In the world of semiconductor testing, terms like CP and FT are commonly used by professionals, but many non-testers may not fully understand their meaning or significance. This article aims to clarify what CP (Chip Probe) and FT (Final Test) tests are, especially for those who work with testing but aren’t testers themselves.
According to industry standards, CP stands for Chip Probe, which refers to testing a chip while it is still on the wafer. During this stage, the chip’s performance is evaluated using probe needles that make contact with its pads. This process is also known as Wafer Sort. On the other hand, FT, or Final Test, is conducted after the chip has been packaged. Only chips that pass this test are shipped to customers.
Beyond just the timing of the test, CP and FT differ significantly in terms of efficiency, coverage, and cost. These differences are crucial for anyone involved in IC design and manufacturing.
Currently, most CP tests use cantilevered probes—often referred to as epoxy needles due to how they are mounted with 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, making high-speed testing nearly impossible. Additionally, direct contact between the probe and the chip pad can lead to issues like leakage and contact resistance, which negatively impact high-precision measurements. Because of these limitations, CP testing is usually reserved for basic connectivity checks and low-speed digital circuits.
However, high-speed or high-precision signals can theoretically be tested during the CP phase if advanced probe technologies like vertical pins or MEMS probes are used. These solutions, though effective, come at a much higher cost and are often not economically feasible for most projects.
So, is CP testing still necessary? And how should one choose the right test items during the CP phase? To answer these questions, it's essential 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, thus saving significant costs associated with packaging and shipping faulty parts. Based on this, it's wise to focus only on test items that have a major impact on yield. Some tests that are expensive or difficult to perform but have a low failure rate may be better suited for the FT stage, where they can be executed more efficiently.
Secondly, some chip modules may not be accessible after packaging, making them impossible to test during FT. In such cases, CP testing becomes a necessity.
Another special scenario involves packages like SIP (System in Package), where the overall yield is highly dependent on the quality of individual dies. In these cases, all necessary tests must be performed during the CP phase to ensure that only "Known Good Dies" are used.
To summarize, here are some key considerations when deciding on CP and FT test strategies:
1. Any test that can be reliably performed in the FT stage should generally be done there, as CP is optional.
2. CP is typically used for basic DC tests, low-speed digital functions, and other easy-to-test features. More complex tests, like ADC performance, should be confirmed in FT.
3. Due to lower accuracy in CP, test criteria can be more lenient, focusing on initial screening rather than detailed analysis.
4. If packaging costs are low and the chip yield is high, CP testing may be skipped or limited to sampling.
5. For new products entering mass production, it’s important to finalize FT test programs first. Initially, FT takes precedence over CP, and CP testing can be introduced later based on actual needs.
These are some of the fundamental insights I’ve gathered from my experience. In real-world scenarios, each project comes with unique challenges, and solutions often require deeper analysis. The purpose of this article is to provide a clear understanding of CP and FT testing for non-test professionals, helping them make informed decisions. Ultimately, a strong testing team is essential for any company aiming to achieve high yields and cost control.
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