1. Introduction to OSP

 

1. Definition:

OSP: Organic Solderability Preservative was called heat-resistant preflux in the early days.

2. Raw Organism

Early organically coated molecules were imidazole and benzotriazole, and the latest molecules are mainly benzimidazole;

3. Function

Anti-oxidation and thermal shock resistance, good solderability and other functions

4. Protection Mechanism

Firstly, it can protect the copper surface from rusting due to external influences; secondly, its film can be quickly removed by dilute acid or flux before welding, so that the bare copper surface can still show good solderability instantly.

5. Reaction Mechanism

The basic principle is to chemically bond the nitrogen functional group to the copper on the PCB, that is, the imidazole ring in the alkylbenzimidazole organic compound can form a coordination bond with the 3d10 electrons of the copper atom, thereby forming an alkylbenzimidazole-copper complex. things. Moreover, after the first layer is coated, the coating layer adsorbs copper (because copper liquid usually needs to be added to the chemical tank); then the organic coating molecules of the second layer are combined with the copper until twenty or even hundreds of organic coatings. The coating molecules gather on the copper surface, which ensures multiple reflows. In addition, long-chain alkyl groups attract each other through van der Waals forces, thus forming a protective layer with a certain thickness (usually between 0.2∽0.5µm) on the fresh copper surface. In addition, due to the presence of benzene rings, this A layer of protective film has good heat resistance and high decomposition temperature.

 

 

 

6. OSP film thickness range

0.2～0.5µm, the best value is 0.35µm or 14µin

7. The two OSPs currently widely used are nitrogen-containing organic compounds, namely benzotriazoles and imidazoles. They all adhere well to bare copper surfaces and are very specific, only focusing on copper and not adsorbing on insulating coatings such as solder masks.

8. Indazole (benzotriazole) will form a molecular film on the copper surface. During the assembly process, when a certain temperature is reached, this film will be melted away, especially during the reflow soldering process. , OSP is relatively easy to evaporate. The protective film formed by imidazole organic crystalline base on the copper surface is thicker than indazole, which can withstand more thermal cycle impacts during the assembly process.

 

2. OSP characteristics

HAL                                                                                                                                            OSP

The solder joints are uneven and have poor uniformity                               The solder joints are flat and have good coplanarity

Warping is prone to occur due to thermal shock during production.             No thermal shock during production, reducing deformation

There is a lot of noise and pollution during production                                  The production environment is less polluted

Long storage time and relaxed environment                                                  Short storage time and strict environment

3. OSP process

Feeding---acidic degreasing---water washing---micro-etching---water washing---acid washing---water washing---ultrasonic washing---OSP---water washing---blow drying-- -Drying---Discharging

Acidic degreasing: remove oil stains and oxides on the surface of the board

Microetching: Microscopically roughening the copper surface to further remove oxides

Pickling: Remove micro-etching residues and protect the OSP working fluid from being contaminated

Ultrasonic water cleaning: remove residue in the hole

OSP tank: grow organic protective film, film thickness is between 0.2-0.5um

Blow dry: Air knife blows away the remaining water in the hole and reduces the moisture on the board surface

Drying: Dry to remove moisture in the board and holes

4. OSP limitations

1. The disadvantage of the OSP film is that the protective film formed is extremely thin and easy to scratch (or scratch), so it must be carefully operated and operated. At the same time, the OSP film (referring to the OSP film on the unsoldered connection pad) that has undergone multiple high-temperature welding processes will discolor or crack, affecting solderability and reliability.

2. Since OSP is transparent and colorless, it is difficult to inspect and it is difficult to tell whether the PCB has been coated with OSP. During the welding process, OSP requires a more powerful flux, otherwise the protective film cannot be eliminated, resulting in welding defects.

3. OSP itself is insulating and does not conduct electricity. Benzotriazoles type OSP is relatively thin and may not affect electrical testing, but for Imidazoles type OSP, the protective film formed is relatively thick and will affect electrical testing. OSP cannot be used to treat electrical contact surfaces, such as keyboard surfaces for keys. ,

4. During the storage process, the OSP surface should not be exposed to acidic substances, and the temperature should not be too high, otherwise the OSP will volatilize.

5. OSP precautions

1. Generally, the effective storage period of OSP boards is 6 months (environment: 20~30℃, 40~70% relative humidity). Exceeding 6 months may affect the tin application performance of the board and needs to be reprocessed to reduce the Tin bad risk.

2. Do not bake it like a tin spray board before going online to avoid damaging the OSP film, causing oxidation of the copper surface, and affecting the soldering properties.

3. When designing with tin, the test points should be tinned. (Because when the board passed through two IR and one tin wave peaks, the OSP burst and caused the copper surface under the OSP to oxidize, making the test difficult or wrong).

4. After the board is folded and packaged, 2 IRs and 1 wave solder must be completed within 24 hours. After completing the first side of the package, it is best to perform the other side of the package within 8 hours. After the first side is completed, the second side must be pre-packaged. Baking is not allowed to avoid affecting the soldering properties.

6. Application of OSP

It is estimated that about 50%-80% of PCBs currently use organic coating processes, and this proportion has been rising (it is likely that organic coating has now surpassed hot air leveling in the first place). The organic coating process can be used on low-tech PCBs or high-tech PCBs, such as single-sided TV PCBs and high-density chip packaging boards. For BGA, there are also many organic coating applications. If PCB does not have functional requirements for surface connection or storage period restrictions, organic coating will be the most ideal surface treatment process.