SMT flux (Flux) composition, the role of flux, flux types

Do you know why you need to clean after circuit board assembly (PCBA)? What is the purpose of cleaning? Why do most of the past/current PCBAs do not need to be cleaned? But why do customers still ask for board cleaning?

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

The main purpose of cleaning the board after circuit board assembly (PCBA) is to remove residual flux from the surface of the PCB. As far as the SMT process is concerned, the biggest difference between the "water washing process" and the "no-clean process" is that the composition of the flux in the solder paste is different, while the wave soldering process is purely the composition of the flux in front of the furnace. This is because the main purpose of flux is to eliminate the surface tension and oxide of the soldered object to obtain a clean solder surface, and the best substances to remove oxidation are chemicals such as "acid" and "salt". However, "acids" and "salts" are corrosive, and if they remain on the surface of the PCB, they will corrode the copper surface over time, causing serious quality problems.

In fact, even if the board is produced using a no-clean process, if the flux is not formulated properly (usually when some solder paste of unknown origin is used, or when the effect of eating tin or solder paste can be removed, because the flux of these pastes usually adds weak acid) or too much flux residue, the solder paste may cause corrosion to the copper surface of the circuit board after mixing with moisture and pollutants in the air over time. When there is a risk of corrosion on the board, cleaning is still necessary. Therefore, it does not mean that the board of the "no-clean process" does not necessarily need to be cleaned, of course, if it can be washed, try not to wash it, after all, washing is very troublesome.

A brief introduction to the composition of flux

Flux basically consists of the following four main components:

①    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.

②  Activator: 2 - 5%. The main function is to clean and remove the oxide layer on the metal surface, and reduce the surface tension of the solder, which helps the solder.

③  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. Solvents are 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 drying out, and affecting soldering.

④Thixotropic modifier: 5%. It is used to adjust the viscosity of the solder paste to make it paste-like, enhance the printability of the solder paste, and make the solder paste still maintain its original shape without collapsing after printing on the circuit board.

In addition, in some special cases, the board of the no-clean process will also be required to be washed with water, such as: Sell the PCBA separately to the end customer, hoping that the surface of the board is clean and gives the customer a good appearance impression. In the subsequent process of PCBA, it is necessary to increase the adhesion of the circuit board surface. For example, conformal coating needs to pass the 100-grid test. Or to avoid unwanted chemical reactions in solder paste residues, such as in the potting process.

The flux residue of the no-clean process will produce micro-conduction (impedance reduction) in a humid environment, especially for fine-pitch parts, such as the bottom of passive components below the size of 0201, especially for small-pitch BGA packages, because the solder joints are set at the bottom of the part, it is easy to retain too much flux, and the moisture is easy to adhere to it after heating up during use and cooling down when not in use, forming micro-conduction over time, resulting in leakage currentOr increase the retention current to consume power.

So, whether the 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 explanations of PCBA flux Since the biggest difference between the "washing process" and the "no-clean process" of PCBA is the flux, and the biggest purpose of washing is to "remove the residue of flux" and other contamination, then we need to understand what types of flux there are. Fluxes are basically divided into the following broad categories:

1.Inorganic flux series In the early days, inorganic acids and inorganic salts (such as hydrochloric acid, hydrofluoric acid, zinc chloride, ammonium chloride) were added to the flux, which was called "inorganic flux". Because inorganic acids and inorganic salts belong to medium and strong acids, the cleaning effect is very good, and good soldering effect can be obtained, and the flux is good, but the disadvantage is that it is very corrosive, and the soldered object must have a thick coating or thickness to withstand the cleaning of strong acids, so it must be strictly cleaned immediately after using this kind of "inorganic flux" to avoid it from continuing to corrode the copper foil of the circuit board, and the practicability is greatly limited.

2.Organic flux Therefore, some people add less acidic organic acids (such as lactic acid and citric acid) to the flux to replace strong acids, which is called "organic flux". Although its cleanliness is not as good as strong acid, as long as the surface pollution of the welded object is not too serious, it can still play a certain cleaning effect, the important thing is that the residue after welding can be retained on the welded object for a period of time without causing serious corrosion, but the weak acid is also acid, so it still needs to be washed after welding, so as not to cause undesirable phenomena such as line corrosion in the future.

