inspections, parametric test procedures can be used to evaluate the digital image and on the
basis of this they classify the inspected PWB, component or solder joint. Automatic Optical
Inspection systems offer a reliable, flexible, fast and cost-effective solution when inspecting
each step of the manufacturing process. Using AOI systems also has financial advantages.
Detailed calculations show this in [1]. Further works give more reasons why AOI should be
used. Several economic, efficiency and suitability studies have been undertaken about these
systems [9]-[29].
3. Sensors, image capturing methods, structure
In the early 1970s, CCD (Charge Coupled Device) and CMOS (Complementary Metal–Oxide–
Semiconductor) sensors were invented. It presented an opportunity to capture digital images
that could be processed and evaluated by a computer. Machine vision was born. The subse‐
quent exponential development resulted in an infinite number of these applications. One such
development was the automatic optical inspection. Comparison between these sensors is
reported in detailed [30]-[36].
Two kinds of methods exist to capture the images: FOI (Field of Interest) based matrix camera
and line scan camera. The first captures several images on an optimized course, the second
scans the whole surface of PWB. Both have their advantages and disadvantages. Line-scan is
the faster method but the design of a proper source of illumination is more difficult or
sometimes not possible at all because the position of components themselves affects the
efficiency of illumination. If the component is parallel or perpendicular to the scanning line,
the captured image could differ. In case of paste inspection, component positioning is out of
question, so line-scan is better choice. For components and meniscus inspection, FOI is better.
A new FOI generation method is shown in [37].
Basically AOI systems have three main parts: optical unit (illumination, cameras), positioning
mechanism, and control system (Fig. 1).
4. Identifying PWB
AOI appliances identify the PWBs with the help of a separate built-in unit i.e. laser-scanner or
by using its inbuilt functionality. On this basis, machines can decide what inspection is
necessary. According to data contained in a barcode, the AOI system loads the appropriate
inspection program. As barcodes (Fig. 2.a) became more widely used, in some cases the amount
of data that could be stored in them was too limited and this became a barrier to its applicability.
To solve this problem, the so-called ‘matrix codes’ (Fig. 2.b) were developed. In [30] 22 types
of linear barcodes and 48 types of matrix codes are described.
Automatic Optical Inspection of Soldering
http://dx.doi.org/10.5772/51699
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