ELECTRONICS
QUALITY CONTROL

The main materials found in the field of electronics are:
• Metals with excellent electrical conductivity or coatings such as copper, silver, nickel and gold,
• Tin alloys for soldering,
• Polymers for printed circuit boards and electronic modules,
• The glass and ceramics (and silicon) that make up the various components.
Inspection requires metallographic preparation. Obtaining an inspection surface requires a succession of operations, each as important as the next, regardless of the material.

These steps are in the following order:
• The removal of the product to be examined (if necessary), called “CUTTING”.
• Standardisation of the geometry of the sample taken (if necessary), called “MOUNTING”.
• Improvement of the surface condition of this sample, called “POLISHING”.
• Characterisation of the sample: revealing the microstructure of the sample by an etching reagent (if necessary) called “METALLOGRAPHIC ETCHING” and microscopic observation (optical or electronic).

In other words, it is essential to avoid heating, deformation or excessive aggressiveness of the cut, which can lead to many different types of damage (cracks/spalling of components, detachment of coatings/metallisations, etc.) to the areas to be inspected. Cutting is a fundamental step which conditions the further preparation and inspection of parts.

PRESI’s wide range of medium and large capacity cutting and micro-cutting machines, and also the range of WELL diamond wire saws, can be adapted to any need with regard to cutting precision, sizing or quantity of products to be cut:

The choice of machine type depends on the geometry of the workpieces to be cut and their sensitivity/fragility, as the clamping possibilities differ. The advantage of wire saws is that they use low support forces which are gentle on the specimens. They also provide very good surface finishes but have longer cutting times.

Figures 12 & 13: Examples of clamping of an electronic circuit – FLEXICUT

Wire saws, on the other hand, are used with a cutting fluid that acts as a lubricant and “cleaner” that extends the life of the wire and allows a better quality of cut to be achieved.

The consumables available for wire saws are stainless steel wires set with diamonds. The characteristics of the wires vary according to requirements and needs. The range is comprised of seven wire diameters from 0.1-0.7 mm and five diamond grit sizes from 20-64μm.

There are two mounting options: hot or cold mounting. In electronics it is always preferable to use the cold mounting since very often the parts are fragile and therefore unsuitable for high pressures, and they are probably also sensitive to heat.

PRESI offers a wide range of manual and automatic polishing machines, with a wide choice of accessories, to cover all needs, from pre-polishing to super-finishing and polishing of single or series samples.

The aim of metallographic preparation of connector or microelectronic samples is often to reach and observe very precise areas of the parts. Therefore, polishing must be carried out in the most controlled manner possible. Manual polishing is therefore more advisable.

=> All the first steps of each range are called “levelling” and consist of removing material quickly to level the surface of the specimen (and resin). Those given below are standard and can therefore be modified as required. In practice, the aim is to reach the area of the sample to be studied after the pre-polishing steps. In this way the polishing is carried out directly on the components to be inspected.

Applied pressures vary according to sample size, but in general the following applies: 1daN per 10mm mounting diameter for the pre-polishing steps (ex: Ø40mm = 4 daN) then reduce force by 0.5daN at each polishing step with an abrasive suspension.

More generally, an “electronics” sample is made up of a multitude of different materials. There is therefore a risk of creating unevenness between the different components during polishing, causing difficulties during observation.

Sometimes, it is not necessary to carry out the polishing range up to super-finishing, particularly in the case of crimping controls, dimensioning or any other inspections on large connector samples.

However finishing must be performed with colloidal silica in the last polishing stage to observe the finest components, intermetallics, porosity brazing, etc…

Micrographs 3 and 4: Polished circuit board up to SPM lens x5

Micrographs 11 and 12: Brass with copper, nickel and gold plating polished up to SPM lens x100