Do you know why the circuit board (PCBA) needs to be cleaned after it is assembled? What is the purpose and intention of cleaning? Why do PCBAs almost no longer need to be cleaned later/now? But why do some customers still ask for boards to be cleaned?

 

 

 

The initial process of circuit board assembly and soldering requires water washing. That is, after the board has undergone "Wave soldering" or "Surface Mount", it must be cleaned with detergent or pure water. After removing the contaminants on the board, as the designs of electronic parts became more and more diversified and became smaller and smaller, some problems gradually appeared in the water washing process. In addition, the cleaning process of PCBA was too troublesome. Later, it was A no-clean process evolved.

 

 

 

The main purpose of cleaning the board after circuit board assembly (PCBA) is to remove the residual flux on the PCB surface.

 

 

 

 

As far as the SMT process is concerned, the biggest difference between the "water-wash process" and the "no-clean process" is the composition of the flux in the solder paste. The wave soldering process is purely the composition of the flux in front of the furnace. This is because the flux (flux) The main purpose is to remove the surface tension and oxides of the objects to be welded to obtain a clean welding surface. The best medicine for removing oxidation is chemical agents such as "acid" and "salt", but "acid" and "salt" It is corrosive. If it remains on the PCB surface, it will corrode the copper surface over time, causing serious quality defects.

 

 

 

In fact, even if the board is produced using a no-clean process, if the flux formula is improper (usually some solder paste of unknown origin is used, or there is a special emphasis on the tin-eating effect or the solder paste that can remove oxides , because the flux of these solder pastes usually contains weak acid) or there is too much flux residue. Over time, the solder paste may also cause corrosion to the copper surface of the circuit board after mixing with moisture and pollutants in the air. When the board is at risk of corrosion, cleaning is still necessary. Therefore, it does not mean that boards with a "no-clean process" do not necessarily need to be cleaned. Of course, it is not necessary to wash them with water. After all, washing with water is very troublesome.

 

 

 

In addition, some special purposes may also require boards with no-clean processes to be washed, such as:

 

 

 

Those who sell PCBA alone to end customers hope that the surface of the board will be clean and give customers a good appearance impression.

 

In the subsequent PCBA process, it is necessary to increase the surface adhesion of the circuit board. For example, conformal coating coating needs to pass a hundred grid test.

 

Or to avoid unnecessary chemical reactions with solder paste residue, such as the potting process.

 

The residual flux from the no-clean process will produce micro conduction (reduced impedance) in a humid environment, especially for fine-pitch parts, such as the bottom of passive components below 0201 size, especially for small-pitch BGA packages. , because the solder joints are set at the bottom of the part, it is easy for too much flux to remain, and moisture is also easy to adhere to the bottom after cooling down during use and not in use, forming micro conduction over time, causing leakage current. ) or increase the retention current power consumption.

 

 

 

Therefore, whether the board needs to be washed depends on individual needs. The important thing is to understand "Why does it need to be washed? What is the purpose of washing?"

 

 

 

Types and explanations of PCBA flux

 

 

 

Since the biggest difference between PCBA "water-washing process" and "no-cleaning process" is flux, and the biggest purpose of water washing is to "remove flux residue" and other contaminants, then we have to understand what types of flux there are.

 

 

 

Flux is basically divided into the following categories:

 

 

 

 

1. Inorganic series flux

 

Inorganic acids and inorganic salts (such as hydrochloric acid, hydrofluoric acid, zinc chloride, ammonium chloride) were added to early fluxes, which were called "inorganic fluxes" because inorganic acids and inorganic salts are It is a strong acid, so it has a very good cleaning effect. It can obtain a good soldering effect and has excellent soldering properties. However, the disadvantage is that it is also very corrosive. The object to be welded must have a thicker coating or thickness to withstand strong acid cleaning, so This type of "inorganic flux" must be strictly cleaned immediately after use to prevent it from continuing to corrode the copper foil of the circuit board, which greatly limits its practicality.

