Magnetic Particle Inspection (MPI)

What is MPI

It is to determine if surfaces of disk drive parts have magnetic particulate contamination such as Neodymium (Nd). SEM/EDX is employed for this test.

Scope

Special care must be taken to prevent cross contamination from magnetic particles that may be present in the lab where outgas testing is performed.

Sampling must be take place in another location, and the analyst must wear a protective garment that is only used for this purpose and has never been exposed to the lab where outgas testing is performed.

Summary of Method

Samples are collected from the surface of the part according to the sampling plan with carbon adhensive tape.

The quantity and size of the magnetic particles are recorded.

The method is intended as a survey; not every particle will be sampled.

Materials and Equipment

Jeol JSM-5600LV Scanning Electron Microscope by Jeol equipped with Energy Dispersive Spectrometer and Back Scattered Electron detector

Brief Procedure

Place the sample into the SEM and obtain a clear image. Operate SEM at a minimum of 15 kV.

Use the BSE mode to discriminate particles with high atomic numbers from those with low atomic numbers.

Scan an area of the carbon tape and find the "brightest" particles by BSE - slow scan mode.

Take an EDS spectrum of the particles.

If a magnetic particle is find, collect a complete spectrum and save the spectrum.

Sample the blank first to confirm it clean.

The entire area of the blank should be imaged to determine if there are any magnetic particles.

Use the method scan at least 10 areas of the tape at 300X.

The test should at least take 45 minutes of SEM time per sample.

Typical Magnetic Particle SEM Image

Typical Magnetic Particle EDX Spectrum

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