The effect of SMT reflow oven temperature tester temperature curve on SMT welding quality!
What is the effect of the SMT reflow soldering temperature tester curve setting on SMT soldering quality? SMT reflow soldering furnace tester comprehensive process monitoring to achieve intelligent closed-loop manufacturing, improve the quality of SMT electronics manufacturing products.
With the rapid development of the electronics industry, high integration and high reliability have become the new trend of the industry. Driven by this trend, SMT (Surface Mount Technology) has been further promoted and developed in China. Many companies have applied SMT technology and surface mount components (SMC/SMD) in production and R&D. Therefore, the welding process is inevitably a large amount of use of reflow soldering. We will talk about some of our experiences and opinions on the reflow soldering temperature curve.
SMT reflow oven temperature tester furnace temperature curve is generally set according to the solder paste and the components on the PCB and the materials used by it, and the temperature curve generated under different PCB environments It is not the same! The temperature curve we tested was actually the temperature on the test PCB board, not the temperature on the stove you saw.
The SMT furnace temperature curve is a coordinate axis of the actual temperature and time of the reflow furnace. It can tell us at what time is the temperature value at that time, so that some important process parameters such as temperature rise slope, constant temperature time, and reflow time are obtained. The return of these process parameters to the entire product until the critical relationship, we can determine the furnace does not meet our solder paste, product process through the furnace temperature curve, and then adjust the furnace temperature, as the different parameters for the entire The influence of the process, I am afraid to slowly accumulate through some theory and then my own experience.
The only way to control your process is to understand your process, and you need to pass a measurement to get a good idea of your process. Evaluate the necessity of the reflow oven temperature curve test in the welding process control.
There are several reasons why your company needs to have its own temperature profile tester on the reflow oven.
First, the use of the temperature profile tester is the key to controlling the process flow throughout the reflow oven operation. Without a temperature curve tester, you won't know if the furnace's function is perfect, if it needs calibration, and so on.
Secondly, the temperature curve tester plays a key role in helping manufacturers to verify and solder all components on the board under standard operation to ensure high reliability and low loss. For example: some components have a maximum melting point of 240C; if there is no temperature curve tester, how can you know if the current state you set meets the melting point requirements of these components?
Third, having a temperature tester can reduce production losses and analyze production losses to avoid recurrence. The direct causes of welding quality are rising slope, immersion tin temperature, wetting time, tin melting time, average temperature and other reflow oven parameters. Without a temperature profile tester, you cannot accurately measure these important characteristics in the reflow oven process.
Finally, when you introduce new boards into different thermal processes, they need to fine-tune the parameters of the reflow oven (zero and chain speed settings) to ensure that the components and solder paste performance parameters are met during soldering. .
SMT reflow soldering is a key component of the surface mount process. Its correct use is undoubtedly to further ensure solder quality and product quality. In the use of reflow soldering, the most difficult to grasp is the setting of the temperature profile of reflow soldering. How can we set the temperature curve of reflow soldering more reasonably?
To solve this problem, we must first understand the working principle of reflow soldering. From the temperature curve (see Figure 1-1), the principle of reflow soldering is analyzed: when the PCB enters the heating zone (drying zone), the solvent and gas in the solder paste evaporate, and at the same time, the flux in the solder paste wets the pad, The terminals and pins of the components, the solder paste softens, collapses, covers the pads, the terminals of the components and the pins are isolated from the oxygen. When the PCB enters the insulation zone, the PCB and components are fully preheated to prevent the PCB. Suddenly entering the high temperature area of the soldering and damaging the PCB and components → When the PCB enters the soldering area, the temperature rises rapidly and the solder paste reaches the molten state. The liquid solder wets, diffuses, and flows over the pads, component terminals and pins of the PCB. Or reflow mixing to form a solder joint → PCB into the cooling zone to solidify the solder joint. At this point, the soldering process of the reflow oven was completed.