 

 

 

2. Organic series flux

 

Therefore, some people add less acidic organic acids (such as lactic acid and citric acid) to the flux to replace the strong acid, which is called "organic flux". Although its cleaning degree is not as good as that of the strong acid, it only needs to be soldered. The surface pollution is not too serious. It can still have a certain degree of cleaning effect. The important thing is that the residue after welding can remain on the object to be welded for a period of time without serious corrosion. However, weak acid is also an acid, so when welding It still needs to be washed with water afterwards to avoid defects such as line corrosion over time.

 

 

 

 

3. Resin and rosin series fluxes

 

Because the washing process is too troublesome, and not all electronic parts can be washed. For example, buzzers, coin batteries, and pogo pins are not recommended for washing. .

 

Later, some people added rosin to the flux to replace the original acidic cleaner, which can also remove oxides to a certain extent. However, when rosin is a monomer, its chemical activity is weak and it is often not sufficient to promote wetting of the solder. Therefore, it is practically necessary to add a small amount of active agent to improve its activity. Another characteristic of rosin is that it is inactive when it is solid and becomes active only when it becomes liquid. Its melting point is about 127°C, and its activity can last up to a temperature of 315°C. At present, the optimal temperature for lead-free soldering is 240 to 250°C, which is just within the active temperature range of rosin, and its welding residue does not have corrosion problems. These characteristics make rosin a non-corrosive flux and is widely used in electronics. The equipment is being welded.

 

IPC-J-STD-004 defines four major fluxes based on flux ingredients: organic (OR), inorganic (IN), rosin (RO), and resin (RE).

 

 

 

 

Trouble and disadvantages of PCBA washing process:

 

The so-called "water washing" is to use liquid solvent or pure water to clean the board. Because general acidic substances can be dissolved in water, "water" can be used to dissolve and clean it, so it is called water washing, but no-clean flux uses rosin. It cannot be dissolved in "water" and requires the use of "organic solvents" for cleaning, but it is still called "water washing". Most water washing processes use "ultrasonic" vibration to enhance the cleaning effect and shorten the time. These During the cleaning process, the cleaning agent is very likely to penetrate into some electronic parts or circuit boards with small pores and cause defects.

 

§ Some functions may fail due to the inability to dry out after liquid penetration, such as reed switches and spring thimble connectors (pogo pins).

 

§ Or after cleaning, dirt is brought into the parts, causing them to operate erratically or have poor contact, such as buzzers, speakers, micro switches, etc.

 

§ Some may be defective because they cannot withstand shock cleaning, such as button batteries.

 

These electronic parts that have doubts about the water washing process must generally be arranged to be welded after washing to avoid permanent damage during cleaning. This increases the production process steps, and the more production processes, the easier it is to have good quality. Damage, which invisibly causes waste to the production process, and welding after washing is usually done by hand, making it difficult to control the welding quality. So, the same saying goes, “If you can wash it without water, don’t wash it with water.”

 

 

 

 

Additional instructions:

 

 

 

Solder paste and flux are divided into water-washable and no-clean. The ingredients used in general water-washable solder paste and flux can be dissolved in water, while the flux used in the no-clean process cannot be dissolved in water and can only be cleaned with organic solvents. Therefore, if you have decided to clean the PCBA, it is recommended to use water-washed solder paste and flux beforehand, which will help clean the flux.

 

 

 

Introduction to the ingredients of flux

 

 

 

The contents of flux basically include the following four main ingredients:

 

 

 

 

 

Resin: 40~50%.

Rosin is sticky and can form a protective layer on the surface of the metal to be welded to isolate air, reduce contact with oxygen during the heating process, and reduce the oxidation rate.

 

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 to help solder.

 

Solvent: 30%.

Solvents can help dissolve and mix different chemicals in the flux, and allow the flux to be evenly mixed into 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 evaporating and the solder paste drying out, affecting the welding.

 

Thixotrope (rheology modifier): 5%.

 

It is used to adjust the viscosity of the solder paste to achieve a paste-like state, enhance the printability of the solder paste, and prevent the solder paste from collapsing after being printed on the circuit board.