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Cross-sectioning

     

Cross-sectioning or microsectioning is a failure analysis technique for mechanically exposing a plane of interest in a die or package for further analysis or inspection. It usually consists of sawing, grinding, polishing, and staining the specimen until the plane of interest is ready for optical or electron microscopy. The conventional method of microsectioning requires the encapsulation of the specimen in plastic to give it stability, support, and protection. A relatively newer technology utilizes specific tools and procedures to allow non-encapsulated microsectioning.

     

Conventional microsectioning starts with sample preparation. This consists of cleaning, mounting, and encapsulation of the sample in polyester or epoxy resin. Sometimes, a sample is sawed to reduce its size prior to encapsulation. This is usually done to fit the specimen perfectly into the mold, as well as to reduce the grinding needed during actual sectioning.

               

Fig. 1. Examples of Sample Mounting Presses from Buehler

                

The positioning 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 usually poured inside a vacuum impregnator to minimize bubbles or air pockets, which affect the quality of the microsectioned sample. The sample is then allowed to cure at ambient pressure. Quick-cure resins are not recommended for specimens suspected of delaminations or microcracks, since the rapid curing of the resin can heal these defects.

     

Sample preparation is then followed by sawing of the encapsulated specimen with the use of a diamond wheel cutter. 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.

                 

Fig. 2. Examples of Precision Saws from Buehler Fig. 3. Examples of Diamond Wheel Saw Blades

               

Grinding is done after the specimen has been cut to its optimum size. A typical grinder/polisher has a platen (or a set of platens) over which the grinding material (SiC paper, polishing cloth, diamond paste, etc.) is placed. Grinding is often started using a 120 or 240 Grit SiC paper. The grinding process then progresses through 320, 400, and 600 Grit SiC paper. Each step should remove all the scratches from the previous step. Rinse the specimen between each step. A properly ground specimen will only have one surface plane.

    

Polishing follows grinding. Polishing is very similar to grinding, except that a Texmet, nylon, or silk cloth with diamond or alumina paste or powder on the surface is used instead of SiC paper. Rough polishing is usually done using 6- or 1-micron diamond particles. Fine polishing is usually done using 1-micron, then 0.3-micron, then 0.05-micron alumina particles. The total fine polish time should be short, i.e., less than 30 seconds. All scratches on the cross-section surface should already have been removed after this step.

             
Fig. 4. Examples of Grinders/Polishers

      

See Also:  Failure AnalysisAll FA Techniques Optical Inspection;

Xray Radiography Decapsulation Focused Ion Beam SEM/TEM Acoustic Microscopy

FA Lab EquipmentBasic FA Flows Package FailuresDie Failures

   

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