Micro-section of Failed Solder Joint
Cross-sectioning or micro-sectioning is a failure analysis technique for mechanically exposing a plane of interest in a die or package for further analysis or inspection. It consists of sawing, grinding, polishing, and staining the specimen until the area of interest is ready for optical microscopy and generation of digital images. The method described in IPC TM650-2.1.1 of micro-sectioning requires the encapsulation of the specimen in plastic to give it stability, support, and protection.
Conventional micro-sectioning starts with the preparation of the PCB to be examined. This consists of cleaning, mounting, and encapsulation of the sample in an epoxy resin. Typically, a sample is sawed to reduce its size prior to encapsulation. This is usually done to fit the specimen perfectly into the mold.
The location of the specimen in the mold during encapsulation is critical. It must be chosen well to minimize the sawing and grinding needed to expose the plane of interest. The resin is poured inside a vacuum impregnator to minimize bubbles or air pockets, which can affect the quality of the micro-sectioned sample. The sample is then cured.
Micro-section of Through-hole on PCB
Sample preparation is then followed by sawing of the encapsulated specimen usually with a diamond blade saw. The sawing in this step is usually done along a plane parallel to the plane of interest. Proper sawing minimizes the amount of grinding needed to expose the plane of interest in the specimen.
Grinding is done after the specimen has been cut to its optimum size, with the sample being polished thereafter. Each step should remove all the scratches from the previous step. Rinse the specimen between each step. Polishing follows grinding. Polishing is very similar to grinding, except that a fine cloth with diamond or alumina paste or powder on the surface is used instead of sand paper. All scratches on the cross-section surface should already have been removed after this step.
The sample is then ready for review, inspection and photographing.
BGA Cross Section