PCHOLO Evaluation of Mechanical Stability in Electronics and

Stringent requirements for reliability in electronics and photonics packaging places extreme demands for dimensional stability on a wide range of components and joining materials. However, properly evaluating a design for mechanical stability is difficult due to the need to measure extremely small (sub-micron) movements or distortions on complex mechanical structures spanning several millimeters or more¹.

To address this issue, a measurement technique is required which can capture submicron movements and distortions that may take place on a complex object over time. Various interferometric methods have been widely used for evaluating mechanical stability in electronic components, printed circuit boards and solder joints^2,3. Electronic Holography^4,5,6is a class of methods that can be used to evaluate the mechanical stability of a wide range of opto-electronic and photonic devices.

Using electronic holography, an interference pattern is created, typically by illuminating the device under test with a laser beam and combining the reflected beam with a reference beam from the same laser. The resulting interference pattern is stored electronically using a CCD camera. After perturbing or stressing the device under test in some way, a new interference pattern is generated and combined electronically with the original image. Any differences in the shape of the object resulting from the stress can be detected by comparing the two interference patterns. Changes in shape which are on the order of a fraction of the wavelength of the light used can be detected and analyzed in this way. Image processing methods can then be used to represent the information to allow identification and analysis of any relative movements or distortions.

Using this method can enable a wide range of tests and studies that are simply not possible with other techniques. Evaluating relative movements that occur in an electronic or opto-eletronic module under the influence of a thermo-electric cooler, for example, can be carried out in situ on the test station.

Figure 1- Electronic holographic fringe pattern showing distortion of a thermoelectric cooler under bias (square region in middle). Surrounding region indicates distortion of underlying plate on which the cooler is mounted.

Figure 1 shows a digitized holographic fringe pattern from a thermoelectric cooler under bias mounted on a flexible plate. The circular fringe pattern indicates distortion (bulging) resulting from the temperature gradient (in this case the cooler is biased to heat the top side). By appropriate image processing, we can extract a topographic profile from the fringe image as shown in Figure 2 (contour map) and Figure 3 (3D topograph). The total time for recording the image and processing the data is less than a few minutes.

Figure 2 - Contour map of cooler top showing out-of-plane displacements corresponding to figure 1. Each contour line corresponds to approximately 750A

Figure 3 - 3-D topograph corresponding to contour map of Figure 2.

In these figures, the cooler is about 4cm on a side, and the total out-of-plane deviation from the highest to the lowest point is approximately 1 micron, corresponding to a contour spacing in Figure 2 of about 750A. The apparent asymetry in the distortion pattern is a result of asymmetries in the mounting method for the cooler and the underlying plate.

Electronic holographic systems can also be configured to detect in-plane displacements. This can be used to directly measure thermal expansion rates with significantly better sensitivity than conventional mechnical methods.

In conclusion, given the stringent requirements for mechanical stability placed on components used in the communications industry, it is critical that companies developing these components have the means to rapidly evaluate new product designs. Techniques such as electronic holography should be an essential part of the developers toolbox enabling rapid introduction of new designs with high confidence. With the advent of the HG7000/K100 system, this is now possible.