OSP is a heat-resistant, solderable organic coating. OSP process is surface cleaning, dipping anti-oxidation agent, drying, simple process, convenient control, uniform surface coating, low cost, is a commonly used process in the design and development of consumer electronics circuit board.

The essence and key of OSP technology is in the “OSP liquid” station (work station), which can provide a small amount of Cu2+ in the bath while OSP can dissociate in the acidic liquid. When this pre-processed printed board enters the OSP liquid, Cu+ is formed on the surface of bare copper (such as lands or vias, etc.) of the printed board, and then the empty electron pairs of the dissociated OSP and the Cu+ surface of the printed board are formed. The complex forms a coordinate bond with copper to form a protective film.

The composition of OSP

The composition of OSP liquid currently used in printed boards is roughly as follows:

Alkylbenzimidazole 8-18g/L
Organic Acid 20～1.0g/L
Copper chloride 0.1-1.0g/L
Deionized water 0.1~1.0g/L
Alkylbenzimidazole
It is the main component of the OSP solution and is the root of the solderability and heat resistance of the printed board. Therefore, the development process is quite confidential, and the alkylbenzimidazoles of each OSP manufacturer are synthetically manufactured by themselves, which results in a difference in the OSP performance of each manufacturer. Such as heat resistance (from the discoloration of heat, or the temperature of discoloration, etc.), decomposition temperature (each product can be from 270 to 300 °C), etc., these properties for surface mount welding performance (especially whether it can withstand three or three times Above welding) is crucial.
The imidazole ring in the alkylbenzimidazole organic compound can form a coordination bond with the 2d10 of the copper atom, thereby forming a complex, and the long-chain alkyl group attracts each other through the van der Waals force, so that a certain degree is formed on the copper surface. Thickness of the protective layer, which has the presence of benzene rings, so this layer of protective film has good heat resistance and high thermal decomposition temperature.
The choice of alkyl in the alkyl benzimidazole will determine whether the OSP can be used as a printed board. In OSP liquids, when the content of alkyl benzimidazole is in the range of 1% to 5%, there is no significant change in the speed of the formation of the complex protective film; and when the concentration is greater than 10%, the excess of the alkylbenzimidazole in the aqueous solution is exceeded. Solubility will cause the oil to precipitate. Therefore, the content of alkyl benzimidazole should be controlled within 10%, and the content of alkyl benzimidazole used in practice is much smaller than this value.

Copper chloride
Adding an appropriate amount of cupric chloride in the preflux solution can promote the formation of the complex protective film and shorten the dipping time. It is generally believed that the alkylbenzimidazole has a certain degree of coordination with copper ions in the preflux solution due to the presence of copper ions. When the complexes with a certain degree of aggregation re-deposit on the copper surface to form a coordination film, a thick protective layer can be formed in a relatively short period of time, thereby acting as a coordination accelerator. When the amount of CuCl added on the surface exceeds 0.1%, the pre-flux solution will be prematurely aged. Generally, it should be controlled at 0.03% to 0.05%.

Organic acids
The addition of organic acid can increase the solubility of alkyl benzimidazole in aqueous solution and promote the formation of coordination protective film. The use of too much amount results in the dissolution of the protective film deposited on the copper surface. Therefore, it is important to control the added value of the organic acid (ie, the pH value). Generally, the pH of the pre-flux solution should be controlled to about 3.5. Formic acid and acetic acid or a mixture of two acids was used in the test and no significant difference was found.

OSP usage conditions

temperature The concentration of alkylbenzimidazole in the OSP solution was 1%, and the pH value was 3.5±0.1. A 3 mm×3 mm copper clad plate was used for the sample, and the dipping time was 1 minute. The thickness of the complex protective film formed on the copper surface was observed. The thickness of the complex film formed on the copper surface is affected by the temperature. Within the scope of the test, the dip coating time was constant and the film thickness increased with increasing temperature. Control temperature is 30 ~ 40 °C, dip coating 1min, you can form a dense, uniform and moderate thickness of the complex film.

pH
The acidity of the OSP solution is regulated by the addition of organic acids. To maintain the pH during dip coating, a certain amount of buffer must be added. Using a 3mm x 3mm copper-clad laminate sample, the concentration of alkylbenzimidazole in the preflux solution was 1%, the pH of the solution was changed, dip-coating at 40°C for 1 minute, and the pH was higher than 5, the alkylbenzene was The solubility of imidazole decreases and oily substances precipitate, which is disadvantageous for dip coating. The pH is controlled at 4.0±0.5, and a dense, uniform and moderate thickness complex film can be obtained. If the pH is too low, the solubility of the complex film increases, so that the complex deposited on the copper can be dissolved and the film of a desired thickness cannot be formed.

Dipping time
The thickness of the complex protective film formed at the determined OSP bath composition, temperature, and pH conditions will start to increase linearly (usually within 30 s) as the dipping time increases. Then, as the time goes by, the film thickness will increase slowly, and after more than 2 minutes, the film thickness will not increase substantially. Therefore, the OSP dip coating time can be completed in about 1min.

What are the advantages of OSP?

In recent years, the application practice and experience of OSP in printed boards have shown that the OSP film has achieved gratifying results through wet test and solderability test. Because it forms a 0.3-0.5 μm thick film only on the bare copper to be soldered, it has excellent properties such as high thermal stability, compactness, and hydrophobicity, and it has been rapidly applied. Its advantages include:

The use of an OSP film can greatly increase the production capacity of the printed circuit board. Since the coating of the OSP film is performed continuously, the intermittent production of the hot air leveling process is avoided, and the production efficiency is greatly improved.
A uniform thickness protective film can be formed on the bare copper portion of the printed circuit board, thereby improving the flatness (coplanarity) of the printed board surface and the pad, making solder paste printing easier, and significantly reducing precision The probability of part displacement increases the qualification rate and reliability of high-density surface mount technology assembly.
The process is simple, easy to operate, compatible with other welding techniques, and easy to operate and maintain. Operating environment is good, less pollution, easy to automate.
With low cost, it can be said that it is the lowest cost process for all surface solderability of printed boards. The disadvantage is that the protective film formed is very thin, easy to scratch (or scratches, etc.) and must be carefully handled and shipped. At the same time, it is only a protective film of bare copper solderability. Once the protective film is destroyed, the solderability of bare copper is not guaranteed.

OSP Process Quality Inspection
Visually inspecting the appearance of the organic flux protective film should be uniform and flat.
After the membrane can be detected by a spectrophotometer. It can also be observed visually to see if the surface of the plate is discolored after 30 minutes of film formation. If the color changes, the thickness of the organic film is not enough, resulting in oxidation of the copper surface.
Solderability testing can be performed without immersing flux for immersing tin solder at 260°C for 10 seconds. All solder pads should be filled with tin and scratched with tin solder.