Necessary knowledge of flux

First, the role of flux:

The functions of the flux are: "assisting heat conduction", "removing oxides", "reducing the surface tension of the welded material", "removing the oil on the surface of the welded material, increasing the welding area", and "preventing reoxidation".

1. Understanding of the role of "auxiliary heat conduction"

In the welding, the solder is basically in a completely molten high temperature state, in this high temperature state, the welded components and pads will inevitably undergo a certain high temperature test, as for the highest temperature of the thermal shock, people in the actual operation will use a variety of countermeasures to prevent, while requiring the welding material heat resistance to be relatively strong, Generally according to the temperature requirements of the standard process, the temperature limit that the material can finally withstand (also known as heat-resistant temperature) is designed to be about 20-300C above the maximum temperature line that may be suffered, which should be said to be a relatively safe range. Therefore, once the material of the welded material is determined, the possibility that it will eventually withstand thermal shock is basically within the safety permit range, but there are too many variables in the actual process operation, such as the error between each machine and the standard process, which may cause all parameters of the entire welding process to change, even if the maximum temperature is within the preset safety range. However, if the heating rate is too large, it will make all the parts or parts that may come into contact with the tin liquid suddenly heat up, and the sudden rise or fall of temperature can cause the creep of material properties. Almost all detection methods are powerless to do anything about the creep of this material properties in the short term, and the harm caused by it is long-term, potential, and difficult to identify the cause. This kind of harm for some precision electronic information products, can be regarded as a fatal internal injury.

Based on the above description, we can easily understand the role of the flux "auxiliary heat conduction", currently all the components of the flux, the solvent is basically indispensable, while the solvent also has a high boiling point of additives, these substances in the heat can absorb part of the heat, at the same time after reaching the boiling point of the temperature began to gradually volatilize, while taking away part of the heat, So that the welded material will not produce sudden temperature changes in an instant; In addition, because of the coating of flux on the surface of the welding material, the heat condition of the entire surface can be uniform. Therefore, we understand the situation as "auxiliary heat conduction", and the whole process it assists can be regarded as the process of delaying the thermal shock and making the welding material heated evenly, rather than such a process or effect of destroying the heat conduction or helping the heat conduction rapidly.

2. Understanding of the effect of "removing oxides".

The welding process is the process of brazing head or solder joint forming, and this process is also the process of alloy structure change and alloy restructuring. The structural state of the solder alloy itself is basically stable, so it is not so easy to refuse with other metals or other alloys in a very short time and form a new alloy structure. At present, the traditional solder alloy is Sn63/Pb37, which can refuse and form a new alloy with many other metals or alloys. Such as copper, aluminum, nickel, zinc, silver, gold, especially copper is easy to fuse with tin lead alloy solder, but when these metals are oxidized or reacted by air or other substances, an oxide layer will be formed on the surface of these metals, although tin lead solder and these metals themselves are easier to form an alloy structure, but with the oxides or compounds of these substances to form a new solder joint. The chance of refusion is very low. Almost all welding material designers, when demonstrating the weldability of solder, set the welding material and process environment in an ideal state, and all ideal states almost do not exist in the actual process, in terms of circuit boards, components, or other welded materials manufacturing and storage, transportation, reproduction and other links. The possibility of oxidation of the surface of various welding materials is 100%.

Therefore, the processing of the oxide layer on the surface of the welded material before welding is particularly critical, and the process of flux "oxide removal" is actually a REDOX process, and the active agent in the flux is usually a variety of organic acids or organic acid salts, at least one or two acids or salts, usually by a variety of acids and salts mixed to use. In the process of REDOX reaction, the "speed" of the reaction and the "ability" of the reaction are of more concern to people, these two are internal problems, and the external performance is usually what people call "welding speed" and "welding ability", in the case of material, process and other circumstances, the choice of different active flux is very important: The welding speed or welding ability of the flux with weak activity or too weak activity is relatively poor, the ability of the flux with strong activity to remove the oxide film is strong, and the tinning speed is fast, but if there are too many active agents in the flux, too strong or the overall structure is not compatible, it is likely to lead to the residue of active substances after welding, then there is the possibility of continued corrosion after welding. The safety performance of the product has caused a considerable hidden danger.

3. Understanding of the effect of "reducing the surface tension of the welded material".

In the welding process, the solder is basically in a liquid state, and the element pin or pad is a solid state, when the two substances contact, due to the effect of the surface tension of the liquid substance, it will directly cause the reduction of the contact interface of the two substances, our surface summary of this phenomenon is "poor tin liquid fluidity" or "small expansion rate", The presence of this phenomenon affects the area, volume or shape of the alloy formed. At this time, what is needed is the role of "surfactant" in the flux, "surfactant" usually refers to a substance that can significantly reduce the surface tension of other substances at very low concentrations, its molecules have two group structures at both ends, one end of the hydrophilic oil-repellent and the other end of the hydrophilic water-repellent, through its external performance can be seen, it is composed of solvent soluble and solvent insoluble two parts. These two parts are located at opposite ends of the molecule, forming an asymmetrical structure that only significantly reduces surface tension because of this special structure.

