1. process window of SMT processing factory
Process window is usually used to describe the available limit range of process parameters, and it is the technical term of “USL-LSL” concept in SMT process field.
For example, according to experience, the lowest temperature of reflow soldering is generally 11~12ºC higher than the melting point of solder. When Sn63P-
At b37, the melting point of the alloy is 183ºC, and the lowest reflow soldering temperature is about 195ºC. In J-STD-020B, the maximum temperature of components is specified to be 245ºC, so that the available process window of lead-containing process in SMT factory is 50ºC, instead of the theoretical 62ºC obtained from “245-183”. Be sure to pay attention to the word “available” here.
2. Process Window Index (PWI) of SMT processing plant
Process Window Index,PWI) of SMT factory is an index to measure the adaptability of process capability within the process limit value determined by users. In other words, it is the maximum percentage of process window used to simply explain whether the process meets the requirements of technical specifications, and its value is basically the percentage of the reciprocal of Cp. The greater the PWI, the worse the process stability, and vice versa.
PWI=100xMax{ (measured value-average limit value)/(maximum limit range /2)}
For example, the process window index of the mounter (see Figure 1-64).
Taking the in-flow welding curve of SMT processing 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 maximum PWI of the four parameters is taken as the PWI of the temperature curve.
3. Process Capability Index (Cp) of SMT processing plant
The process capability index Cp of SMT factory is called process capability index by Taiwan Province enterprises. It reflects how many 6σ there are in the specification range () of users. 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-shaped graph with normal distribution. The narrower it is, the stronger the process capability of SMT processing plant is. USL is the upper limit of user specification, and LSL is the lower limit of user specification.
Select the core process indexes of general SMT processing plants for measurement. Such as reflow soldering furnace, we can measure the fluctuation of peak temperature under different load rates.
The process management index Cpk reflects the center of the bell-shaped graph with normal distribution, that is, the minimum value of Cpk =(USL-u)/(3σ) or (u-LSL)/(3σ). Where u is the center value of the user specification.
Pay attention to the concept of minimum value.