then the appropriate parts would seem to be defective. These are the pseudo-failures which
can reduce productivity so their number should as close to zero as possible. [164] ALSO
indicates some other image processing problems. The problems of AOI systems will be
described in more detail later in the chapter.
Another disadvantage is that they are usually in the ‘bottle-neck’ of the manufacturing
production line because they are not able to inspect the whole circuit board as fast as the line
can produce them. Therefore, the practice is usually to place more machines behind each other
to enable inspections to take place in parallel. Of course, this also has financial implications
which should be taken into consideration.
7. Special AOI solutions — Inspection of lead-free solder joints, flexible
substrates, wire bonding and semiconductors
According to RoHS and WEEE directives, lead-free solder alloys have to be used in commercial
electronics. This has presented a new challenge for AOI systems because of the differing optical
properties of lead-free alloy. Some solutions are shown in the following studies [166]-[173].
AOI has several further application possibilities in electronic device manufacturing e.g.
semiconductor and wire-bonding inspection. These appliances need extremely high-resolu‐
tion cameras to detect defects in the μm scale. Another interesting area is flexible substrate
inspection. Some of these special inspections are described in [174]-[179].
7.1. Differences between lead-based and lead-free solder alloys
Solders that contain lead are available with a tin content of between 5% and 70%. The compo‐
sition of the most commonly used lead solder is 63/37 Sn/Pb; this was the main type used in
electronics manufacturing until strict controls were imposed on its use for environmental
reasons. The homogeneity of the solder meniscus that formed was beneficial in that the melting
point of eutectic solder really is manifested as a single point on the phase diagram; in other
words the molten alloy solidifies at a specific temperature, rather than within a broader
temperature range. The solidified alloy can be broken down into tiny lead and tin phases of
almost 100% purity, without intermetallic layers.
In the case of non-eutectic solders, the crystallisation begins around cores of differing compo‐
sition and crystal structure, and at differing temperatures, so that during the accretion of the
individual cores the composition of the residual melt also changes. Due to this, in the case of
lead-free, non-eutectic solder alloys, certain phases solidify earlier, and these solid cores do
not form a completely mirror-like, smooth surface on the face of the solder meniscus (and
naturally, they also cause differences in the volume of the material).
Lead-free solders usually contain tin, silver and copper. Compared to lead-based solders they
have several negative properties: they are more expensive, their melting point is higher, and
they give rise to problems that do not occur when soldering with lead (the phenomenon of
whisker formation has still not been fully explored). Because their surface differs from that of
Materials Science - Advanced Topics
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