Most MIB failures and processing defects are not observable on
the surface of the completed boards. Sectioning techniques employing either
metallurgical polishing or chemical etching are generally used. The choice of
technique is dependent on the analytical equipment available, the failure
being investigated, and the inventiveness of the investigator.
Cross-sectioning is generally useful for dielectric and
conductor thickness, dielectric structure, and for investigation of shorts.
Since exposing shorts is a particularly sensitive procedure, detailed
information is provided. Cross- sectioning is basically a mechanical technique
employing scribing, cutting, and polishing to remove material until the defect
is exposed. A combination of polishing and chemical etching has been very
successful for the investigation of shorts.
Exposing shorts for analysis requires adherence to the following
technique. The protuberance type of short is first located to within one
centimeter accuracy by judiciously scribing the ceramic blank. Artwork is used
to advantage for shorts involving signal lines. Following fracture, the
segment containing the short is hand-held and ground with a #400 grit on a
sectioning wheel. Potting is not necessary. As the suspected short location is
approached, the progress is checked both optically and electrically with
increasing frequency. Since the protuberances are usually less than 0.1 mil in
diameter, great care and restraint must be exercised lest the short vanish
before it is located. A good 500X microscope is necessary for examination. The
final operation is removal of the obscuring dielectric surrounding the short.
A 7-second hydrofluoric acid etch is followed by cleaning in an ultrasonic
bath for 1 second, then a rinse, and a gentle dry with nitrogen.
Shorts caused by copper precipitates are more difficult to
expose. They appear most frequently at signal line edges where continuous
voids are most likely to form. No etch is used. Patience and self discipline
are all important because of the numerous optical, electrical, and scanning
electron microscopy inspections required. Once located and exposed, both types
of shorts require a scanning electron microscope in order to be observed
Surface sectioning is useful for investigating opens, conductor
grain size, and via alignment. Sectioning from the surface on down can be
accomplished by either mechanical polishing or chemical etching. Chemical
etching for dielectric removal is generally the preferred method; however,
once exposed, the defect or object of interest can be coated with mounting
compounds and metallurgically polished.
Blisters and delamination are generally observable visually with
the naked eye or low power magnification. If further analysis is required, the
ceramic blank can be scribed through the desired location with a diamond
point. The cross section can then be studied using a microscope or a scanning
Cracks can be observed optically with a 30X-100X power
microscope using various lighting conditions. Directional, halo, and polarized
light all have useful applications for this purpose.
The basic equipment necessary includes sectioning tools,
microscopes with up to 500X capability, and the scanning electron microscope.
For removing dielectric, use either 48% hydrofluoric acid (HF), or a solution
of 20 grams of ammonium bifluoride (HN4HF2), 7.5 ml of 48% hydrofluoric acid,
and 45 ml of water.
Dye penetrant can be useful in determining the depth or severity of cracks
in both the dielectric and alumina substrate. Ultraviolet-sensitive penetrants
have been found to be particularly useful in examining alumina edge chips.