Judging from the current situation of China's SMT electronic welding manufacturing industry, many users simply monitor the furnace temperature with a furnace temperature tester and cannot judge the true status of the welding furnace and whether the process conditions are set reasonable. As a result, quality defects cannot be circumvented, so there are often disputes about process problems or equipment problems.
1) Equipment problem or process problem?
We need to identify this problem, we must have a comprehensive understanding of the equipment. When we use a thermometer to confirm the furnace temperature, in fact, in most cases, it is carried out under no-load conditions, and the actual production situation will be more complicated. Under full load, the heat feedback capability of the welding furnace, the heat compensation capability and the control of the board entrance interval will directly affect the heating temperature environment of the actual product.
Only when we fully control the temperature changes in each temperature zone: Incoming PCB---heat absorption---temperature drop---feedback---compensation---returning temperature---feeding board; We can achieve accurate control of the equipment. However, the problem comes again:
1) How should we fully control the change in heat caused by each PCB passing through each temperature zone?
2) Is this thermal change controlled in terms of long-term stability?
3) If it is not under control, how to tell if there is a problem with process control?
4) Or is it caused by the instability of the device itself?
This series of problems are all severe challenges facing the current SMT welding process.Next, analyze the equipment problem or the process problem with a case:
Among the SMT factories currently in service, one of them has occasional cold welding, and there is no fixed rule. Let’s first understand its process capability:
From the CPK matrix table of the 125 production boards shown in the figure to monitor the various process parameters of each thermocouple probe in real time, the CPK of its peak temperature is 1.16, which is less than 1.33 (4Sigma). This shows that the process is not robust. However, whether it is caused by the equipment or the process problem needs further analysis, and then the equipment stability is analyzed:
First of all, we have to understand the stability of the welding furnace. We need to define the input of the product. For the current single board, what is the reasonable boarding interval? With the help of the software function "Capacity Planning" (see Part 3 for details), we know that at least 45 seconds of boarding interval is guaranteed.
Therefore, a mass production of 125 pieces of boards with a 45-second loading interval control was carried out. To analyze the minimum temperature of each board when it passes through each temperature zone, the minimum temperature of the corresponding zone when the veneer passes through the furnace is taken as the specification, and the stability of the furnace is fully loaded and continuous production is evaluated.
The evaluation found that the CPK of the 10 heating zones, except for the first zone CPK 1.67, the other heating zones CPK are above 2.0 (see the example in Figure 3 below). These data clearly show that the furnace is quite stable when the process is set within the furnace's tolerance.
However, the question is here again, can the actual production be able to strictly control the input interval of each input board like we do experiments? This can be confirmed from the record of the interval between each board and the previous board monitored in real time.
The above data shows that in actual production, the interval between two adjacent plates is only 3 to 5 seconds. How does the temperature in the furnace change under such frequent plate loading?