In the field of SMT electronic manufacturing, "quality and stability, efficient automation, solder joint reliability" is always a hot topic. With the wave of German Industry 4.0, the entry of smart interconnection and other topics, SMT factory is even more chaotic, more and more The more users are blindly pursuing, of course, there are also many such as: Huawei, ZTE and other leading representatives have been at the forefront of the times, exploring the integration of intelligence, IoT, high-speed transmission and big data, in order to achieve the ultimate goal of unmanned chemical plants. I am tireless to explore.
Talking about Industry 4.0, as far as China's overall level is concerned, we are still between 2.0 and 3.0, which is far from 4.0. Based on China's current industrial development status, we should start from strengthening the foundation and do standardization work. Popularize comprehensive process capability monitoring, and then transition to the intelligent interconnection platform, step by step, and finally achieve the goal.
As far as the current situation of China's SMT electronic welding manufacturing industry is concerned, many users simply monitor the temperature of the welding furnace with a furnace temperature tester. It is impossible to judge whether the actual condition of the welding furnace and the process conditions are reasonable. Therefore, quality defects are not circumvented, so there is often a debate about process problems or equipment problems.
1) Equipment problems or process problems?
We need to identify this problem, we must have a comprehensive understanding of the equipment, when we use the thermometer to confirm the furnace temperature, in fact, in most cases, it is carried out under no-load conditions, and the actual production situation It will be more complicated. Under full load, the heat feedback capability of the furnace, the thermal compensation capability and the interval control of the inlet plate will directly affect the heating temperature environment of the actual product.
(Fig. 1) Heating structure
Only when we completely control the temperature change in each temperature zone: into the board --- heat absorption --- temperature drop --- feedback --- compensation --- back to temperature --- into the board; we can do it Accurate manipulation of the device. However, the problem is coming again:
1) How should we fully control the change in heat generated by each production panel through each temperature zone?
2) Is this heat change controlled in terms of long-term stability?
3) If it is not controlled, how to judge the problem of process control?
4) Or is the problem caused by the instability of the device itself?
This series of problems is a serious challenge facing the current SMT welding process. Next, we will analyze the equipment problem or the process problem in one case:
Among the SMT factories that are currently serving, one of them has occasional cold welding, and there is no fixed law. Let us first understand the process capability status:
(Figure 2) CPK matrix table
From the 125-sheet production board shown in Figure 2, the CPK matrix of each thermocouple probe is monitored in real time. The CPK of the peak temperature is 1.16, which is less than 1.33 (4 Sigma). This indicates that the process is not stable. However, whether it is caused by equipment or process problems, further analysis is needed, and then the stability of the equipment is analyzed:
First of all, we need to understand the stability of the furnace, we have to define the input of the product, for the current board, what is the reasonable board spacing? With the software function "Capacity Planning" (see section 3), we learned that at least 45 seconds of board spacing is ensured.
Therefore, 125 batches and 45 seconds of batch-to-board interval control are used to analyze the minimum temperature of each panel passing through each temperature zone. The minimum temperature of the corresponding zone is ±3 °C as the specification. Perform a stability assessment of the continuous production of the furnace at full load.
The evaluation found that the CPK of the 10 heating zones, except for the first zone CPK 1.67, the other heating zones CPK were above 2.0 (see the example in Figure 3 below). These data clearly indicate that the furnace is fairly stable when the process is set within the furnace's affordability.
(Figure 3) Zone 9 CPK
However, the problem is coming again. Can the actual production strictly control the board spacing of each input board as we do the experiment? At this point, we can record the interval between each board and the previous board from real-time monitoring. Confirmed.
(Figure 4) Incoming board interval recording
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 entry conditions?
(Fig. 5) Temperature drop in the recirculation zone
From the point of view of the refining capacity of a reflow zone for continuous feeding of the furnace, it is obvious that the furnace temperature continues to decrease, the thermal compensation capacity is simply insufficient, and the maximum drop is 4 to 5 ° C. This temperature is not only in a certain A recirculation zone occurs, and all 10 heating zones have a temperature drop of 4-8 °C, which is the root cause of occasional cold welding.
