Zero-defect IC inspection: anyway you look at it, camera view plays a key role in defect detection.

Ed.: For the full article, please see circuitsassembly.com/cms/content/view/2293/

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Automatic optical inspection systems are often stationed after soldering for post-reflow inspection, where they can supply the greatest error coverage. IC component packages show a broad spectrum of defects. Yet with a typical pitch of up to 0.5 mm, and sometimes even less, the parts often have defects that are either difficult or impossible to detect in subsequent electrical and functional tests.

Even when AOI inspection is used, IC solder joint defects and IC assembly defects make considerable demands on the performance of the inspection equipment and have a decisive influence on the depth of the AOI inspection. It is therefore necessary to differentiate between AOI systems with purely orthogonal camera views, and those that are also equipped with cameras capable of inclined views.

This article looks at potential IC defects, and discusses advantages and disadvantages of inspections performed with orthogonal and inclined cameras. Factors such as camera resolution, software and conveyors are discussed. (The results are valid for tin-lead and Pb-free soldering.)

Defect types. Defects typical of ICs may be classified according to the manufacturing phases involved, namely solder paste print, component assembly and soldering. During soldering the component has an additional influence over its own (soldering) characteristics.

The following defects can be assigned to the solder paste or printing step:

* Deficient tin or solder joints that are too thin.

* Solder bridges.

* Solder droplets (balls).

The following defects can be assigned to component placement:

* Missing components.

* "Stray" IC components.

* Displaced components.

* Twisted components.

* Reverse-polarity components.

* Wrong components.

* Bent leads.

The following defects can be assigned to soldering or the components themselves:

* "Lifted lead"; i.e., IC lead raised or bent upward.

* Coplanarity of an IC lead; e.g., due to lack of wettability.

* Unmelted solder paste.

Depending on the package type, certain kinds of defects occur more or less frequently. Lifted leads and lead coplanarity occur more frequently with fine-pitch QFPs than with SOICs, for instance.

The objective of AOI is to cover the types of defects described without exception, so that all defects that occur during production are actually detected. At the same time, the number of pseudo-defects (false alarms, bogus defects) must be reduced to a minimum with system throughput at its maximum rate.

When inspections of solder joints and components are being performed by AOI, certain factors not connected with the AOI have to be taken into consideration. One important influencing factor is pad design.

Peter Krippner is head of the SP division, Viscom AG (viscom.de); ([e-mail]).