PCB flux in the wave soldering over the causes of fire and improvement measures

When the PCB flux is wave soldered, some customers find that the PCB flux will catch fire and burn, especially in the summer when the temperature is high and the air is dry, the incidence of this situation is relatively high. The fire of PCB flux will not only cause certain economic losses, but also easily cause fire, which will cause injury to working equipment or personnel. Based on years of experience in PCB flux R&D technology and soldering process, we analyze the causes of PCB flux fire as follows, and put forward corresponding countermeasures.

In general, the reasons for this can be summarized from three aspects: one is the problem of the PCB flux itself, the second is the process problem, and the third is the reason for the working state of the equipment.

Except for water-based PCB fluxes or some special PCB fluxes, most of the solvents of commonly used PCB fluxes are mainly alcohols, mainly including methanol, ethanol and isopropanol. Due to the toxicity of methanol and its relative instability in PCB fluxes, it is rare to use methanol alone as a single solvent, so there are many manufacturers that use ethanol, isopropanol or isopropanol and methanol mixed solvents. Alcohols themselves are flammable and explosive, the flash points of the three alcohols are not much different, all at 11 - 12 °C, and their vapor pressures are: 13.33 kPa for methanol at 21.2 °C, 5.33 kPa for ethanol at 19 °C, and 4.40 kPa for isopropanol at 20 °C. It can be seen that under the same operating environment, methanol is the most flammable (or explosive), followed by ethanol, and isopropanol is the weakest. In the ratio of PCB flux, by adding different substances (especially flame retardants), the flash point of PCB flux itself can be improved, and the flash point of PCB flux can be increased to about 20.8 °C, which is the reason why the flash point of PCB flux is not equal to the flash point of solvent. Therefore, it is understandable that products with flame retardants added to PCB fluxes are less likely to be flammable (or explosive) than products without flame retardants.

However, although the flammability (or explosive) performance of PCB flux can be reduced by adding substances such as flame retardants, PCB fluxes with flame retardants are not non-flammable (or non-explosive), but are less flammable (or explosive) than PCB fluxes without flame retardants. Therefore, PCB flux itself is still flammable (explosive). This requires users to pay special attention to keep away from open flames when using PCB flux, and eliminate all kinds of hidden dangers that may cause PCB flux to burn (explode).

Through the above analysis of the flammable (explosive) properties of the PCB flux itself, we can realize that commonly used PCB fluxes (except for water-based PCB fluxes or special PCB fluxes) are inherently flammable. In fact, in actual production operations, the cause of PCB flux fire and combustion is more due to the operating process and the working state of machinery and equipment.

In the case of the soldering process of the "PCB flux overwave soldering furnace", the causes that may cause the PCB flux to catch fire during wave soldering are as follows:

<!--[if !supportLists]-->1.<!--[endif]-->Too much PCB flux is applied and drips onto the heating tube during preheating.

<!--[if !supportLists]-->2.<!--[endif]-->There are too many strips on the PCB, or the strips fall off the heating tube or plate, which ignites the beads.

<!--[if !supportLists]-->3.<!--[endif]-->When the board is too fast and the PCB flux is coated with a large amount, the PCB flux continues to drip onto the heating tube or heating plate before it completely evaporates.

<!--[if !supportLists]-->4.<!--[endif]-->The speed of the board is too slow, resulting in the temperature of the board surface being too high.

<!--[if !supportLists]-->5.<!--[endif]-->The preheating temperature is too high.

<!--[if !supportLists]-->6.<!--[endif]-->The flame retardant performance of the PCB itself is poor, and the distance between the heating element and the PCB is too close.

Among the reasons that may cause PCB flux fire in the above process aspects, except for the sixth reason may be the quality problem of the plate itself, several other aspects can be improved by process adjustment. The second cause is a fire caused by too much strip on the PCB or strip detachment, which may be improved by reducing the use of strips or using more adhesive strips that are more adhesive and resistant to high temperatures. Both the fourth and fifth causes indicate that the preheating temperature is too high, and lowering the preheating temperature may effectively reduce the potential fire hazard. The first and third reasons mainly involve too much flux coating, and the reason for the excessive coating amount is that in addition to the large amount of spray or foaming, the role of the air knife in wave soldering has to be considered, which 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 lead to the fire of PCB flux may be as follows: 1. The wave soldering machine is not equipped with an air knife, or the angle of the air knife is incorrect, the air knife hole is blocked, and the air pressure is insufficient to cause the wind to be too weak to work normally, etc., resulting in excessive coating amount of PCB flux. In the wave soldering machine, whether it is a spray or foam wave soldering machine, after the PCB flux is coated, it has to go through the air knife blowing.

The normal working air knife blows on the PCB coated with PCB flux at an angle of about 40 degrees, and the good blowing effect can make the PCB flux coating on the board surface more uniform on the one hand, and blow off the excess PCB flux on the other hand, so as to ensure that when passing through the preheating area, there will be no excess PCB flux dripping in the preheating area; It not only reduces the risk of PCB flux fire, but also protects the heating element from erosion and prolongs its service life.

