Recent design changes in fabrication of area array packages (e.g., Ball Grid Arrays (BGAs) and Chip Scale Packages (CSPs)) have resulted in a change in the reflectivity of the solder balls placed on these devices. In previous area array package designs, solder ball surfaces were relatively uneven and rough. When illuminated by a laser beam, for instance, the uneven and rough solder ball surfaces resulted in a strongly diffused and scattered incident light. More recent area array packages are characterized by solder balls that maintain a smooth and highly reflective surface.
Laser triangulation systems for the measurement of solder ball height and position on area array devices typically use a light emission source (e.g., a laser) and a receiver arrangement (e.g., a laser beam sensor). The laser projects a laser beam vertically down onto the surface of the area array device such that the surfaces of the solder balls are illuminated. Position sensing devices then record the position of the reflected light. The position sensing devices in the receiver arrangement of the sensor typically are arranged to operate at an angle between 20° to 30° from the normal plane of the solder ball, but may also operate in a range from 10° to 45°. When the solder ball surfaces are highly reflective, as in the case of new area array devices, the intensity of the reflected light in the receiver section varies over a very large range as the laser beam traverses across the surface of the solder ball.
When the laser beam reaches the apex (the highest elevation) of the solder ball, the solder ball surface is generally normal to the incoming laser beam. As a result of the positioning of the laser beam relative to the solder ball surface, as well as the reflectiveness of the solder ball surface, most of the laser beam light is reflected back towards the laser. The smooth surface of the solder ball results in minimal scattering of light toward the receiver arrangement. The receiver arrangement signal reception intensity and signal-to-noise ratio are low, resulting in very low signal quality. When the laser beam reaches a point on the solder ball surface where the ball surface normal is at approximately half the receiver angle, in most instances 10° to 15°, the laser beam is strongly specularly reflected directly into the receiver arrangement. This strong reflection results in a high amount of reflected laser beam light and a high signal intensity at the receiving arrangement. The large amount of reflected laser light often saturates the receiver arrangement and consequently the position sensing devices, resulting in poor overall signal quality at this location on the solder ball surface. Beyond this point, (i.e. progressing down the sides of the solder ball towards the bottom of the solder ball) most of the laser light is reflected more horizontally, and eventually towards the device floor, away from the receiver arrangement and corresponding position sensing devices. A single laser beam intensity may not be able to provide high quality receiver signals over the entire ball surface.
There is therefore a need to provide a method to accurately measure highly reflective solder balls placed upon area array packages.
U.S. Patent No. 4,991,968 describes conventional methods and apparatus for obtaining three dimensional object surface determination. As can be seen in FIG. 1 of U.S. Pat. No. 4,991,968, included as FIG. 1 of the present application, the prior art system 1 includes a projector assembly 3 with a projector 3A, variable power supply 3B, power controller 3C, video amplifer 8, processing unit 9, master timing control 29, and control circuitry 14. The method and apparatus disclosed in U.S. Pat. No. 4,991,968 have a significant drawback of being costly to produce due to the number of components used. Additionally, these conventional system are bulky to operate and do not adapt to chip scale packages used today. There is therefore an additional need to provide a method and apparatus for scanning the surface of an object that is not cumbersome to the user.