No-clean concept for fluxes in electronic soldering

1. Definition of no-clean In the production process of electronic assembly, a flux with low solid content and non-corrosive is used, and soldering is carried out in an inert gas environment, which is called no-clean. In this case, there is very little residue on the post-soldered circuit board, there is no corrosion, and it has a very high surface insulation resistance (SIR). Under normal circumstances, the ionic cleanliness standard (MIL-P-228809 of the American military standard can be achieved without cleaning, and the ionic contamination level is divided as follows: first-class ≤1.5ugNaCl/cm², no pollution; Secondary ≤ 1.5 ~ 5.0ugNACl/cm², high quality; The third-level ≤ 5.0 ~ 10.0ugNaCl/cm², meet the requirements; The fourth stage >10.0ugNaCl/cm², not clean), so that it can directly enter the next process, which is the no-clean process technology.

To be clear, "no-clean" and "no-clean" are two completely different concepts. The so-called "no cleaning" refers to the use of traditional rosin flux (RMA) or organic acid flux in the production of electronic assembly. After soldering, although there will be a certain residue left on the surface of the circuit board, the quality requirements of some products can be met without cleaning, such as household electronics, professional audio and video equipment, low-cost office equipment and other products are usually produced in a "no-clean" way, but this is by no means "no-clean". ”。

 When it comes to no-clean printed circuit boards and components, the stringent requirements for solderability and cleanliness also create operational challenges. To ensure weldability, it is necessary to cooperate with the supplier and the precise control of the storage environment and time of the production plant; Ensuring cleanliness requires strict environmental controls and operating practices to avoid any possible anthropogenic contamination. However, it is not easy to fully meet these requirements during mass production. For example, in the production of some small electronic devices, due to the limited production space, it is difficult to achieve complete constant temperature dry storage, which may affect the solderability of circuit boards and components.

IIThe advantage of no-clean

 (1) Improve economic benefits: After realizing no-cleaning, the most direct benefit is that there is no need for cleaning. In this way, the manpower, equipment, site, materials (water, solvents) and energy consumption required for cleaning can be significantly saved. At the same time, due to the shortened process flow, man-hours are saved and production efficiency is improved.

(2) Improve product quality: Because of the application of no-clean technology, it is necessary to strictly control the quality of materials, such as the corrosion performance of flux (halides are not allowed), the solderability of components and printed circuit boards, etc.; In the assembly process, some advanced technological means need to be adopted, such as spray coating flux, soldering under inert gas protection, etc. The implementation of a no-clean process avoids damage to the welded components due to cleaning stress, so no-clean is extremely beneficial for improving product quality.

 (3) Conducive to environmental protection: After adopting no-clean technology, the use of ozone-depleting substances (ODS) can be stopped, and the use of volatile organic compounds (VOCs) can be greatly reduced, which has a positive significance for the protection of the ozone layer.

Material requirements (1) No-clean flux In order to achieve the specified quality level without cleaning the soldered PCB board, the choice of flux is the key.

(4) Although the application of no-clean process in the field of electronic assembly has many advantages, it also faces some challenges in the actual implementation process.

As mentioned in the article, no-clean fluxes require low solids, non-corrosive, solderable, and environmentally friendly. This means that when it comes to selecting fluxes, companies need to go through rigorous screening and testing. For example, the corrosivity test of flux involves a variety of methods, such as copper mirror corrosion test, silver chromate test strip test, etc., which not only increases the cost of testing, but also challenges the technical ability of enterprises. From a material point of view, although no-clean flux can theoretically meet the quality requirements after soldering, in actual production, different formulations will affect its activity and reliability, and the pH value of each manufacturer is also slightly different, which requires enterprises to adjust according to the characteristics and needs of their own products.

In general, the following requirements are placed for no-clean fluxes:

 (1) Low solid content: less than 2% The solid content of traditional fluxes is higher (20 ~ 40%), medium (10 ~ 15%) and lower (5~).  10%), the PCB surface after soldering with these fluxes will leave more or less residues, while the solid content of no-clean fluxes is required to be less than 2%, and it cannot contain rosin, so there is basically no residue on the board surface after soldering.

 (2) Non-corrosive: halogen-free, surface insulation resistance >1.0×10¹¹Ω Due to the high solid content of traditional fluxes, some harmful substances can be "wrapped" after soldering, isolating them from the air and forming an insulating protective layer. However, no-clean fluxes are unable to form an insulating protective layer due to their very low solid content, and if a small amount of harmful components remain on the board, it can lead to serious adverse consequences such as corrosion and leakage. Therefore, no-clean fluxes are not allowed to contain halogens.

Fluxes are typically tested for corrosiveness using the following methods:

 a.Copper mirror corrosion test: used to test the short-term corrosiveness of fluxes (solder paste).

 b.Silver chromate test strip test: used to test the halide content in fluxes.

 c.Surface Insulation Resistance Test: It is used to test the surface insulation resistance of the PCB after soldering to determine the reliability of the long-term electrical properties of the flux (solder paste).

d. Corrosivity test: It is used to test the corrosiveness of the residue on the surface of the PCB after soldering.  e.Test the degree to which the conductor spacing on the PCB surface decreases after soldering.

 (3) Solderability: expansion rate≥80% Solderability and corrosion are a pair of contradictory indicators. In order for the flux to have some ability to eliminate oxides and remain active throughout the preheating and soldering process, it must contain some kind of acid. Amongno-clean fluxes, the most commonly used are the non-water-soluble acetic acid series, which may also contain amines, ammonia, and synthetic resins in the formulation, which can affect their activity and reliability depending on the formulation. Different companies have different requirements and internal control indicators, but they must meet the requirements of high welding quality and non-corrosive use.

The activity of the flux is usually measured by the pH value, and the pH value of the no-clean flux should be controlled within the technical conditions specified in the product (the pH value varies slightly from manufacturer to manufacturer).

 (4) Meet the requirements of environmental protection: non-toxic, no strong irritating odor, basically do not pollute the environment, safe operation.

No-clean printed circuit boards and components When implementing a no-clean soldering process, the solderability and cleanliness of printed circuit boards and components are key aspects that need to be controlled. In order to ensure weldability, the manufacturer should store it in a constant temperature and dry environment under the premise of requiring the supplier to ensure the weldability, and strictly control the use of it within the effective storage time. In order to ensure cleanliness, the environment and operating specifications should be strictly controlled during the production process to avoid man-made pollution, such as handprints, sweat stains, grease, dust, etc.

(5) Despite the challenges, with the continuous development of technology and the continuous improvement of enterprises' attention to production quality, environmental protection and economic benefits, the application prospect of no-clean process is still very broad. Companies can overcome these challenges by strengthening their collaboration with suppliers, improving their own technology, and optimizing their production processes to take advantage of the benefits of no-clean processes.