OSP stands for Organic Solderability Preservatives; in Chinese, it is known as an organic solderability preservative and is also commonly referred to as a solder mask or copper protector.
In short, OSP involves chemically depositing an organic film onto the bare copper surface of a PCB. This film provides properties such as oxidation resistance, heat resistance, and moisture resistance, ensuring that the copper will not oxidize or undergo sulfidation under normal environmental conditions.
However, during the subsequent high-temperature welding process, this protective layer must dissolve quickly and easily in the welding fumes in order to form a strong weld.
1. The OSP Process Flow
The OSP process typically includes the following steps:
1.1. Degreasing
This initial step removes any grease or contaminants from the copper surface, ensuring a clean starting point.
1.2. Water Rinse
After degreasing, rinse with water to further clean the board.
1.3. Micro-erosion
Micro-etching is performed to prepare the copper surface for OSP treatment.
1.4. Acid Pickling
Pickling further cleans the copper surface.
1.5. Rinsing with deionized water (DI water)
Before performing OSP treatment, rinse the board with deionized water.
1.6. OSP Coating
Apply the OSP material to the copper surface of the PCB to form a thin organic film.
1.7. Rinsing with deionized water (DI water)
After applying the OSP, perform another rinse with deionized water.
1.8. Drying
Finally, dry the board to complete the OSP process.
2. The Principles Behind OSP
In PCB manufacturing, OSP forms a protective organic film that firmly protects the copper surface at high temperatures, preventing oxidation and contamination.
Its key principles include:
- OSP films are typically 0.2–0.5 micrometers thick.
- OSP provides excellent wetting properties, allowing the solder to bond directly to the copper surface.
- This is a low-temperature process that offers cost advantages over methods such as HASL (Hot Air Solder Leveling).
- OSP is suitable for low-tech PCBs and high-density chip packaging substrates.
3. Key Features of OSP
OSP offers the following advantages in PCB manufacturing:
- Smooth surface: OSP provides a smooth, even surface.
- No IMC formed: No intermetallic compounds (IMCs) form between the OSP coating and copper, allowing for direct soldering.
- Low-Temperature Processing: It has excellent wetting properties.
4. OSP Material Types
There are various types of OSP materials to choose from. At Jingpeng Electronics, the most commonly used OSP material is azole-based OSP.
5. Limitations of OSP
Although OSP is a valuable method for protecting PCBs, it also has some limitations:
5.1. Difficulty of Visual Inspection
The protective film on OSP is difficult to inspect visually and is not suitable for multiple reflow soldering cycles (it is generally recommended to perform three or fewer reflow soldering cycles).
5.2. Prone to Scratches
The OSP coating on the surface of a PCB is prone to scratches, so it must be handled with care.
5.3. Storage Environment Requirements
OSP PCBs must be stored in a controlled environment with a temperature between 15 and 35°C and a relative humidity below 60%.
5.4. Limited Shelf Life
OSP PCBs have a relatively short shelf life.
6. Storage Guidelines for OSP Boards
- OSP boards should be vacuum-packed and stored for up to six months at temperatures between 15 and 35°C and relative humidity below 60%.
7. SMT Assembly Requirements
There are certain requirements when assembling OSP PCBs using surface-mount technology (SMT):
7.1. Storage Conditions
OSP boards must be stored in an environment with low temperature and low humidity (15–35°C, RH ≤ 60%) and must be assembled within 48 hours of unpacking.
7.2. Single-Sided Assembly
For single-sided assembly, it is recommended to use OSP boards within 48 hours and store them in a low-temperature cabinet rather than in vacuum packaging.
7.3. Completing the DIP
After completing the SMT double-sided assembly, it is recommended to complete the dual-in-line package (DIP) assembly within 24 hours.
OSP (Organic Solder Preservative) is a critical process in PCB manufacturing that ensures the copper surface remains protected from corrosion prior to soldering. Although it offers numerous advantages, including cost-effectiveness and excellent wetting properties, it is important to understand its limitations and follow proper storage and assembly guidelines.