If there is no air knife or the air knife cannot work normally, the air knife should be installed, adjusted and repaired in time. The production of the air knife is not complicated, you can use a stainless steel pipe with a length of about 50cm and an inner diameter of 1 - 1.5cm, block one end of it, connect the air pump pipe at the other end, and drill a small hole of about 0.3mm evenly with a fine drill bit in the middle of the stainless steel pipe, and then fix it behind the nozzle or flux groove with a clamp card;

2. When passing through the preheating zone, the steam concentration of PCB flux in the furnace is too large, which causes deflagration. There are three main reasons for excessive vapor concentration of PCB flux:

<!--[if !supportLists]-->1.<!--[endif]-->The PCB flux is coated too much, or there is no air knife, and too much steam is volatilized in the preheating section. If this is the case, the flux vapor concentration in the furnace can be reduced by installing the air knife or adjusting the air knife to normal working condition as described above.

<!--[if !supportLists]-->2.<!--[endif]-->Based on the above analysis of the causes of PCB flux fires, we can cite the ideas in this article to further elaborate on the importance of coping strategies. As pointed out in the article, the flammable nature of PCB flux itself is difficult to fundamentally change by adding flame retardants, so in practice, the control of processes and equipment is particularly critical.

<!--[if !supportLists]-->3.<!--[endif]-->The exhaust (exhaust) device of the wave soldering machine cannot work normally, or the exhaust effect is poor, resulting in a high concentration of PCB flux in the furnace. The exhaust (exhaust) device on the wave soldering machine can be strengthened, and it is recommended to use a powerful and powerful exhaust device to ensure that the concentration of PCB flux in the furnace (especially in the preheating section) is not too high.

<!--[if !supportLists]-->4.<!--[endif]-->If the wave soldering machine itself has a poor extraction (exhaust) device and has a protective cover above the preheating section of the wave soldering, it is likely to cause the concentration of PCB flux to be too high. If various improvement measures are not effective, it is recommended to remove the protective cover, which will reduce the flux vapor concentration in the furnace and reduce the possibility of PCB flux deflagration.

To sum up, the main reasons for the fire of PCB flux during wave soldering are as follows:

<!--[if !supportLists]-->1.<!--[endif]-->The PCB flux itself does not add flame retardant, or the amount of flame retardant added is too small, and the type of flame retardant is incorrect, etc.; (This is a complex and difficult problem to judge, as we mentioned earlier that even with the addition of flame retardant, the PCB flux itself is still flammable and explosive, but the degree of deflagration may be relatively low compared to the non-flame retardant PCB flux.) At the end of this article, I would like to say from the bottom of my heart that this is not an obvious effect and does not change the flammable and explosive nature of the PCB flux itself. ）

<!--[if !supportLists]-->2.<!--[endif]-->Process issues with PCB flux overwave soldering. In addition to the poor flame retardancy of the PCB itself, other defective conditions such as too fast or too slow board running speed, too high preheating temperature, etc., can basically be effectively improved by adjusting the process parameters.

<!--[if !supportLists]-->3.<!--[endif]-->Problems with the working condition of the machine itself. In fact, process problems and the working condition of machinery and equipment are two factors that are closely related and inseparable and affect each other. Because we have previously analyzed that commonly used PCB fluxes (except for water-based PCB fluxes or special PCB fluxes) are inherently flammable and explosive, more often than not, we hope that the user manufacturers can improve the problem of PCB flux fire by adjusting the process and equipment condition.

<!--[if !supportLists]-->4.<!--[endif]-->In terms of process, the amount of PCB flux must be strictly controlled to avoid over-coating. For example, in the process of board walking, the speed of board walking should be accurately set according to the specific situation of the PCB, so that the flux can not be completely volatilized and drip onto the heating tube or heating plate too quickly, nor can the temperature of the board surface be too high too slowly. At the same time, the preheating temperature should also be accurately controlled to avoid the risk of fire caused by too high preheating temperature. In addition, it is necessary to reduce the amount of adhesive strip used on the PCB, and ensure the quality of the adhesive strip to prevent the adhesive strip from falling off the heating element or heating plate.

<!--[if !supportLists]-->5.<!--[endif]-->From the equipment point of view, the correct installation and proper operation of the air knives is crucial. Enterprises should regularly check whether the air knife has problems such as incorrect angles, blocked holes, or insufficient air pressure to ensure that the air knife can effectively blow off excess PCB flux and make the coating more uniform. At the same time, the exhaust (exhaust) device of the wave soldering machine should also be kept in good working condition, if the exhaust effect is not good, it should be repaired or replaced with a more powerful exhaust device in time. For those cases where there is a protective cover above the wave soldering preheating section and the exhaust device is ineffective, decisively removing the protective cover is also an effective measure to reduce the flux vapor concentration and reduce the possibility of deflagration.

<!--[if !supportLists]-->6.<!--[endif]-->In the actual PCB production process, both the manufacturer and the user should attach great importance to the problem of PCB flux fire. Manufacturers can try to explore safer and more stable formulations when developing and producing PCB fluxes, although it is difficult to change their flammable and explosive nature, but there is still room for improvement. The user manufacturer should strictly follow the operation specifications, start from the process and equipment, and comprehensively and meticulously investigate the possible fire hazards, so as to ensure the safety and stability of production, and avoid economic losses and damage to personnel and equipment.