Three-dimensional imaging system using a varifocal mirror at video frame rate

为了实现高质量的三维成像和精确的形状测量，设计了两种基于完美投影的成像系统，该系统使用变焦镜来实现高质量的三维成像和精确的形状测量。第一种是基于正射投影的恒倍成像系统，第二种是基于透视投影的广角成像系统。第一个系统采用压电式变焦反射镜，工作在20赫兹的视频帧速率，用于测量10 mm正方形大规模集成电路封装基板上的焊点阵列，其边缘排列着直径为300μm的208个焊点，间距为500μm，可以精确地获取焊点上部的形状。高度误差小于12μm，因此精度足以用于焊料凸点的自动检测。据估计，焊料凸起的一些水平部分有足够的信息。为了获得指定高度的截面数据，我们设置了一对焦点H1和H2，沿高度轴具有指定的间隔。通过将焦点H1和H2的两幅图像与每个像素进行比较，并选择具有相同焦点测量值的像素，我们可以检测位于焦点H1和H2之间的截面轮廓上的点。我们假设焊料凸起是一个半径恒定的球体。通过使用指定高度的水平截面将球体适配到凹凸中，可以有效地计算凹凸顶部的高度。利用球面拟合法从截面上估计凸块的高度，可以成功地应用于球凸块阵列的共面性测量。使用个人计算机测量一块LSI封装电路板的时间估计为7.2秒。

Two types of imaging systems based on perfect projection using a varifocal mirror were devised to achieve high-quality three-dimensional imaging and precise measurement of shape. The first was a constant-magnification imaging system based on orthographic projection and the second was a wide-angle imaging system based on perspective projection. The first system using a piezoelectric-type varifocal mirror operating at an almost video-frame-rate of 20 Hz was used for an experimental setup for measuring an array of solder bumps on an LSI package board 10-mm square in size, on the edges of which 208 bumps with a diameter of 300 μm were arrayed at a pitch of 500 μm. Shape of an upper part of a solder bump could be acquired precisely. Height errors were less than 12 μm, so that precision was sufficient for automated inspection of solder bumps. Some horizontal sections of solder bumps have been estimated to have sufficient information. To obtain sectional data at a specified height, we set a pair of focal points, H1 and H2, with a specified interval along the height axis. By comparing the two images for focal points H1 and H2 with every pixel and selecting pixels with the same focus measure values, we can detect points on the contour of the section located at the mid-point between focal points H1 and H2. We assumed the solder bump was a sphere with a constant radius. By using the horizontal section at a specified height to fit a sphere into the bump, the height of the bump top was efficiently evaluated. Sphere fitting to estimate the bump height from the section could be successfully applied to measuring the coplanarity of a ball bump array. The time to measure an LSI package board was estimated to be 7.2s, using a personal computer.