OSP PCB Technology

What is OSP PCB?

OSP is a process for treating printed circuit board (PCB) copper foil. It meets RoHS standards.

OSP surface finish is the abbreviation of Organic Solderability Preservatives. It is also called “organic solder protection film”, “copper protecting agent”, or Preflux.

OSP has chemically grown a layer of organic film on the surface of cleaning bare copper. The film resists oxidation, thermal shock, and moisture. It protects copper from rust in normal environments.

But, it must be quickly removed by the heat of follow-up welding. This exposes the clean copper so it can quickly bond with molten solder to form a solid joint.

OSP-PCB

Common PCB Surface Treatment Methods

Printed circuit boards are now more precise and thin. Electronic products are also lighter, thinner, and smaller. The rapid growth of SMT and hot air leveling has not kept up with the high-density boards. At the same time, hot air leveling using Sn-Pb solder does not comply with environmental requirements.

Since the EU RoHS directive took effect on July 1, 2006, the industry is urgently seeking lead-free alternatives to PCB surface treatments. The most common are organic solder protection(OSP), electroless nickel immersion gold (ENIG), immersion silver, and immersion tin.

The figure compares some common PCB surface treatments. It shows hot air leveling (Sn-Pb, HASL), immersion silver, immersion tin, OSP, and electroless nickel plating gold (ENIG). The last four are lead-free processes. It can be seen that OSP is more and more popular in the industry because of its simple process and low cost.

Physical Properties HASL Immersion Sliver Immersion Tin OSP ENIG
Storage Life 12 months 6 months 6 months 6 months 12 months
Experience reflow times 4 5 5 4 4
Cost Medium Medium Medium Low High
Process complexity High Medium Medium Low High
Process temperature 240°C 50°C 70°C 40°C 80°C
Thickness range(um) 1-25 0.05-0.2 0.8-1.2 0.2-0.5 Au:0.05-0.2,Ni 3-5
Flux compatibility Good good good good good

Manufacturing Process of OSP

Degreasing—>Water cleaning—>Microetch—>Water cleaning—>pickling—> DI water cleaning—>Formed film and drying—>DI water cleaning—>Drying

1. Degreasing

The effect of oil removal directly affects the quality of film formation. The film thickness is uneven when the oil is not clear.

On the one hand, the concentration can be controlled within the range of the process by analyzing the solution.

On the other hand, often check if the degreasing worked. If not, replace it with an oil remover.

2. Microetch

The purpose of etching is to form a roughened copper surface to facilitate film formation. The thickness of the etching directly influences the rate of film formation.

Therefore, it is very important to maintain a stable thickness of the film to maintain the thickness of the etched film.

Generally, it is appropriate to control the etching thickness at 1.0-1.5um. The micro corrosion rate can be determined before each production. The etching time is based on the micro etching rate.

3. Film formation

DI water is best used for washing before film formation, so as to prevent contamination of the film forming fluid.

After washing the film, use DI water. The pH should be 4.0-7.0 to avoid contamination and damage. The key to the OSP process is to control the thickness of the oxide film. The film is too thin. Its heat shock resistance is low.

In reflow soldering, the film must resist high temperatures (190-200 °C). This affects the welding performance on the assembly line.

Film is not very good in the flux of dissolved, welding performance. The average control film thickness is between 0.2-0.5um.

Disadvantages of OSP PCB

  1. OSP, of course, has its drawbacks. For example, there are many kinds of practical formulas and different performances. That is to say, the certification and selection of suppliers should be done well enough.
  2. The OSP surface finish process has a disadvantage. The resulting protective films are very thin. They are easy to scratch. So, they must be carefully operated and used with operational amplifiers. After many high-temp welds, the OSP film on the non-welded plate will change color or crack. This will affect weldability and reliability.

Packaging and Storage Requirement of OSP PCBs

OSP is a thin organic coating on the PCB surface. If exposed to high temperature and humidity for a long time, it will oxidize. This affects weldability.

After the reflow soldering process, the coating will be thinner, which will oxidize the copper foil on the PCB.

So, the preservation methods and use of OSP PCB and SMT semi-finished products must follow these principles:

  • The OSP PCB shall be packed in a vacuum with a desiccant and humidity display card attached. Isolation paper should be used for transport and storage of the PCB with OSP. It will prevent friction from damaging the OSP surface finish.
  • Do not be exposed to direct sunlight. Store in a good environment: 30-70% humidity, 15-30°C. Use within 6 months.
  • After opening the package at the SMT site, be sure to check the humidity indicator card and leave it online for 12 hours. Never open a large number of packages at one time.
  • If not finished, or is it a device to solve the problem for a long time, it is prone to problems. After printing the furnace as soon as possible not to stay, because there is a very strong flux solder paste on the OSP coating corrosion. Maintain a good workshop environment: relative humidity 40~60%, temperature: 22~27 degrees Celsius.
    In production, avoid touching the PCB surface with your hands. Sweat can oxidize it.
  • The second side of the SMT assembly must be completed within 24 hours of the first side’s completion.
    Finished the DIP in as short a time as possible (the longest 36 hours) after completing the SMT.
  • OSP PCB surface finish can’t be baking. High-temperature baking easily deteriorates color of OSP. If bare board exceeds the storage life, it can be returned to OSP manufacturer for rework.

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