IR and Other Assembly-screening Tools

Failure analysis success hinges on its effectiveness in detecting and isolating manufacturing defects, which generally fall into five major categories:

1. Missing parts (presence)
2. Backward or wrongly oriented parts, e.g., reversed electrolytic capacitors, diodes, transistors or integrated circuits (polarity)
3. Wrong components placed, e.g., a 100 Ω instead of a 1,000 Ω chip resistor (value)
4. Parts-attachment defects, e.g., solder opens, bridges and lifted leads as well as faulty components and breaks in conductor traces (electrical integrity)
5. Packaging or mounting problems, e.g., components contacting box sidewalls or improperly placed heat sinks (mechanical soundness).

In the past, failure analysis has depended on data generated by one or more of the following standard tests:

• In-circuit test (ICT) is an excellent diagnostic tool and aids in performing simple functional tests of mounted components. ICT can spot defects such as opens, shorts, and incorrect or wrongly oriented — or missing — components. ICT detects such defects by probing the circuit physically.
• Manual visual inspection (MVI) is an inexpensive method for detecting soldering flaws such as lifted leads and misaligned or missing components. However, MVI is very operator-dependent and requires a highly trained inspector.
• Automated optical inspection (AOI) is a good in-process automated tool for detecting missing components. AOI also can find solder cracks, lifted leads, misaligned components and tombstoned (dislodged) parts. The technique's value is enhanced in high-volume production scenarios with numerous PCBA samples that can be used to build good models. Also, lower false-call rates are associated with AOI than with MVI because AOI is not based on subjective operator decisions.
• X-ray also is excellent for screening defects associated with ball grid array (BGA) packages and other area-array devices. X-ray has been successful in detecting cracked BGA moldings, missing solder bumps and solder bridgings. Its advantage over visual inspection lies in the capability to penetrate devices and "see" solder joint quality underneath the component.
• Functional test (FT) generally is a product-specific test that "exercises" the assembly in its actual application. This provides helpful "go/no-go" information but may not detect latent or marginal performance problems. Also, FT usually requires a high skill level to run and generally requires substantial test time.