System Provides Interior Packaging Deformation

Traditionally, Moiré technology has been used to measure packaging strain. A disadvantage for in-plane deformation measurement is that Moiré requires a grating be adhered to the sample, to produce fringe patterns for measuring in-plane deformation. Other — Moiré and non-Moiré— systems allow the accurate external measurement of out-of-plane package deformation (warpage), using the projection of two gratings (shadow Moiré) or by measuring the height of the package's four corners relative to the center.

What has been needed is the capability to look at in-plane deformation in the cross section of the actual package, rather than the deformation of a grating adhering to it. Manufacturers must see what happens at the die attach interface between the silicon die and the leadframe.

For the measurement of in-plane deformation by Moiré methods (interferometric or geometric), as opposed to shadow or projection Moire's out-of-plane deformation measurement, a phase grating must be placed on the sample. This has difficulties.

Optical Metrology Innovations (OMI, Cork, Ireland) has developed a system that uses white-light imaging to correlate package images. The OMISTRAIN platform uses 3-D fast image correlation, based on an image stacking and, if necessary, compositing technology that allows inspection of samples that are fairly rough and ready, and have a topography. Precise focus is unnecessary because the system renders the image in focus using 3-D digital image correlation technology.

The platform looks directly at the sample and does not require structures such as a grating and adhesive. Using moderate heating (20-70°C or 20-90°C), it creates a thermal stress resulting in a deformation motion. The image is taken of an area where materials meet. Then the system divides the main image in focus, a high-magnification 660 × 850 µm digital image (1200 × 1600 µm with a larger camera), into sub-images — 32 × 32 pixels or smaller, selected to partially overlap. Using an 8 bit grayscale (10 bit with the larger camera), it produces a high-resolution "fingerprint" generated by the sample's texture and its response to top-down diffused lighting — green-filtered white light — of a local area 30 × 30 µm or smaller. Each sub-image is then analyzed to locate that same grayscale pattern in the second image.

The upper left diagram shows what is being analyzed, the image of the area where several materials meet: the die, the molding compound, the die attach adhesive, the solder resist, and the die paddle. On the right is the main output, a vector map, with arrows showing the motion of the materials. It is shown that the die paddle has a motion toward the left, while the die itself has a strong motion downwards and to the right. There is a strain building up across the die attach adhesive, where the die is pulling downwards and the die paddle is pulling upwards. This is visible in the strain map underneath, in the red area and the shear map. (Source: OMI)

Using correlation algorithms, the system locates the motion of a pixel pattern at deep sub-pixel resolution. This provides a correlation function — as the function of the distance necessary to shift the sub-image to get it to correlate in the second sub-image — resulting in a deformation vector. The detailed effects obtained with the system's analysis capabilities are difficult, if not impossible with Moiré technology. The platform displays the motion change across one of the strain vectors. Each vector calculation is performed independently, making it possible to detect a motion change between one vector and another, over the typical 20 µm thickness of die attach layers. Differential motion equations are unnecessary.

With Moiré, the adherence of the grating and sample preparation takes about a day. In OMISTRAIN's case, if the surface is polished, it needs to be slightly roughened to create a texture that can be detected in a grayscale image, which takes about 30 seconds. Results are typically available in an hour; less for simpler thermal cycles.

For additional information on inspection, measurement and test, go to www.semiconductor.net/imt .