Cause Analysis and Countermeasures of PCB Flux Ignition During Wave Soldering
When PCB flux is over wave soldering, some customers find that the PCB flux will catch fire, especially when the temperature is high in summer and the air is dry, the situation is relatively high. The fire of PCB flux not only brings certain economic losses, but also easily causes fire to cause injury to work equipment or people. Based on the summary of years of PCB flux research and development technology and soldering process experience, we analyzed the following reasons for PCB flux ignition and proposed corresponding countermeasures.
In general, the reasons for this situation can be roughly summarized from three aspects, one is the problem of the PCB flux itself, the other is the process problem, and the last is the cause of the equipment working conditions.
Except for water-based PCB fluxes or some special PCB fluxes, the solvent part of commonly used PCB fluxes is mostly alcohol-based substances. There are mainly three types of methanol, ethanol, and isopropyl alcohol because of the toxicity of methanol. , And its relative instability in PCB flux, the use of methanol as a single solvent is less, so there are more manufacturers using ethanol, isopropanol or a mixture of isopropanol and methanol. Alcohols themselves are flammable and explosive. Refer to the following performance parameter comparison table for the three alcohols (Table 1). The three alcohols have similar flash points, all at 11-12 ° C, and their vapor pressures. They are: methanol is 13.33kPa / 21.2 ° C, ethanol is 5.33kPa / 19 ° C, isopropyl alcohol is 4.40kPa / 20 ° C. It can be seen that, under the same operating environment, methanol has the most flammable (or explosive) performance. Strong, followed by ethanol, and weakest is isopropanol. In the proportion of PCB flux, the flash point of PCB flux itself can be improved by adding different substances (especially flame retardants). Generally, the flash point of PCB flux can rise to about 20.8 ℃, which is why The reason for the flash point of PCB flux is not the same as the flash point of solvent. Therefore, it can be understood that the flame retardant added to the PCB flux is less likely to be flammable (or explosive) than products without the flame retardant.
Although the flammable (or explosive) performance of PCB flux can be reduced by adding flame retardants and other substances as mentioned above, it is not that the flame retardant (or non-explosive) PCB flame retardant is not added. It's just that the degree of flammability (or explosion) is lower than that of PCB flux without added flame retardant. Therefore, the PCB flux itself is still flammable (explosive). In this way, our users are required to pay special attention to keep away from open flames when using PCB flux, and eliminate various hidden dangers that may cause PCB flux to ignite (explosive).
Through the above analysis of the flammable (explosive) properties of the PCB flux itself, we can recognize that the commonly used PCB flux itself is flammable (except for water-based PCB flux or special PCB flux). Actually, in practice, In the production operation, the reasons for the PCB flux to ignite and burn are more from the operating process and the working conditions of the machinery and equipment. The following two points focus on the reasons that cause the "PCB flux to ignite during wave soldering".
As far as the welding process of "PCB flux over-wave soldering furnace" is concerned, the reasons that may cause PCB flux to fire during over-wave soldering are as follows:
1. The PCB flux is applied too much and drips onto the heating tube during preheating.
2. There are too many adhesive strips on the PCB, or the adhesive strips fall off on the heating tube or heating plate, igniting the adhesive strips.
3. When the board speed is too fast and the PCB flux is applied in a large amount, the PCB flux is continuously dripped onto the heating tube or the heating plate before it is completely evaporated.
4. The walking speed is too slow, causing the board temperature to be too high.
5. The preheating temperature is too high.
6. The flame retardancy of the PCB itself is poor, and the distance between the heating tube and the PCB is too close.
The above points are the reasons that may cause the PCB flux to ignite. In addition to the sixth reason, which may be the quality of the plate itself, other aspects can be improved through process adjustment. The second reason is the fire caused by too many adhesive strips on the PCB. Reduce the use of adhesive strips or use more adhesive and high temperature adhesive strips. This situation may be improved. The fourth and fifth reasons indicate that the preheating temperature is too high, and lowering the preheating temperature may effectively reduce the risk of fire. The first and third reasons are mainly about the amount of flux coating, and the reason for the excessive coating amount is the role of the air knife in wave soldering. This is also one of the reasons for the use of machinery and equipment to be discussed below.
