Patent Application
20080141795
1. Field of the Invention
The invention relates to a device for visual inspection of micro-surfaces. More particularly, this invention relates to a device for visually inspecting solder connections for microelectronic components, weld joints, braze joints, machined surfaces, castings, biological samples, crystal structures, molded components, fibers, wear analysis, finishes and coatings.
2. Description of the Prior Art
A ball grid array (BGA) is a chip package that is assembled on a printed circuit board (PCB). Conductive metal spheres (balls), arranged in a grid pattern on the underside of the BGA, form the electrical connections between the electronic components on the BGA and the PCB. The BGA is placed on the PCB, the pads of which have been coated with solder paste, and then heated to the point where the solder paste becomes semi-liquid. The board is then cooled, at which point the solder forms rigid current-conducting connections between the various components on the chip package and the PCB.
Many features or characteristics of the ball grid array, such as ball dimensions, count, position, diameter, and body shape are inspected, in order to ensure that bridging between balls or other faults that result in a failed or faulty electrical connection are not likely to occur. The solder joints are difficult to inspect because they are located between the chip package substrate and the PCB, where they are partially obscured from direct viewing and where ambient light is obscured by the BGA itself.
Conventional tools for inspecting sphere shaped solder joints are known. Typically additional lighting is used to illuminate the area to be inspected with front and back lighting. In the present art, cameras used for inspection are high-lux, expensive, bulky, and also require additional lighting. The additional lighting adds to the cost and bulk of the tool, and also adds to the difficulties of inspecting the solder joints. The solder joints are silver and reflect the light. This causes an over-bright spot that can obscure surface details on the joints. Furthermore, additional lighting devices may be burdensome, because it is not always possible to aim the light directly into the area between the BGA substrate and the PCB. Current handheld video micro inspection units weigh in excess of 2 pounds, with a prism that is mechanically unprotected. It is difficult to hold this type of unit manually in a vertical orientation in close proximity to a printed circuit board, particularly without touching the board. Typically, the narrow edge of the prism rests on the PCB, whereby tremendous pressure is exerted on it, pressures on the order of 80,000 psi. The result is that the prism frequently fractures or shatters. In an effort to overcome these disadvantages, other inspection devices have been developed that rely on the use of x-rays to inspect joints. These devices are costly and difficult to use, because of the precautions required when working with x-ray technology.
What is needed, therefore, is a tool for inspecting microelectronic components and assemblies, particularly solder joints on BGAs, that is easy to use, light weight, inexpensive, and enables close inspection of micro-surfaces. What is further needed is such a tool that requires only minimal ambient light.
The present invention is an inspection tool that inspects a miniature or microscopic object under ambient light, and simultaneously provides a view of the object from two different angles. The inspection tool is particularly useful for inspecting microelectronic components and connections, but is also suitable for close inspection of myriad types of objects, surfaces, spaces, etc., particularly in poorly illuminated environments.
The inspection tool according to the invention comprises a low-lux video camera and a data transmission means for transmitting data to a visual display device. The low lux sensitivity of the camera allows the inspector or operator to view microelectronic components and connections in ambient light, even when the ambient light is poor. The inspection tool according to the invention eliminates the need for an external light source to provide back and/or front lighting of the object being inspected. The low-lux video camera also works with the near-infrared (NIR) spectrum and is able to provide images for detailed viewing that are not possible with standard inspection-tool cameras.
The images projected onto the image sensor of the camera are transmitted via a data transfer cable or wireless transmission to a visual display device, which presents the images in greatly magnified dimensions for easy viewing and inspection. The degree of magnification is determined by the size and resolution of the visual display device.
The inspection tool is, for most applications, a handheld tool that the operator maneuvers to view particular objects. For this purpose, a housing for the inspection tool is formed as a holder or gripper, that is easily held in the hand and light enough to be readily maneuverable. Depending on the intended use of the inspection tool, a prism may be mounted on the tool to allow the camera to capture an image that is at some predetermined angle relative to the focal axis of the camera lens. This ability to view the object from two angles increases the reliability of the inspection. It is within the scope of this invention, however, to encompass an inspection tool that is mounted in a fixture and manipulated by software-controlled location means, so that the operator is viewing the images on the display, but is not manually manipulating the tool.
A particularly well-suited use of the inspection tool according to the invention is to inspect the integrity of solder joints on an assembled PCB. A primary example is the inspection of sphere shaped solder joints on a BGA. These solder joints form a grid between the BGA and the PCB and provide the electrical connection between components on the BGA with components or conductors on the PCB. The purpose of inspection is to ascertain that the shape and surface of the joints are properly formed and dimensioned, that no bridges have formed between adjacent joints, and that other faults are not present. The operator maneuvers the inspection tool such that the objects to be inspected are presented on the visual display device. Objects that are in shadow or are in poorly illuminated areas are clearly imaged by the low-lux video camera and appear clearly on the visual display device.
The present invention is described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. The drawings are not necessarily drawn to scale.
The present invention will now be described in detail with reference to the accompanying drawings, in which the preferred embodiments of the invention are shown. This invention should not, however, be construed as limited to the embodiments set forth herein; rather, they are provided so that this disclosure will be complete and will fully convey the scope of the invention to those skilled in the art.
The camera 102 is capable of capturing images in the normally visible range, as well as in the Near-Infrared (NIR) spectrum. This enables the operator to use the inspection tool 100 without having to provide external or additional sources of illumination, which is particularly desirable when inspecting components that are reflective. The solder balls SB, for example, are silver and, as such, reflect light, which appears on the captured image as a bright spot. This bright spot washes out or obscures some of the features on the surface of the solder ball SB, and, particularly, may obscure a close view of the solder joints SJ1 and SJ2. Much greater detail on the solder joints SJ1 and SJ2 is actually visible when the area to be inspected is not brightly illuminated, but is, instead, dimmed. Small details, paint chips, etc. are more readily visible in a dimmed environment. The inspection tool 100 according to the invention is able to capture images using only light in the NIR range, thereby providing the operator with more information on the important features of the solder joint and improving the reliability of the inspection. The elimination of any external or additional light sources reduces the cost of the inspection setup. It also makes the setup less bulky and allows greater maneuverability of the tool.
It is understood that the inspection tool 100 according to the invention may be mounted in a fixture that is manually or automatically controlled to scan a particular object. The operator, viewing the images presented on the video monitor, is able to interrupt the automatic scanning process and review or record the image.
It is understood that the embodiments described herein are merely illustrative of the present invention. Variations in the construction of the inspection tool may be contemplated by one skilled in the art without limiting the intended scope of the invention herein disclosed and as defined by the following claims.
