The WS phase is the testing that’s typically done by an outsourced semiconductor assembly and test (OSAT) supplier, a third-party company that is in charge of applying parametric tests given by fabless companies (chip developer) onto the wafers while on a tester. In this stage, there can be thousands of tests that are run depending on how elaborate the testing is. The wafer is then diced into individual chips and placed into packages during the ASSY phase. FT and SLT can consist of many of the same tests that were performed during WS but often consists of additional tests as well.
Historically, WAT, WS, and FT were the most common sources of data actively sought by fabless companies for analysis primarily because those were the data sources that data analytics solutions focused on or knew how to handle. This is somewhat still true today. However, BUMP, ASSY, and SLT data are quite important in knowing the full history of the chip and can be invaluable when uncovering issues previously masked by not analyzing this data. A good data analytics solution should be able to gather and analyze all data from each of these predominant test phases.
A simple rule of thumb is the later in the manufacturing process that chips fail testing, the more costly it is due to the wasted tester time spent through the earlier test phases, the expensive packaging now going to waste and, ultimately, the lost revenue of not being able to sell those failed chips. Identifying faulty chips as early as possible, therefore, becomes critical to the financial health of companies developing chips. This can only be achieved with access and visibility to all of the data.