From the perspective of the use of machinery and equipment, the reasons that may cause PCB flux to catch fire may be the following:
First, the wave soldering machine is not equipped with an air knife, or the angle of the air knife is not correct, the air knife hole is blocked, the air pressure is not enough, the wind is too weak to work properly, and so the PCB flux coating amount is too large. In the wave soldering machine, whether it is a spray type or a foaming wave soldering machine, after the PCB flux is applied, it must be blown by an air knife. Refer to Figures 1 and 2 below for the effect diagrams of the air knife of spray wave soldering machine and foam wave soldering machine.
As can be seen from the above figure, the normal working air knife blows the PCB coated with the PCB flux at an angle of about 40 degrees. The good blowing effect can make the PCB flux coating on the board more uniform. On the other hand, the excess PCB flux can be blown off to ensure that there will be basically no excess PCB flux dripping down in the preheating zone when the preheating zone is overheated; both reducing the risk of PCB flux fire, At the same time, it can protect the heating pipe from corrosion and prolong its service life.
If there is no air knife or the air knife can't work normally, it should be installed or adjusted and repaired in time. The production of the air knife is not complicated. A stainless steel tube with a length of about 50 cm and an inner diameter of 1-1.5 cm can be used to plug one end and connect the air pump tube to the other end. Use a thin drill in the middle of the stainless steel tube to evenly punch a small 0.3 mm. Hole, and then fix it behind the nozzle or soldering slot with a clip; adjust the blowing angle according to the method described in the diagram above.
Twenty-two, when the preheating zone is over, the vapor concentration of PCB flux in the furnace is too large, which causes deflagration. There are three main reasons for the excessive PCB flux vapor concentration:
1. Too much PCB flux is applied, or there is no effect of air knife, too much steam is volatilized in the preheating section. If it is caused by this situation, according to the method mentioned above, the air knife can be installed or adjusted to the normal working state to reduce the concentration of the steam in the furnace;
2. The exhaust (exhaust) device of the wave soldering machine cannot work normally, or the exhaust effect is poor, which causes the PCB flux concentration in the furnace to be too high. It can strengthen the air extraction (exhaust) device on the wave soldering machine. It is recommended to use a powerful and powerful air extraction device to ensure that the PCB flux concentration in the furnace (especially the preheating section) is not too high;
3. If the wave soldering machine's exhaust (exhaust) air device itself is ineffective, and there is a protective cover on the preheating section of the wave soldering (refer to Figure 3), in this case, it is likely to cause PCB flux The concentration is too high. If the various improvements are not effective, it is recommended to remove the protective cover. This can reduce the concentration of the flux vapor in the furnace and reduce the possibility of PCB flux detonation.
In summary, the main reasons for the PCB flux to ignite during wave soldering are as follows:
1. The PCB flux itself is not added with a flame retardant or the amount of flame retardant is too small or the type of addition is incorrect; etc. (This is a more complex and difficult to judge problem, because we have mentioned above that even if it is added Flame retardant, PCB flux itself is still flammable and explosive, but the degree of deflagration may be relatively lower than PCB flux without added flame retardant. At the end of this article, I sincerely say, In fact, the effect of this behavior is not obvious, and it cannot change the flammable and explosive characteristics of the PCB flux itself.)
2, the process problems of PCB flux over wave soldering. In addition to the poor flame retardancy of the PCB itself, the other aspects, such as the board running speed is too fast or too slow, the preheating temperature is too high and other bad conditions can basically be effectively improved by the process parameters.
3, the working condition of the machine itself. In fact, process issues and the working conditions of machinery and equipment are two closely related and indivisible factors that affect each other. Because we have analyzed above, the common PCB flux itself is flammable and explosive (in addition to water-based PCB flux or special PCB flux), so more often, we hope that manufacturers can use the process and equipment conditions. Adjustment to improve the problem of PCB soldering flux.
The ultimate purpose of writing this article is to hope that all PCB flux manufacturers can completely solve or avoid the situation that "PCB flux catches fire during wave soldering"; all PCB flux manufacturers are also expected to be able to use PCB flux The safety during the use of the flux is more fully considered, and the degree of detonation of the PCB flux is reduced as much as possible. In view of the situation that the PCB flux is on fire, the flux manufacturer cannot completely shirk its responsibility to the user, and the PCB flux user cannot completely blame the PCB flux manufacturer. In such a situation, it is still hoped that production and Using both parties, you can calmly and objectively find the cause of the problem from multiple aspects, and finally find out countermeasures so that the problem can be completely resolved.