What are the types of fluxes in the SMT industry? What are the components of flux?

In circuit board assembly and soldering, the initial process involves washing. That is, after the circuit board has undergone "wave soldering" or "surface mounting", the contaminants on the board must be removed with cleaning agent or pure water. Later, as the design of electronic parts became more and more diverse and smaller, the washing process gradually exposed some problems, and the cleaning process of PCBA was too cumbersome, so the no-clean process came into being.

As far as the SMT process is concerned, the biggest difference between the "water washing process" and the "no-clean process" is the difference in the composition of the flux in the solder paste, while the wave soldering process is purely the difference in the composition of the flux in front of the furnace. This is because the main function of flux is to eliminate the surface tension and oxide of the soldered object to obtain a clean soldering surface, and the best ingredients to remove oxidation are chemicals such as "acid" and "salt". However, "acids" and "salts" are corrosive, and if left on the surface of the PCB, they can corrode the copper surface over time, leading to serious quality problems.

For those electronic parts that have doubts about the washing process, such as buzzers, button batteries, spring thimble connectors, etc., special attention should be paid to arranging the sequence of welding and cleaning during the production process to avoid damage to the parts. In actual production, even the circuit board of the no-clean process may need to be cleaned if there are high requirements for the cleanliness of the circuit board, such as selling the PCBA separately to the end customer, improving the adhesion of the circuit board surface in the subsequent manufacturing process, or avoiding unnecessary chemical reactions in the solder paste residue. However, there are many troubles and disadvantages in the washing process, such as the cleaning agent may penetrate into the electronic parts or circuit boards and cause adverse effects, increase the production process, reduce the yield rate, and it is difficult to control the quality by manual soldering.

In fact, even if the circuit board is produced using the no-clean process, if the flux is not formulated properly (usually when using some solder paste of unknown origin, or when the solder paste is particularly emphasized to eat the effect of tin or can remove oxides, because the flux of these solder pastes often adds weak acid) or too much flux residue, the copper surface of the board may be corroded by mixing the solder paste with moisture and pollutants in the air over time. When there is a risk of corrosion on the circuit board, cleaning becomes a necessary operation. Therefore, circuit boards that are not "no-clean" processes do not necessarily need to be cleaned, and of course, if they can be washed as much as possible, they should not be washed, after all, washing is very troublesome.

In addition, there are special cases where the board for the no-clean process is required to be washed with water, such as:

<!--[if !supportLists]-->·<!--[endif]-->In the case of selling PCBA alone to end customers, it is desirable that the surface of the circuit board is clean and leave a good appearance impression on the customer.

<!--[if !supportLists]-->·<!--[endif]-->The subsequent process of PCBA needs to improve the adhesion of the circuit board surface. For example, the coating of conformal adhesive (Conformalcoating) needs to meet the requirements of the 100-grid test.

<!--[if !supportLists]-->·<!--[endif]-->Or to avoid unnecessary chemical reactions in solder paste residues, such as in the potting process.

Flux residues from the no-clean process can cause microconduction (impedance reduction) in wet environments, especially on fine-pitch parts, such as the bottom of passive components up to size 0201, and especially in small-pitch BGA packages. Because the solder joints are located at the bottom of the part, it is easy to retain too much flux, and when the temperature changes during use (heating up during use and cooling down when not in use), moisture tends to adhere to the bottom of the part, and over time it can form microconduction, resulting in leakage current or increased power consumption for retention current.

Therefore, in the design of the whole process of circuit board assembly and soldering, it is necessary to fully consider the type of flux, the characteristics of electronic parts and the requirements of the final product, and reasonably choose the water-washed process or no-clean process, and carefully select the composition of the flux to ensure that the potential risks and problems are minimized and the production efficiency and product quality are improved while meeting the soldering quality requirements. This requires manufacturers to have an in-depth understanding of the PCBA process, not only the mastery of surface knowledge such as washing and no-cleaning processes, types of fluxes, etc., but also the interaction between various links and the impact on the quality of the final product. For example, when choosing a flux, in addition to its flux performance, it is also necessary to consider factors such as its cleaning characteristics and compatibility with other electronic parts. Only in this way can optimal decisions be made in the complex process of circuit board assembly and soldering, and high-quality circuit board products can be produced.

Therefore, whether the circuit board needs to be washed or not depends on the specific needs, and it is important to understand "why wash? What is the purpose of washing?".

Types and analysis of PCBA flux Since it is mentioned that the biggest difference between the "washing process" and the "no-clean process" of PCBA is the flux, and the main purpose of washing is to "remove flux residues" and other contaminants, then we need to understand what types of flux there are.

Fluxes can be broadly divided into the following categories:

<!--[if !supportLists]-->1.<!--[endif]-->Inorganic flux series Early fluxes would add inorganic acids and inorganic salts (such as hydrochloric acid, hydrofluoric acid, zinc chloride, ammonium chloride), which were called "inorganic fluxes". Because inorganic acid and inorganic salt belong to medium and strong acids, the cleaning effect is very good, and good welding results can be obtained, and the welding performance is good. However, the disadvantage is that it is very corrosive, and the soldered object must have a thick coating or thickness to withstand strong acid cleaning, so this kind of "inorganic flux" must be cleaned strictly immediately after use to prevent it from continuing to corrode the copper foil of the circuit board, which limits its practicability to a large extent.

