In the world of semiconductor testing, terms like CP and FT are well-known to professional testers, but they may be less familiar to those outside the field. This article aims to clarify what CP (Chip Probe) and FT (Final Test) tests are, especially for individuals who work with testing but are not test engineers themselves.
According to international standards, CP refers to the testing of a chip while it is still in the wafer stage. During this process, the chip's performance is evaluated using probe needles that make contact with the chip's pads. This step is also known as Wafer Sort. On the other hand, FT is the final test conducted after the chip has been packaged. Only chips that pass this test are shipped to customers.
The differences between CP and FT go beyond just their stages in the production process. They also vary in efficiency, test coverage, and complexity. Understanding these distinctions is crucial for anyone involved in IC development or manufacturing.
Currently, in many regions, including China, the probes used for CP testing are often cantilevered needles, also referred to as epoxy needles because they are fixed using epoxy resin. These needles are long and suspended, making signal integrity difficult to maintain. As a result, the maximum data transmission rate is typically limited to 100–400 Mbps, which makes high-speed signal testing challenging. Additionally, direct contact between the probe and the pad can lead to issues such as leakage and increased contact resistance, which can significantly affect precision measurements. Therefore, CP testing is usually reserved for basic connectivity checks and low-speed digital circuits.
While high-speed or high-precision signals could theoretically be tested during the CP stage, this would require advanced solutions like vertical pin or MEMS probe technology—both of which come with significant cost increases. In most cases, this is not economically feasible.
So, is CP testing still necessary? How should one decide which tests to include during the CP phase? To answer this, it's important to understand the primary purpose of CP testing.
First and foremost, the main goal of CP testing is to identify and eliminate defective chips before packaging, thereby reducing overall costs. Based on this, during the CP phase, it’s wise to focus only on test items that have a major impact on yield. Tests that are expensive, difficult to perform, and have a low failure rate may be better suited for the FT stage, where they can be executed more efficiently without unnecessary cost.
Secondly, some module pins may not be accessible after packaging, meaning certain functions cannot be tested at the FT stage. In such cases, testing must occur during the CP phase, making it an essential step.
There's also a special case when the chip uses a complex package type like SIP (System in Package). Due to lower testability at the FT stage and the sensitivity of multi-die integration, it's common to ensure each die is functional before packaging. In these situations, comprehensive CP testing is often required, regardless of difficulty.
To summarize, here are some key guidelines:
1. Chip functionality that can be fully tested in the FT stage should generally be done there. CP testing is optional.
2. CP testing typically focuses on basic DC parameters, low-speed digital functions, and other easy-to-test items. Any test that is hard to perform in CP but easier in FT should be moved to FT.
3. Since CP testing may lack precision, the criteria can be slightly relaxed for initial screening. Detailed and rigorous tests should be left for FT.
4. If packaging costs are low and the chip has a high yield, CP testing might not be necessary, or it could be limited to sampling for process monitoring.
5. When introducing new products into mass production, the FT test program should be developed first. Initially, FT is more critical than CP, and CP testing can be added later based on actual needs.
These points represent some of the fundamental concepts I've encountered through experience. However, real-world projects often involve more complex scenarios, and specific solutions require detailed analysis. The purpose of this article is to provide non-test professionals with a clear understanding of CP and FT testing basics. Ultimately, a skilled test team plays a vital role in ensuring product quality, yield, and cost control in any design company.
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