a printed circuit board onto a linear sensor array. The work surface is typically 300 mm in width, which is significantly larger than the camera lens diameter. Thus, light from the work piece edge strikes the camera lens at angles as large as thirty degrees. Such large angles of incidence give rise to parallax, which causes features near the edge of the work surface to appear distorted in the image plane. Also, features can protrude as high as 10 mm from the surface. Such protrusions obscure other small features present on the work surface from being detected by the linear sensor array.
In accordance with the invention, the longitudinal distance between the object plane and image plane is reduced as a result of the use of just one lens set, thereby reducing sensitivity to lateral fabrication errors.
These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:
Reference will now be made in detail to the present embodiments in accordance with the invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures.
The object distance can be altered according to the needs of the particular application. Automatic optical inspection systems used to line scan printed circuit boards need at least 40 mm between the object surface and the imaging lens assembly in order to allow for the height of components on the object surface. In the present embodiment, the object distance is approximately 67 mm so as to ensure adequate imaging with the optics used.
The number of tube-like baffles and thus the number of lens assemblies and image sensors making up the single lens array depend on the size of the work surface being scanned. Automatic optical inspection systems used to line scan printed circuit boards typically span a width of 300 mm in order to image the entire circuit board. In the present embodiment, 42 tube-like baffles 11a through 11pp are used to span a width of 300 mm.
Each image segment 36a through 36d is inverted and de-magnified. The demagnification allows each image segment to be contained within a baffle without obscuration. Also, each image is undersized relative to the linear image sensor 35a through 35d and, accordingly, the lateral image position can vary somewhat and still be entirely captured by the linear image sensor 35a through 35d. Each lens set design can be optimized and additional lenses added for different magnification values. In the present embodiment, there is one doublet lens set 34a through 34d in each baffle.
The baffles 11a through 11d further isolate each lens set 34a through 34d from adjacent lenses and reduce the field of view of each lens, thereby reducing off axis optical aberrations and thus enabling good imaging performance. Each lens set design can be optimized and additional lenses added for more aberration reduction or different fields of views. In the present embodiment, the field of view of each lens set 34a through 34d is approximately 5 degrees.
The length of the baffles 11a through 11d can also be altered to suit the requirements of the focal length and optimize imaging. In the present embodiment, the length of each baffle 11a through 11d is 54.52 mm.
The distance between each baffle 11a through 11d can also be altered according to the pixel spacing of the image sensors in order to optimize imaging. In the present embodiment, there is 1 mm spacing between individual baffle 11a through 11d.
The baffles 11a through 11d can be manufactured from any absorbing material with structural strength, such as anodized aluminum.
As described earlier, each lens set 34 is disposed at the end of its respective baffle closest to the object plane. The lens set 34 is so arranged to provide inverted and de-magnified imaging. It may be held in place by use of an adhesive or other suitable means. Additional lenses may be added to each lens set as appropriate, or to reduce aberration.
The lighting of the header assembly comprises a near on-axis LED array 72 disposed under the baffle array assembly 10 and a further LED array 74 disposed at approximately 45 degrees, in the present embodiment, with respect to the object plane. This results in both on-axis illumination as well as diffuse illumination resulting in better feature detection of components on the object surface.
The image segments formed by the single lens array are stitched together by using a featureless substrate that reflects light with uniform intensity into the optical imaging system. Dark pixels in the image sensors are electronically monitored and referenced so as to be eliminated in future images allowing the remaining pixels to be stitched together.
According to the present invention as described above, by using just one lens set, the longitudinal distance between the object plane and image plane is reduced, and, therefore, sensitivity to lateral fabrication errors is reduced. By isolating each lens set, off axis aberrations are reduced thus enabling good imaging performance. By de-magnifying a received image, obscuration is reduced.
Although a few embodiments in accordance with the invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.