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What is OSP Surface finish?And are the advantages and disadvantages?

2017-05-18

OSP(Organic solderability Preservative)

Using chemical method grow a dislocation organic copper compound (complex compound) of the surface of the copper film.This layer of organic film can protect the clean bare copper on the PCB no longer rusting(Vulcanization or oxidation) in contact with air when they are preserved in normal storage environment.Also can easily efficiently cleaned by flux and dilute acid to show clear copper surface to form welding with molten solder during flow soldering.

Basically this "OSP" is a transparent protective film,and really hard to be detected by visual check.But pros can see it from refracted through the reflective copper to say is there is a film.Otherwise there is no big different between a bare cooper and OSP surface finished board,this makes it difficult for the board factory to check and measure the value.

If OSP film unfortunately have a hole just on the copper surface,copper film will start oxidation from hole area,and then become another bad factor to influence SMT mounting .Therefore ,the more OSP thicker,the better the protection for copper foil .Considering need to be clean by flux and dilute acid ,so the thickness of OSP film is between :0.2-0.5 um.

Advantages of OSP (Organic Solderability Preservative) Surface finished

1.The price is lower than other surface finished.

2.Good welding strength. The welding strength of the OSP copper base is basically better than that of the ENIG nickel base.

3.Boards that have expired (three or six months) can also be resurfaced, but usually only once, depending on the board.

Disadvantages of OSP (Organic Solderability Preservative) Surface finished

1. OSP is a transparent film, it is not easy to measure its thickness, so the thickness is not easy to control. The film thickness is too thin to find the effect of not protecting the copper surface. If the film thickness is too thick, it will not be welded.

2. In the case of secondary reflow, it is recommended to operate in an environment with open nitrogen, which can get a good welding effect.

3. The shelf life is insufficient. Generally speaking, after the completion of the OSP, the shelf life of the OSP is up to six months, and some are only three months. Depending on the capabilities of the board factory and the quality of the board, some boards that exceed the shelf life can be returned to the board. The factory washes off the old OSP on the PCB surface and then re-adds a new OSP. However, washing out the old OSP requires more corrosive chemicals, which will damage the copper surface more or less. Therefore, if the solder pad is too small to be processed, it must communicate with the board manufacturer whether it can be resurfaced.

4. It is susceptible to acid and humidity. When used in secondary reflow , it needs to be completed within a certain period of time. Usually, the effect of the second reflow soldering will be poor. It is generally required to use up within 24 hours after opening the package (after reflow). The shorter the time of one reflow and the second reflow, the better. It is generally recommended to complete the second reflow within 8 hours or within 12 hours.

5. OSP is an insulating layer, so the test points on the board must be printed with solder paste to remove the original OSP layer to contact the pin for electrical testing

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