Page 152 - Six Sigma for electronics design and manufacturing

4.4.1 PCB test strategy
Figure 4.5 is an example of the test methodologies available for PCB
assemblies. These methodologies can be summarized as follows:
1. Visual test and inspection. These tests use trained operators to in-
spect PCBs for defects using the naked eye or visual magnification
such as microscopes or enlarging lenses. They concentrate on geo-
metrical defects that are easily observed by the human eye, but
may be difficult for machines to detect, such as solder shapes and
shorts. Determining the Manufacturing Yield and Test Strategy 121
2. In-circuit test (ICT). This type of test is used to eliminate defects
that result from individual components not meeting their specifica-
tions. The defects either due to the components being defective as
supplied, or becoming defective through the PCB assembly opera-
tions. They could either be missing, wrong, placed or inserted into
the PCB incorrectly, or become defective because of PCB assembly
operations exceeding manufacturing specifications.
The ICT test consists of a machine with electronic means of com-
paring the components to a preprogrammed value. The compo-
nents, already soldered in place on the PCBs, are reached through
a bed of nails fixture that provides contact of the component pads
on the PCB to pins in the fixture. Many sources of electronic noise
may be present, such as stray capacitance and resistance in the fix-
ture and its wires. In addition, some components in the circuit are
used in parallel with other components, so that it is difficult for the
tester to isolate the individual component to be tested.
The ICT is not always capable of detecting all component defects
because of the tester connections to the circuit. This inability to de-
tect all of the component defects is called defect or test coverage. A

Figure 4.5 PCB test alternatives.