The addition of surfactants in the flux is very small, but the effect is very critical, reducing the "surface tension of the material to be welded", which shows a strong wetting effect, which can ensure that the tin liquid on the surface of the welded material smooth expansion, flow, infiltration, etc. Usually the solder joint into a ball, false welding, pulling tip and other similar bad conditions are not enough to have a certain relationship with the surface activity, and this reason is not necessarily the addition of too little flux "surfactant", it may also be caused in the production process of its decomposition, failure, etc., thus greatly weakening the surface activity.

4. Understanding of the role of other aspects.

As mentioned above, among the several functions of flux, the most important is "removing oxides" and "reducing the surface tension of the welded material", through the above analysis, the role of other aspects of flux is actually easier to understand. "Removing the oil on the surface of the welded material and increasing the welding area" is more easily accepted by people. Under normal circumstances, the surface of the pad or other welding materials that we observe with the naked eye is smooth or flat, in fact, when we observe with a high-magnifying glass, we can find that in fact, the surface of these materials is not smooth or flat, but there are many pits and pits, and there will be oil or other dust distribution in these pits and pits. The organic solvent and surface activator in the flux can easily remove these oil or dirt, so that the surface of the welded object still shows a bumpy condition, so that the welding surface will increase and the welding strength will be enhanced.
Regarding the role of "preventing reoxygenation", it is especially obvious in the early rose-type flux. The current no-cleaning flux has been accepted by the majority of users, and the reduction of the solid content and resin content of no-cleaning flux has greatly changed the role in this regard. However, there are still some fluxes that add a small amount of efficient film forming substances. This substance can replace the protective effect of early rosin or resin, so that the degree and probability of oxidation of the new solder joint alloy after welding are reduced.

Second, the working principle of flux:

Through the above analysis of the related effects of flux, the working principle of flux is easy to understand, generally speaking: In the entire welding process, the flux removes the oxide layer on the surface of the welding material through its own active substance, while reducing the surface tension between the tin liquid and the welded material, enhancing the flow and infiltration of the tin liquid, and helping the welding to complete, so its name is called "flux".
If you want to carry out a full analysis of the working principle of the flux, that is, through the active substance in the flux to clean the oxide on the surface of the welding material, so that the solder alloy can be well combined with the welded material and form a solder joint, in this process, the main role is the activator in the flux and other substances. These substances quickly remove oxides from pads and element pins, and sometimes protect the material from oxidation until the welding is complete.

In addition, while removing the oxide film, the surfactant in the flux also begins to work, which can significantly reduce the surface tension of the liquid solder reflected on the surface of the welded material, strengthen the fluidity and spreading ability of the liquid solder, and ensure that the tin solder can penetrate into every fine brazing gap; In the soldering process of the tin furnace, when the welded body leaves the surface of the tin liquid, the excess tin solder will flow down the pin because of the wetting effect of the flux, so as to avoid bad phenomena such as pulling the tip and welding.

Third, some basic requirements of common flux:
1, with a certain chemical activity (to ensure the ability to remove the oxide layer);
2. It has good thermal stability (to ensure that it does not decompose and fail at a high soldering temperature). ;
3, with good wettability, to promote the expansion of solder (to ensure better welding effect);
4. The flux residue retained on the substrate is non-corrosive to the material after welding (based on safety performance considerations, water cleaning or clearly cleaning type flux should be considered to have lower corrosion in the process of delayed cleaning, or to ensure that the corrosion of a longer delay period is weak). ;
5, need to have good cleaning (no matter what kind of flux, whether it is cleaning type flux, it should have good cleaning, if it is really necessary to clean, it can ensure that there is an appropriate solvent or cleaning agent for thorough cleaning; Because the fundamental purpose of the flux is to help the welding finish, not to create an irrevocable coating on the surface of the welded material. ;
6, each type of flux should basically meet or exceed the relevant national standards, row standards or other standards for some basic parameters of the relevant flux specification requirements; (A flux that does not meet the requirements of the relevant standards is, strictly speaking, an unqualified flux.) ;
7, the basic components of the flux should be no obvious public hazards to the human body or the environment, or known potential hazards; (Environmental protection is a worldwide issue at present, which is related to the health and safety of the human body and the environment, and also relates to the possibility of sustainable development of the industry.) .

Four, commonly used flux components:
1, solvent: it can make the various components of the flux evenly and effectively mixed together; At present, the common solvents are mainly alcohols, such as ethanol, isopropyl alcohol, etc. Although methanol has a low price and cost, it has a strong toxic effect on the human body, so at present, methanol has rarely been used by regular flux production enterprises.
2, surfactants: alkanes or fluorocarbons and other high-efficiency surfactants.
3, activator: mainly organic acids or organic acids, inorganic acids or inorganic acids are basically not used in electronic coupling flux, and sometimes used in other special fluxes.
4, rosin (resin) : rosin itself has a certain degree of activation, but when added to the flux, it is generally used as a carrier, which can help other components effectively play their due role.
5, other additives: In addition to the above components, the flux often adds different additives according to the specific requirements, such as brightener, matting agent, flame retardant and so on.