PCB Tech - Why should the circuit board be washed after soldering?

Do you know why cleaning was necessary after PCBA? What is the purpose and intention of cleaning? Why is it that almost all of the PCBA afterwards/now does not need to be cleaned? But why is there still a customer request to clean the board?

PCB assembly and soldering, the initial process requires water washing, that is, after the "Wave soldering" or "Surface Mount" of the board is completed, it is finally cleaned with a detergent or pure water. The contaminants on the board, later as the design of electronic parts became more and more diverse, and smaller and smaller, some problems gradually appeared in the washing process, and it was because the cleaning process of PCBA was too troublesome, and it evolved later. Out of the no-clean process.

The main purpose of cleaning the board after PCBA is to remove the residual flux on the surface of the PCB

For the SMT process, the biggest difference between the "washing process" and the "no-cleaning 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 of the flux. The main purpose is to remove the surface tension and oxides of the welded material to obtain a clean welding surface, and 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 surface of the PCB, it will corrode the copper surface over time and cause serious quality defects.

In fact, even if the board is produced using a no-clean process, if the flux formulation is improper (usually when some unknown solder paste is used, or when there is a special emphasis on the effect of eating tin or the solder paste that can remove oxides, Because the flux of these solder pastes usually add weak acid) or too much flux residue, after a long time the solder paste mixed with moisture and pollutants in the air may also cause corrosion to the copper surface of the circuit board. When the board is at risk of corrosion, cleaning is still necessary. Therefore, it is not to say that the "no-washing process" board does not necessarily need to be cleaned. Of course, it can be washed without water. After all, washing with water is very troublesome.

In addition, some special purpose cases will require the boards of the no-wash process to be washed with water, for example:

▪ Sell PCBA only to end customers, hoping that the surface of the board will be clean and give customers a good appearance impression.

▪ In the follow-up process of PCBA, it is necessary to increase the surface adhesion of the circuit board. For example, conformal coating needs to pass the 100 grid test.

▪ Or to avoid unnecessary chemical reactions caused by solder paste residues, such as the potting process.

▪ The flux residue from the no-clean process will produce micro-conduction (resistance reduction) in a humid environment, especially fine-pitch parts, such as the bottom of passive components below 0201 size, especially small-pitch BGA Packaging, because the solder joints are set at the bottom of the part, too much flux is likely to remain, and moisture is also easy to adhere to the bottom of it after cooling when it is not in use, and it will form a micro-conduction over time, resulting in leakage current (leakage current) or increase the retention current (retention current) power consumption.

Therefore, whether or not the board needs to be washed depends on individual needs. It is important to understand "Why washing? What is the purpose of washing?".

Introduce the basic knowledge of [solder paste]

Types and explanations of PCBA flux

Now that the biggest difference between PCBA "washing process" and "no-washing process" is flux, and the biggest purpose of washing is to "remove flux residues" and other pollution, then we have to understand what types of flux are there.

Fluxes are basically divided into the following categories:

1. Inorganic series flux

In the early soldering flux, inorganic acids and inorganic salts were added (for example, hydrochloric acid, hydrofluoric acid, zinc chloride, ammonium chloride), which was called "inorganic flux", because inorganic acids and inorganic salts were medium Strong acid, so it has a very good cleaning effect, a good soldering effect can be obtained, and the soldering ability is excellent, but the disadvantage is that it is also very corrosive. The welded object has a thicker coating or thickness than the virtual can withstand the cleaning of strong acid, so After using this type of "inorganic flux", strict cleaning must be carried out immediately to prevent it from continuing to corrode the copper foil of the circuit board, and its practicality is greatly limited.

2. Organic series flux

Therefore, some people add weakly acidic organic acids (such as lactic acid, citric acid) to the flux to replace the strong acid, which is called "organic flux". Although its cleanliness is not as good as the strong acid, it only needs to be soldered. The surface pollution is not too serious, it can still play a certain degree of cleaning effect, the important thing is that the residue after welding can be retained on the welded object for a period of time without serious corrosion, but the weak acid is also acid, so in the welding After that, it must be washed with water to avoid problems such as line corrosion over time.

3. Resin and rosin series flux

Because the washing process is too cumbersome, and not all electronic parts can be washed, such as buzzer, coin battery, pogo pin connector (pogo pin) is not recommended for washing .

Troubles and disadvantages of PCBA washing process:

The so-called "water washing" is to use liquid solvents or pure water to clean the board. Because general acidic substances can be dissolved in water, you can use "water" to dissolve and clean, so it is called water washing, but no-clean flux uses rosin It can’t be dissolved in "water" and needs to be washed with "organic solvent", but it is also called "washing". Most of the washing process will use "ultrasonic" vibration to enhance the cleaning effect and shorten the time. 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 of them may fail due to the inability to dry after the liquid penetrates, such as reed switches and pogo pins.

▪ Or after cleaning, the buried objects are brought into the parts to make them move or contact poorly, such as buzzers, horns, micro switches... and other parts.

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

These electronic parts that have doubts about the washing process must generally be arranged after washing to avoid permanent damage during cleaning. This increases the production process links, and the more processes produced, the easier it is to be good. It is actually a waste of the production process, and the welding after washing is usually hand welding, and it is difficult to control the welding quality. Therefore, the phrase "can be washed without water".

Supplementary note:

Solder paste and flux are divided into water wash and no-clean. The general water-washed solder paste and flux can be dissolved in water, while the flux of no-clean process cannot dissolve in water and can only be cleaned with organic solvents. Therefore, if the PCBA has been decided to be cleaned, it is recommended to use water-washed solder paste and flux before it will help to clean the flux.