PCB News - Advantages and disadvantages of OSP (organic solder protection film) in the surface treatment of circuit board factories

Advantages and disadvantages of OSP (organic solder protection film) in the surface treatment of circuit board factories

[OSP (OrganicSolderabilityPreservative, organic solder protection film)] is a chemical method to grow a layer of organic copper complex compound (complexcompound) film on the surface of copper. This organic film can protect the clean bare copper on the circuit board from rusting (vulcanization or oxidation) in contact with the air under normal storage conditions, and can be easily fluxed and diluted during the PCBA circuit board assembly process. The acid is quickly removed and the clean copper surface is exposed to form a weld with the molten solder.

This OSP is basically a transparent protective film. It is generally very difficult to detect its existence with the naked eye. Experts can see through refraction and reflection to see if there is a transparent film on the copper foil. There is not much difference between the OSP circuit board and the ordinary bare copper board in appearance, which also makes it difficult for the circuit board factory to check and measure the value.

If the organic copper protective agent (OSP) has a hole just on the copper surface, the copper surface will start to oxidize from the hole, which will affect the failure of SMT assembly. The thicker the organic copper protective agent, the greater the thickness of the copper foil. The better the protection, but relatively it also requires a stronger active flux to remove it for soldering, so the OSP film thickness is generally required to be between 0.2-0.5um.

OSP (organic solder preservative) production flow chart

AcidCleaner (degreased):

The main purpose is to remove the copper surface oxides, fingerprints, grease and other pollution that may appear in the pre-processing process to obtain a clean copper surface.

Micro-etch:

The main purpose of micro-etching is to remove serious oxides on the copper surface, and produce a uniform and bright micro-rough copper surface, so that the subsequent OSP film can grow more finely and uniformly. Generally, the gloss and color of the copper surface after OSP film formation have a positive correlation with the selected micro-etching chemicals, because different chemicals will cause different roughness of the copper surface.

AcidRinse (pickling):

The pickling function completely removes the residual material on the copper surface after microetching to ensure that the copper surface is clean.

OSPcoating (organic solder preservative treatment):

A layer of organic copper complex compound is grown on the copper surface to protect the copper surface from oxidation due to contact with the atmosphere during storage. Generally, the OSP film thickness is required to be between 0.2-0.5um.

The factors that affect OSP film formation are:

The pH value of OSP bath solution

The concentration of OSP bath solution

The total acidity of OSP bath

·Operating temperature

Reaction time

Washing after OSP should strictly control its acid-base value above pH 2.1 to avoid over-acid washing to bite and dissolve the OSP film, resulting in insufficient thickness.

Dry (drying):

In order to ensure the drying of the coating layer on the board surface and the holes, it is recommended to use hot air at 60-90°C for 30 seconds. (This temperature and time may have different requirements due to different OSP materials)

OSP (OrganicSolderabilityPreservative, organic solder protection film) surface treatment circuit board advantages:

▪ The price is cheap.

▪ Good welding strength. The welding strength of OSP copper base is basically better than ENIG nickel base.

▪ Overdue (three or six months) circuit boards can also be resurfaced, but usually only once, depending on the condition of the board.

OSP (OrganicSolderabilityPreservative, organic solder protection film) surface treatment circuit board disadvantages:

▪OSP is a transparent film which is not easy to measure its thickness, so the thickness is not easy to control. If the film thickness is too thin, it will not protect the copper surface. If the film thickness is too thick, it will not be able to be welded.

▪ It is recommended to operate in an environment with open nitrogen during the secondary reflow, which can get a good welding effect.

▪ The shelf life is insufficient. Generally speaking, after the OSP is completed in the circuit board factory, its shelf life is up to six months, and some are only three months. It depends on the capacity of the circuit board factory and the quality of the circuit board. Some boards that exceed the shelf life can be Send it back to the board factory to wash off the old OSP on the PCB surface, and then put a new layer of OSP on it. However, washing off the old OSP requires more or less corrosive chemicals, which will damage the copper surface more or less. Therefore, if the solder pad is too small, it will not be able to be processed. It is necessary to communicate with the circuit board factory whether the surface treatment can be done again.

▪ Easily affected by acid and humidity. When used in secondary reflow soldering (Reflow), it needs to be completed within a certain period of time. Generally, the effect of the second reflow soldering is relatively poor. It is generally required to use it up within 24 hours after opening the package (after reflowing). The shorter the time between the first reflow and the second reflow, the better. Generally, it is recommended to finish the second reflow within 8 hours or 12 hours.

▪OSP is an anti-oxidation insulating layer, so the test points on the board must be printed with solder paste to remove the original OSP layer to contact the pins for electrical testing. Related reading: What is ICT (In-Circuit-Test)? What are the advantages and disadvantages?

▪ The OSP anti-oxidation circuit board is a copper base. The benign IMC of Cu6Sn5 will initially be generated after soldering, but after time aging, it will gradually change to the inferior IMC of Cu3Sn, which will affect the reliability, so if it needs to be used in a high temperature environment for a long time Or products that require a longer service life must consider the long-term reliability of OSP.