3.Resin, rosin series flux Because the washing process is too cumbersome, and not all electronic parts can be washed, such as buzzer, coin battery, and pogo pin, it is not recommended to wash with water. Later, some people added rosin to the flux to replace the original acid cleaning agent, which can also play a certain role in removing oxides, but when rosin is a monomer, the chemical activity is weak, and the wetting of the solder is often not sufficient, so a small amount of active agent needs to be added in practical applications to improve its activity. Another property of rosin is that it is inactive in a solid state and only active when it becomes liquid, with a melting point of about 127 °C, and the activity can last up to a temperature of 315 °C. At present, the optimal temperature of lead-free solder 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 and widely used in the soldering of electronic equipment.

4.IPC - J - STD - 004Four major fluxes are defined based on flux composition: organic (OR), inorganic (IN), rosin (RO), and resin (RE).

Troubles and disadvantages of PCBA washing process: The so-called "washing" is to use liquid solvent or pure water to clean the board, because the general acidic substances can be dissolved in water, so you can use "water" to dissolve and clean, so it is called washing. However, the use of rosin in no-clean flux cannot be dissolved in "water" and must be cleaned with "organic solvent", but it is also called "water washing". Most of the water washing process uses "ultrasound" 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.

 Some may fail due to the inability to dry after liquid penetration, such as reed switches and pogo pins. Or after cleaning, it will bring dirt to the inside of the parts, so that it does not run smoothly or has poor contact, such as buzzers, horns, micro switches and other parts. 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 usually must be arranged after washing before they can be welded, so as to avoid permanent damage during cleaning, which increases the production process, and the more links in the production process, the more likely it is to lose good products, which virtually causes waste to the production process, and the welding after washing is usually hand welding, and the welding quality is also difficult to control. Therefore, it is still the same saying, "If you can wash it, don't wash it".

Although the no-clean process can meet the production needs in many cases, the cleaning of PCBAs is still an indispensable part of some demanding electronic device manufacturing. For example, in aerospace, medical equipment, and other fields, the reliability and stability of circuit boards are critical. In these areas, even the slightest flux residue or microconduction risk can have serious consequences. Just like in aerospace, circuit boards can be exposed to extreme temperatures, humidity, and complex electromagnetic environments, which can affect the safety of the entire flight system if the board fails due to flux residues.

In terms of medical equipment, the normal operation of circuit boards in precision equipment such as pacemakers is related to the safety of patients. Microconduction due to flux residue may cause misoperation or malfunction of the equipment, which is absolutely unacceptable. Therefore, in these special fields, whether from the perspective of safety or reliability, the PCBA must be cleaned, even if the board adopts the no-clean process, additional cleaning procedures must be carried out. Moreover, in the cleaning process, it is also necessary to select the appropriate cleaning method and cleaning agent according to the specific circuit board structure and the characteristics of electronic components to ensure the cleaning effect and avoid damage to the circuit board and components. This requires manufacturers to test and evaluate each possible cleaning method and detergent in detail, which undoubtedly increases production costs and cycle times, but is necessary to meet the demanding quality standards.

In addition, with the continuous development of electronic technology, the integration of electronic components is getting higher and higher, and the layout of circuit boards is becoming more and more complex. This presents new challenges for PCBA cleaning, such as in multi-layer circuit boards, where flux residues may be hidden in the tiny gaps between layers that may not be effectively removed by traditional cleaning methods. In response to this situation, some new cleaning technologies are being developed and applied, such as plasma cleaning technology. Plasma cleaning technology uses the chemical reaction of active particles in plasma with flux residue to break them down into small molecules and remove them, which can be cleaned deep into tiny crevices with less damage to circuit boards and components. However, plasma cleaning technology also has some limitations, such as high equipment cost and relatively low cleaning efficiency. Therefore, in practical applications, it is necessary to comprehensively consider the specific production needs and cost-effectiveness to select the most suitable cleaning method.

Solder paste and flux are divided into two types: water washing and no-clean, the general water-washed solder paste and flux use ingredients can be dissolved in water, while the no-clean process of flux can not be dissolved in water, can only be cleaned with organic solvents. Therefore, if you have decided that the PCBA should be cleaned, it is recommended to use water-washed solder paste and flux at the beginning, so as to help clean the flux.