Furthermore, it is investigated for the period of continuous excessively frequent board entry. This period is just in the white night shift period. A batch of boards are stacked in front of the furnace. In order to increase the production capacity, the artificial push plate is put into the furnace to make the placement machine empty. The placement machine can enter the working state as soon as possible. This phenomenon is a prohibited behavior in production management.
At present, the case has come to an end, but have we ever thought about a problem: how much fluctuations in the furnace will occur, and the interval between the boards is difficult to avoid human intervention, then is there a suitable solution? Let's share the process risks and mitigation methods together.
2) Is the furnace process robust enough?
Many users use the thermometer to measure the furnace temperature. As long as the temperature curve is qualified, the production is ordered. Is the process robust? We can clearly see through the following two examples!
(Figure 6) Process A: PPI Process Risk Warning
(Figure 7) Process B: PPI process controlled
In Figure 6 and Figure 7, all the curve data are within the scope of the process specification, but the process B of Figure 7 is more robust than the process A of Figure 6, how to determine?
Firstly, the PPI (ProcessPerformance Index) is analyzed. The test results of each parameter of each channel need to meet the process specification requirements. The test result is located at the upper and lower limits of the process specification, which is the main factor affecting the robust process. The following is the solution to the peak temperature of the second channel:
Peak specification: 230 ° C (PPI = -100) ~ 250 ° C (PPI = 100);
1) If the measured value of the A process is: 230.0 ° C (PPI = -100);
2) If the measured value of the B process is: 240.0 ° C (PPI = 0);
It can be seen that the A and B processes are all satisfactory, but the A process has reached the lower limit of the specification. A slight temperature fluctuation of the furnace or an increase in the frequency of the plate will result in insufficient peak temperature and cold welding.
The B process is not the case, the resistance of the soldering furnace temperature fluctuation, the full load capacity is very strong, even if the board is too fast, there is still room for some fluctuations.
(Figure 8) PPI schematic
Therefore, through real-time statistical analysis of the risk PPI of each production board, you can grasp whether the current product has the upper or lower limit risk of the specification, and you can know whether your current process is robust enough, and whether there is room for optimization. On a board, the device Delta T is too large to optimize the robust process, and it is especially important to control the tempo of each product. So how do you determine the rhythm of different sizes and weight plates?
3) How to scientifically plan production capacity and intelligently control the board?
Faced with different sizes, different thicknesses, weights and materials, how should the capacity be distributed to meet both quality requirements and maximum capacity?
The planning of production capacity needs to have a scientific basis, rather than planning by experience, imagination, and production tasks. We must study the heat absorption and heat recovery of each board in each temperature zone in order to correctly plan production capacity.
As shown in the figure below, you can understand the thermal compensation capability of this area by knowing the time required for the board to pass through each zone from the heat absorption temperature to the heat recovery temperature to the initial state. Through the “capacity planning” function, the thermal compensation capability of each heating zone can be grasped, and the time required for each zone to return to the initial state is known. By comparison, find the longest rewarming time as the ideal entry interval.
(Figure 9) Scientific Capacity Planning
But when the planned ideal board spacing is too large, how should we choose to meet the maximum capacity? Here are a few ways to share or optimize:
1) Increase the wind frequency, optimize the heating efficiency, and improve the thermal compensation capability of each temperature zone to reduce the compensation temperature recovery time;
2) Check whether the temperature sensing probe in each temperature zone is at the tuyere, improve the temperature sensing sensitivity, shorten the feedback time, and reduce the compensation warming time;
3) The compensation temperature recovery time of the important temperature zone is mainly. If the first zone needs 90 seconds to warm up, and the other zone only needs 60 seconds, then 60 seconds can be considered as a reasonable board spacing.
With scientific capacity planning, can it be done once and for all? Actually, then, who can go to control the board of each production board at reasonable intervals? People always make mistakes, so it is especially important to control the welding plate signal through SMEMA and automatically control the incoming plate according to the different board spacing of each board plan.
The above has been introduced to the temperature fluctuation, compensation, and planning sections, but the main focus is on the temperature level. In fact, in addition to temperature, which affects the quality and reliability of welding, there are chain speeds, fans and track vibrations. Do we have any countermeasures in these areas?
