The Economics of Test, Part - IV

Detecting a defective unit is often only part of the job. Another important aspect of test economics that must be considered is the cost of locating and replacing defective parts. Consider again the board with 10 integrated circuits. If it is found to be defective, then it is necessary to locate the part that has failed, a time-consuming and error-prone operation. Replacing suspect components that have been soldered onto a PCB can introduce new defects. Each replaced component must be followed by retest to ensure that the component replaced was the actual failing component and that no new defects were introduced during this phase of the operation. This ties up both technician and expensive test equipment. Consequently, a goal of test development must be to create tests capable of not only detecting a faulty operation but to pinpoint, whenever possible, the faulty component. In actual practice, there is often a list of suspected components and the objective must be to shorten, as much as possible, that list.

One solution to the problem of locating faults during the manufacturing process is to detect faulty devices as early as possible. This strategy is an acknowledgment of the so-called rule-of-ten. This rule, or guideline, asserts that the cost of locating a defect increases by an order of magnitude at every level of integration. For example, if it cost N dollars to detect a faulty chip at incoming inspection, it may cost 10N dollars to detect a defective component after it has been soldered onto a PCB. If the component is not detected at board test, it may cost 100 times as much if the board with the faulty component is placed into a complete system. If the defective system is shipped to a customer and requires that a field engineer make a trip to a customer site, the cost increases by another power of 10. The obvious implication is that there is tremendous economic incentive to find defects as early as possible. This preoccupation with finding defects early in the manufacturing process also holds for ICs.27 A wafer will normally contain test circuits in the scribe lanes between adjacent die. Parametric tests are performed on these test circuits. If these tests fail, the wafer is discarded, since these circuits are far less dense than the circuits on the die themselves. The next step is to perform a probe test on individual die before they are cut from the wafer. This is a gross test, but it detects many of the defective die. Those that fail are discarded. After the die are cut from the wafer and packaged, they are tested again with a more thorough functional test. The objective? Avoid further processing, and subsequent packaging, of die that are clearly defective.

About the Author:
Name: Joachim Bauer, Test Engineer
Experience: 13+ Yrs
Location: Nice, France