<!--[if !supportLists]-->2.<!--[endif]-->Organic flux So some people add less acidic organic acids (such as lactic acid and citric acid) to flux to replace strong acids, and this flux is called "organic flux". Although its cleanliness is not as good as that of strong acid, as long as the surface pollution of the welded object is not too serious, it can still play a certain cleaning role. It is important that the residue after it has been welded can remain on the welded object for a period of time without causing severe corrosion. However, weak acid is also acid, so it still needs to be washed after welding to avoid adverse consequences such as line corrosion in the future.

<!--[if !supportLists]-->3.<!--[endif]-->Resin, rosin series flux Because the washing process is too cumbersome, and not all electronic parts can be washed, such as buzzers, coin batteries, and pogo pins, it is not recommended to wash them. Later, some people added rosin to the flux to replace the original acid cleaning agent, which can also play a role in removing oxides to a certain extent. However, when rosin exists alone, the chemical activity is weak, and the wetting of the promoting solder is often insufficient, so a small amount of active agent needs to be added to improve the activity in practical applications. Another property of rosin is that it is inactive in a solid state and is only active when it becomes liquid, its melting point is about 127 °C, and the activity can last up to 315 °C. At present, the optimal temperature of lead-free soldering is 240 - 250°C, which is just within the active temperature range of rosin, and its soldering residue does not have corrosion problems, these characteristics make rosin a non-corrosive flux, so it is widely used in the soldering of electronic equipment. IPC - J - STD - 004 defines four fluxes based on flux composition: organic (OR), inorganic (IN), rosin (RO), and resin (RE).

Troubles and disadvantages of PCBA washing process: The so-called "water washing" is the use of liquid solvent or pure water to clean the circuit board. Because general acidic substances can be dissolved in water, they can be dissolved and cleaned with water, so it is called water washing. However, no-clean flux cannot be dissolved in "water" when using rosin, so it must be cleaned with "organic solvent", but it is also called "water washing". Most of the washing process uses "ultrasonic" oscillation to enhance the cleaning effect and shorten the cleaning time, and these cleaning agents are very likely to penetrate into some electronic parts or circuit boards with fine pores during the cleaning process, causing adverse effects.

<!--[if !supportLists]-->·<!--[endif]-->Some may fail to function because the liquid does not dry after penetration, such as reed switches, pogo pins.

<!--[if !supportLists]-->·<!--[endif]-->Or after cleaning, it will bring dirt into the parts, so that they do not run smoothly or have poor contact, such as buzzers, horns, micro switches and other parts.

<!--[if !supportLists]-->·<!--[endif]-->Some may be defective because they cannot withstand shock cleaning, such as coin cell batteries.

These electronic parts that have doubts about the washing process must generally be arranged after washing before soldering to avoid permanent damage during cleaning, which increases the production process. Moreover, the more links in the production process, the more likely it is that the yield rate will be damaged, which will virtually cause waste in the production process. Moreover, the welding after washing is usually manual welding, and it is difficult to control the welding quality. So, it's still the old saying, "If you can't wash it, don't wash it."

Solder paste and flux are divided into water-washable type and no-clean type, generally water-washable solder paste and flux use ingredients that are soluble in water, while the no-clean process flux cannot be dissolved in water and can only be cleaned with organic solvents. Therefore, if it has been determined that the PCBA needs to be cleaned, it is recommended to use water-washed solder paste and flux in the early stage, so as to help clean the flux.

Ingredient Profile of flux The content of flux basically contains the following four main components: Resin: 40 - 50%. Rosin is viscous and can form a protective layer on the surface of the metal being welded, insulating the air, reducing the contact with oxygen during the heating process and reducing the oxidation rate. Activator: 2 - 5%. The main function is to clean and remove the oxide layer on the metal surface, and at the same time reduce the surface tension of the solder, which helps the solder. Solvent: 30%. The solvent helps to dissolve and mix the different chemicals in the flux and allows the flux to mix evenly in the tin powder. The solvent is volatile, so it is not recommended to expose the solder paste to the air for a long time to avoid the solvent volatilizing, the solder paste will dry out, and the soldering will be affected. Thixotropic modifier: 5%. It is used to adjust the viscosity of the solder paste to achieve the purpose of paste, enhance the printability of the solder paste, so that the solder paste can still maintain its original shape after printing on the circuit board without collapsing.

To sum up, in the field of circuit board assembly and soldering, there are pros and cons to the washable process and the no-clean process, and the choice needs to be weighed according to the specific situation. From a flux perspective, different types of flux correspond to different cleaning needs. For example, inorganic fluxes have good fluxing properties but are highly corrosive and must be washed with water immediately after use; Organic fluxes are relatively less corrosive, but they still need to be washed after soldering; The rosin series fluxes are widely used in the no-clean process because of their non-corrosion problems of soldering residues.