What are the technological windows and capabilities of SMT processing plants?

Process window is usually used to describe the limit range of available process parameters. It is a professional term for the concept of “user specification range (usl-lsl)” in the field of SMT process.
For example, according to experience, the minimum temperature of reflow welding is generally 11–12 ° C higher than the melting point of solder, when sn63p is used-
At b37, the melting point of the alloy is 183 ° C, and the lowest reflow welding temperature is about 195 ° C. In j-std-020b, the maximum temperature of components is specified as 245 ° C, so that the process window available for lead process in SMT processing plant is 50 ° C, instead of the theoretical 62 ° C obtained from “245–183”. We must pay attention to the word “available” here.

The process window index (PWI) of SMT processing plant is an index to measure the adaptability of process capability within the range of process limit value determined by users. In other words, it is the maximum percentage of process window used to simply explain whether the process is satisfied with the requirements of technical specifications. Its value is basically the percentage of the reciprocal of CP. The larger 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 flow welding curve of SMT processing plant 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 value of PWI in the four parameters is taken as the PWI of the temperature curve.

The process capability index CP of SMT processing plant is called process capability index by Taiwan enterprises. It reflects the number of 6 σ in the user’s specification range (∂). The larger the value is, the higher the stability of the process is.
Cp=（USL-LSL）/6σ
Among them, σ standard deviation reflects the average value of the distance from each point of data to its average, that is, the narrower the “bell” shape of normal distribution is, the stronger the technological capacity of SMT processing plant is; USL is the upper limit of user specification and LSL is the lower limit of user specification.
Generally, the core process indexes of SMT processing plant are selected for measurement. For example, in reflow soldering furnace, we can measure the fluctuation of peak temperature under different load rates.
CPK, the process capability management index, reflects the neutrality of the bell shape of normal distribution, that is, the minimum value of CPK = (usl-u) / (3 σ) or (u-lsl) / (3 σ). Where u is the central value of the user specification.
Pay attention to the concept of minimum value.

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