Since the biggest difference between PCBA "rinsing process" and "no rinsing process" is flux, and the main purpose of rinsing is "removing flux residue" and other pollution, then we should know the types of flux.

1. Inorganic series flux

In the early flux, inorganic acids and inorganic salts (such as hydrochloric acid, hydrofluoric acid, zinc chloride, ammonium chloride) were added, which were called "inorganic fluxes" because inorganic acids and inorganic salts were medium. Those strong acid, which can up to good cleaning effect, soldering effect and excellent soldering performance, but the disadvantage is that it is also very corrosive. So the welded board must have a thicker plating layer to withstand strong acid cleaning. Thus, after using such "inorganic flux", it must be strictly rinsed immediately in order to avoid corrode the copper foil of the board continuously, and ensure the function of the board.

2. Organic series flux

Therefore, some people add a weak organic acid (such as lactic acid, citric acid) to the flux to replace the strong acid, called "organic flux". Although the degree of rinse is not as good as strong acid, as long as the contamination of solder surface is not too serious, it can still play a certain degree of cleaning effect.In other words, the residue after welding can remain on the object for a period of time without serious corrosion. However, the weak acid is also a kind of acid. It still need rinse to avoid bad corrosion like line corrosion.

3. Resin and rosin series flux

Because the rinsing process is fairly cumbersome, and not all electronic parts can be rinsed, such as buzzer, coin battery, pogo pin and so on.

Later, some people added rosin to the flux to replace the original acidic detergent due to it can play a certain degree to remove the oxide. However, when the rosin is a monomer, the chemical activity is weak, and the wet process of solder is often insufficient. Therefore, it is practical to add a small amount of an active agent to increase its activity. Moreover, rosin is that rosin is active only when it becomes liquid. Its melting point is about 127 ° C, and the activity can last up to 315 ° C. At present, the optimum temperature for lead-free soldering is 240-250 ° C, which is just within the active temperature range of rosin, and its solder residue won’t lead corrosion problems. These special features contribute to rosin widely is used in the welding of the electronics equipment.

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

"Rinsing" means by using a liquid solvent or pure water to clean the board. Since the general acidic substance can be dissolved in water, "water" can be used to dissolve and wash, so it is called rinsing.

What are the disadvantages of PCBA rinsing process?

As the no-clean flux uses rosin. It can't be dissolved in "water", so it also need to be rinsed with "organic solvent". Most of the rinsing process uses "ultrasonic" vibration to enhance the cleaning effect and shorten the time. Unfortunately, the cleaning agent is likely to penetrate into the electronic parts or boards along the small pores and cause defects.

▪ Function problem. Due to it can’t dry out automatically after the liquid has penetrated, such as reed switches and spring thimble connector.

▪ Malfunction and bad contact.After repeat rinsing, some dirty stuff are easy enter to the inside of the parts, such as buzzer, horn, micro switch and etc.

▪ Production defect. Some parts cannot withstand shock cleaning, such as button batteries.

▪ Manufacturing process added & Welding quality hardly control.

These electronic parts arrange to weld after rinsing in order to prevent possible permanent damage. For one thing, it adds the manufacturing process. For another thing, the welding after water washing is usually hand soldering, the quality is also difficult to control.

What can we learn?

Solder paste and flux are divided into water wash and no wash. The general solder paste and flux components can be dissolved in water, while the no-clean flux can only rinse with organic solvents. Therefore, if you decide to rinse the PCBA, we sincere recommend you to use a water-washed solder paste and flux at very first.