The Economics of Test, Part - II

The table depicted shows test cost broken down into four categories some of which are one-time, non recurring costs whereas others are recurring costs. Test preparation includes costs related to development of the test programs as well as some potential costs incurred during design of the DFT features.

DFT-related costs are directed toward improving access to the basic functionality of the design in order to simplify the creation of test programs. Many of the factors depicted in the Figure imply both recurring and nonrecurring costs. Test execution requires personnel and equipment. The tester is amortized over individual units, representing a recurring cost for each unit tested, while costs such as probe cards may represent a one-time, nonrecurring cost. The testrelated silicon is a recurring cost, while the design effort required to incorporate testability enhancements, listed under test preparation as DFT design, is a nonrecurring cost.

The category listed as imperfect test quality includes a subcategory labeled as tester escapes, which are bad chips that tested good. It would be desirable for tester escapes to fall in the category of nonrecurring costs but, regrettably, tester escapes are a fact of life and occur with unwelcome regularity.

Lost performance refers to losses caused by increases in die size necessary to accommodate DFT features. The increase in die size may result in fewer die on a wafer; hence a greater number of wafers must be processed to achieve a given throughput. Lost yield is the cost of discarding good die that were judged to be bad by the tester.

The column in Figure labeled “Volume” is a critical factor. For a consumer product with large production volumes, more time can be justified in developing a comprehensive test plan because development costs will be amortized over many units. Not only can a more thorough test be justified, but also a more efficient test—that is, one that reduces the amount of time spent in testing each individual unit. In low-volume products, testing becomes a disproportionately large part of total product cost and it may be impossible to justify the cost of refining a test to make it more efficient. However, in critical applications it will still be necessary to prepare test programs that are thorough in their ability to detect defects.

A question frequently raised is, “How much testing is enough?” That may seem to be a rather frivolous question since we would like to test our product so thoroughly that a customer never receives a defective product. When a product is under warranty or is covered by a service contract, it represents an expense to the manufacturer when it fails because it must be repaired or replaced. In addition, there is an immeasurable cost in the loss of customer goodwill, an intangible but very real cost, not reflected in the Figure, that results from shipping defective products. Unfortunately we are faced with the inescapable fact that testing adds cost to a product. What is sometimes overlooked, however, is the fact that test cost is recovered by virtue of enhanced throughput. Consider the graph in the Figure. The solid line reflects quality level, in terms of defects per million (DPM) for a given process, assuming no test is performed. It is an inverse relationship; the higher the required quality, the fewer the number of die obtainable from the process. This follows from the simple fact that, for a given process, if higher quality (fewer DPM) is required, then feature sizes must be increased. The problem with this manufacturing model is that, if required quality level is too high, feature sizes may be so large that it is impossible to produce die competitively. If the process is made more aggressive, an increasing number of die will be defective, and quality levels will fall. Point A on the graph corresponds to the point where no testing is performed. Any attempt to shrink the process to get more units per wafer will cause quality to fall below the required quality level.

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

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