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What are the process windows and process capabilities of the smt factory
2021-10-03
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Author:Frank

What are the process windows and process capabilities of the smt factory
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1. Process window of smt factory
The process window is usually used to describe the available limit range of the process parameters. It is the professional term of the "user specification range (USL-LSL)" concept in the SMT process field.


For example, according to experience, the minimum temperature of reflow soldering is generally 11~12oC higher than the melting point of the solder. When Sn63P-b37 is used, the melting point of the alloy is 183oC, and the minimum reflow soldering temperature is about 195oC. In J-STD-020B, the maximum temperature of components is specified as 245oC, so that the process window available for leaded processes in SMT processing plants is 50oC instead of the theoretical 62oC obtained from "245-183". Here must pay attention to the word "available"

pcb board

2. Process window index (PWI) of smt factory


The Process Window Index (PWI) of the SMT factory is an index that measures the adaptability of the process capability within the process limit value range determined by the user. In other words, it is the maximum percentage of the process window used to simply indicate whether the process is satisfactory According to technical specifications, its value is basically the reciprocal percentage of Cp. The larger the PWI, the worse the process stability, and vice versa.


PWI=100xMax{(measured value-average limit value)/(maximum limit range/2)}

    

Take the in-flow welding curve of the SMT factory as an example. The main control parameters of the process curve are heating rate, preheating time, preheating end time, peak temperature and time above melting point. Through measurement and calculation, the PWI of the four parameters is the largest. The value is used as the PWI of the temperature curve.


3. Process capability index (Cp) of SMT factory


 The process capability index Cp of SMT factories is called the process capability index for Taiwanese companies. It reflects how many 6σ are in the user's specification range (∂). The larger the value, the higher the stability of the process.

                           Cp=(USL-LSL)/6σ

 Among them, σ standard deviation reflects the average value of the distance from each point of the data to its average, that is, the width of the "bell" shape of the normal distribution. The narrower the SMT processing plant, the stronger the process capability; USL is the upper limit of user specifications, LSL is the lower limit of user specifications.

 

Generally select the core process indicators of the pcb factory for measurement. Such as reflow soldering furnace, we can measure the fluctuation of peak temperature under different load rates.

The process capability management index Cpk reflects the centrality of the "bell" shape of the normal distribution, that is, the minimum value of Cpk = (USL-u)/(3σ) or (u-LSL)/(3σ). Where u is the central value of the user specification.