Customer requirement documents often specify a yield standard, such as 99.5% or higher. However, a quick search online reveals no clear distinction between "equipment yield" and "equipment production yield." Yield generally refers to the ratio of the number of good products produced within a given timeframe to the total number of products manufactured. That is: Yield = Number of Good Products / Total Number of Products. Because many factors can cause equipment defects, including not only equipment-related issues but also other non-equipment factors, it's unfair to equipment suppliers to broadly specify product or process yield requirements. This article defines the concept of equipment yield primarily to facilitate communication between equipment suppliers and users, preventing misunderstandings.
Equipment yield
Equipment yield refers to the ratio of the total number of products produced by the equipment within a certain period of time minus the number of defective products caused by equipment problems, to the total number of products produced. This is used as an indicator to evaluate equipment performance and as one of the conditions for equipment acceptance.
Equipment yield calculation formula: Equipment yield = (Total production quantity - Quantity of defective products due to equipment factors) / Total production quantity.
The concept of equipment yield refers neither to the yield rate of the manufacturing equipment itself nor to the overall production line yield. It's crucial to clarify this when discussing yield with clients. Many factors influence production yield, including incoming materials, equipment, operating methods and processes, and product structure. During the equipment validation production phase, when analyzing the causes of product defects, it's essential to separately analyze the types of defects and their causes to identify the root cause. If the problem stems from equipment issues, improvements must be made to the equipment, including readjustment, increased precision, and parameter adjustments.
Yield rates are typically set by customers based on their past production experience and needs. They are both a process capability indicator and directly impact actual production capacity, often serving as a key performance indicator for the production department. Equipment suppliers analyze the product's process during the solution evaluation phase, identifying potential risks that could lead to product defects and selecting appropriate solutions. Equipment-related product defects include surface scratches, deformation, damage, dirt, out-of-specification dimensions, inaccurate positioning, incorrect shape, poor adhesion, inadequate locking, and discrepancies between the final product and specifications. Some of these are secondary defects, primarily caused by issues with clamping, handling, positioning, and pressing, and should be avoided as much as possible.
Some defects are functional, mainly caused by design flaws (implementation method problems), unreasonable structures, and poor equipment precision and stability. During the solution evaluation phase, the handling of incoming material defects and the disposal of defective products should be comprehensively considered. For incoming material defects, automatic detection can be used to screen for defects, preventing defective products from entering the equipment, or clear requirements can be set for incoming materials.
Cpk --- Process Capability Index
Cpk (Constant Pace) is a measure of the degree to which process capability meets technical standards (e.g., specifications, tolerances). It is a modern indicator used by enterprises to represent process capability. It is also used by users to evaluate and accept newly introduced equipment. Process capability is the ratio of the maximum allowable variation in process performance to the normal deviation of the process. Process capability studies aim to confirm the degree to which these characteristics meet specifications, ensuring that the rate of non-conforming finished products is above the required level, serving as a basis for continuous process improvement. After the product passes Gage R&R testing, Cpk value testing begins. A higher Cpk value indicates better quality. When 1.67 > Cpk ≥ 1.33, the condition is considered good, indicating good and most stable process capability. When Cpk = 1.33, it means that the theoretical pass rate of products produced by this process is 99.379%. Calculation formula: CPK = Min((X - LSL/3σ), (USL - X/3σ))
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