OSP Surface Finish Guide

We are aware that the conduction of current on a circuit board relies on copper traces. However, this metal material is susceptible to oxidation when exposed to air, especially in humid and high-temperature environments. This oxidation can lead to poor soldering, resulting in issues such as component detachment. To mitigate this potential problem, a series of surface finish measures are implemented, with the Organic Solderability Preservative (OSP) process being widely adopted due to its relatively low cost and ease of implementation.

osp process in pcb

Table of Contents

What is OSP Surface Finishing

Comparison of surface treatments including OSP

OSP, or Organic Solderability Preservatives, also known as organic solder mask or copper protectant, is a thin and flat organic film applied to the surface of a circuit through chemical means. The purpose of this film is to protect the bare board from oxidation, thermal shock, and moisture before soldering components, significantly enhancing the quality of solder joints.

In the modern electronics manufacturing industry, it is common to separate the manufacturing and assembly processes. After the bare board production is complete, manufacturing facilities promptly implement OSP protection, storing the boards in sealed bags before delivering them to assembly plants. In the assembly plant, once components are installed, the bare boards proceed directly to the high-temperature soldering process. At this elevated temperature, the OSP coating naturally decomposes without the need for additional processes, ensuring higher soldering compatibility.

However, it is important to note that sometimes, due to improper scheduling or other reasons, the bare boards might be delayed from entering the assembly production line promptly. In such cases, attention should be paid to the shelf life of the OSP coating. If the expiration date is exceeded, it may be necessary to re-implement OSP protection.

OSP Process in PCB Manufacturing

After the completion of PCB manufacturing, a comprehensive cleaning of the board surface is necessary to remove grease, dust, chemical contaminants, and other impurities. This process not only helps prevent potential corrosion issues but also ensures the uniform adhesion of the OSP coating, thereby enhancing the overall quality of surface finishing.

Once the board surface is confirmed to be clean, the next step is the activation process. This step involves methods such as acid washing, alkaline washing, and activation agent treatment to optimize the surface condition, providing a robust foundation for the OSP coating.

After all preparations are completed, the PCB is immersed in a solvent or the OSP coating is applied by spraying. After ensuring an even coating, a short pre-baking is performed to remove solvents from the coating solution and accelerate the curing process. Finally, cooling is carried out until the OSP coating is completely cured.

Characteristics of OSP Coating

  • The OSP coating is smooth and uniform, with a film thickness of approximately 0.2~0.5μm, suitable for various high-density PCB.
  • It provides excellent thermal shock resistance, compatibility with various solders, suitable for lead-free and double-sided soldering processes.
  • Used to prevent threats of oxidation and moisture to PCB.
  • The process is simple, has a high success rate, allows for automated production, and is cost-effective.
  • Water-soluble compounds make it environmentally friendly, in compliance with RoHS and WEEE standards.
  • It is flexible, easy for rework and repair.
  • Only applicable to copper surfaces and does not adhere to gold surfaces.
  • OSP PCB shelf life is short and not suitable for certain transportation and long-term storage.
  • It needs to be stored in an environment with a relative humidity of 30-70% RH, temperature between 15-30℃, and protected from direct sunlight.

OSP vs. Other Surface Finish

ItemHASLENIGTinSilverOSP
CostMedium ~ HighHighMediumMediumLow
Temp/°C≥250≥80≥60≥50Room temperature to 40
Time2-3s40min6-10min60-120s30-90s
Thickness/μm3-53-50.8-1.20.3-0.50.1-0.2
Shelf Life/year110.50.50.5
ReworkEasyDifficultDifficultDifficultEasy
High DensityDifficultMediumMediumMediumMedium
Wastewater TreatmentEasyDifficultMediumMediumEasy
Usage Ratio: 200729%14%4%8%45%

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