Monday, September 15, 2008

The Role of the Three Silicon Testing Categories

Proper validation of silicon in support of a production worthy product involves three major categories of testing requirements. These test types include manufacturing, characterization and functional validation. See the table to the lower right for an overview of these test types. Below I will expand upon the requirements of each one of these three essential testing philosophies.

Functional Validation
This type of testing us usually done on a low volume engineering type setup and concentrates on verifying that the silicon meets the original requirements, both electrical as well as functional. There may be specific silicon test modes necessary for functional validation but usually the characterization and manufacturing needs will cover the validation activities as well. The design engineer needs to be highly involved in defining the functional validation requirements, the test fixture definition and the actual validation activities. Remove any barriers (or excuses) that would keep the design engineer(s) away from the bench during the silicon validation activities. Their required participation will provide a minimized path to production and enable the designer's essential "real world" silicon learning cycle.

Once a product is ready for manufacturing the test focus moves from functional verification to the identification of manufacturing defects. Once functionality has been verified via functional validation testing, a defect is the only means that functionality would be altered, hence defects become the primary objective of production testing for manufacturing. Test costs and test coverage to maximize defect exposure along with parametric validation are the principal motivators in decisions about manufacturing test. In digital land this is what motivates an emphasis on scan, which has some very specific silicon requirements to support this mode. In analog land the primary requirement will be visibility and stimulus of internal analog sub-systems, usually accomplished via test modes to mux key internal signals to external pins.

This type of testing involves the ability to margin the design by varying FAB processes, supplies, frequencies and signal levels. For analog, this often requires visibility to internal signals, supplies, references and so on. For digital, the silicon support is often the same as the production test requirement of having a scan capability to inject and view the internal data. The process variance is typically accomplished via process corner splits within a given lot. There may also be some specific requirements necessary in support of ESD and/or latch-up testing that should also be considered.

Again, all three of these testing families are essential for a product prior to release to production. Do these well by facilitating an early plan to define the silicon content necessary for all three, thus removing another layer of potential surprises in your product release plans.

No comments:

Post a Comment