4) Speed, wind, vibration, the three killers, how to monitor?
The most worrying aging problem with chain speed? How is the maintenance cycle defined? What is the long-term stability of CPK?
Through the following statistical analysis of the chain speed aging trend, you can grasp this information in order to fully understand the chain speed variation process.
(Figure 10) Real-time chain speed monitoring
(Figure 11) Chain speed aging analysis
For the hot air reflow oven, in addition to the "heat" factor, "wind" plays a key role, "heat" needs to be transmitted to the board, and the "wind" medium is indispensable, and the size of the wind directly affects the heat transfer. How much, how fast, directly affects the strength of thermal compensation and so on.
Therefore, the real-time monitoring of the actual fan speed and the trend analysis of the aging data help us to judge the health status of each fan, keep abreast of its working status, and avoid bad quality.
(Figure 12) Fan actual speed monitoring
(Figure 13) Fan Fluctuation Analysis
At present, the abnormal quality of the missing parts is mostly caused by the vibration of the track, and it is usually hidden and difficult to monitor. The influence of vibration on the product has become the most critical factor of the quality problem. So how much do we know about the vibration? ?
We can understand the working state of the equipment through the understanding of the real-time vibration of the track and the analysis of the historical vibration data, and the time of the surrounding environment changes to bring about strong vibration time, so as to filter out the specific vibration source.
(Fig. 14) Track vibration monitoring
(Fig. 15) Orbital vibration analysis
For the vibration sources that may cause strong vibrations, summarize the current situations encountered on the client side:
1) vibration from the placement machine;
2) vibration from the cooling fan;
3) vibration from the transmission of the chain;
4) Vibration from the orbital deformation extrusion furnace carrier
5)......
As long as you find the source of vibration, the way to solve the vibration is a matter of opinion!
In summary, we have carried out comprehensive analysis and monitoring from the aspects of temperature, chain speed, wind and vibration; only by comprehensive monitoring of the process and sufficient data support, we can move to the final intelligent self-adjustment stage of intelligent closed-loop control. .
5) Intelligent closed loop manufacturing, are you ready?
How to understand intelligent closed loop manufacturing? Can be divided into three parts to understand:
1) Intelligence: the representative has independent thinking and logic ability;
2) Closed loop: realizes two-way data interaction between uplink and downlink;
3) Manufacturing: serving manufacturing services;
This shows that many of my friends may still not understand much. I will use a case description to help my friends deepen their understanding of intelligent closed-loop manufacturing.
A product A is being produced;
Real-time monitoring of the full load recirculation zone temperature is 5 ° C lower than the no-load;
Real-time monitoring collects and analyzes all relevant information;
Real-time monitoring reports relevant information to the MES brain;
MES based on real-time monitoring and comprehensive analysis results;
The MES issues an upward temperature command to the welding furnace;
The furnace receives the command and raises the recirculation zone by 5 ° C;
Real-time monitoring continues to monitor the status of the furnace;
The entire process optimization adjustment process does not require human intervention;
Realize the realization of unmanned chemical plants.
(Figure 16) Intelligent closed-loop manufacturing relationship diagram
(Figure 17) Schematic diagram of intelligent closed-loop manufacturing
Intelligent closed-loop manufacturing is not a one-step process. Before entering intelligent closed-loop manufacturing, it is necessary to establish a certain degree of standardization system to a certain degree, the degree of automation reaches a certain level, and in the process of transition from automation to intelligence, many external environment support is needed, such as : Big data high-speed network transmission, big data operations, processing; big data storage. And there are many transitional stages in the middle, and finally step by step to the intelligent closed-loop manufacturing of unmanned chemical plants!
From comprehensive process monitoring to intelligent closed-loop manufacturing, it is a long process, a process of accumulation, a trend of the times, and the joint efforts of several generations of people, gradually improving, and finally achieving, let us work together!
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GUS Technology has 20 years of SMT professional manufacturing experience and has trained more than 1,000 technicians and trusted Chinese suppliers in more than 50 countries around the world;
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Kiman chen
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Shenzhen GUS Technology Co., Ltd.
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