Page 121 - Building A Succesful Board-Test Strategy

Inspection as Test 107

Figure 3-11 Untrimmed leads of a through-hole component. (Photo courtesy
Teradyne.)

Some manufacturers use laser or white-light techniques to examine loaded
boards. This approach constructs a three-dimensional profile of the board, detecting
missing and some off-pad components. The technique is generally too slow for this
process stage, however, and one of the others will likely produce more accurate results.
Nevertheless, for a cursory check of certain critical board areas, it may be sufficient.

3.3.1 Manual Inspection
As mentioned earlier, despite the preponderance of incredible shrinking elec-
tronics, the most common inspection technique remains manual inspection, also
known as human visual inspection (HVI) and manual visual inspection (MVI). Its
popularity persists despite a consistent body of evidence that it is less effective than
it used to be, always assuming that it was ever effective at all.
Manual inspection consists of a cadre of people examining boards either with
the naked eye or aided by magnifying glasses or microscopes. Manufacturers like
the technique because it is relatively simple and inexpensive to deploy. Microscopes
and magnifying glasses require little up-front investment, and people costs are easy
to manage and adjust as situations change. Its flexibility stems from the fact that
human beings adapt much more easily to new situations than machines do. Also,
manual inspectors are much less bothered by changes in lighting or other envi-
ronmental conditions. There is no need for programming, and, within the limits of
the inspectors' capability, it can be quite accurate.
On the other hand, operating costs are quite high. Labor costs represent a
considerable expense, and adjusting the workforce as manufacturing throughput