How to choose PCB surface finish process

After we design the PCB board, we need to choose the surface treatment process of the circuit board. Now the commonly used surface treatment processes of the circuit board include HASL (surface spray tin process), ENIG (immersion gold process), OSP (anti-oxidation process), and the commonly used surface treatment processes are How should we choose the processing technology? Different PCB surface treatment processes have different effects. You can choose according to the actual situation. The following are the advantages and disadvantages of the three different surface treatment processes of HASL, ENIG and OSP.

1. HASL (surface spray tin process)

The spray tin process is divided into lead spray tin and lead-free spray tin. The spray tin process used to be the most important surface treatment process in the 1980s, but now, fewer and fewer circuit boards choose the spray tin process. The reason is that the circuit board is developing in the direction of "small and refined". The tin spraying process will lead to tin beads when welding fine components, and the production of spherical tin points will cause poor production. In order to pursue higher process standards and For production quality, the surface treatment process of ENIG and SOP is often selected.

The advantages of lead-sprayed tin are: lower price, excellent welding performance, better mechanical strength and gloss than lead-sprayed tin.

Disadvantages of lead-sprayed tin: lead-sprayed tin contains lead heavy metal, production is not environmentally friendly, and cannot pass ROHS and other environmental protection evaluations.

The advantages of lead-free tin spraying: low price, excellent welding performance, and relatively environmentally friendly, can pass ROHS and other environmental protection evaluation.

Disadvantages of lead-free spray tin: mechanical strength, gloss, etc. are not as good as lead-free spray tin.

The common disadvantage of HASL: it is not suitable for soldering pins with fine gaps and components that are too small, because the surface flatness of the tin-sprayed board is poor. Tin beads are easily generated in PCBA processing, which is more likely to cause short circuits to components with fine gaps.

2. ENIG (Immersion Gold Process)

Immersion gold process is a relatively advanced surface treatment process, which is mainly used on circuit boards with functional connection requirements and long storage periods on the surface.

Advantages of ENIG: It is not easy to oxidize, can be stored for a long time, and has a flat surface. It is suitable for soldering fine-gap pins and components with small solder joints. Reflow can be repeated many times without reducing its solderability. Can be used as a substrate for COB wire bonding.

Disadvantages of ENIG: high cost, poor welding strength, because the electroless nickel plating process is used, it is easy to have the problem of black disk. The nickel layer oxidizes over time, and long-term reliability is an issue.

3. OSP (Anti-Oxidation Process)

OSP is an organic film formed by chemical means on the surface of bare copper. This layer of film has anti-oxidation, thermal shock resistance, moisture resistance, and is used to protect the copper surface from rusting (oxidation or vulcanization, etc.) in the normal environment; it is equivalent to an anti-oxidation treatment, but in the subsequent high temperature welding, The protective film, in turn, must be easily and quickly removed by the flux, and the exposed clean copper surface can immediately bond with the molten solder into a strong joint in a very short time. At present, the proportion of circuit boards using OSP surface treatment process has increased significantly, because this process is suitable for low-tech circuit boards and high-tech circuit boards. If there are no functional requirements for surface connection or storage period restrictions, OSP process will be the most ideal. surface treatment process.

Advantages of OSP: It has all the advantages of bare copper board soldering, and boards that have expired (three months) can also be resurfaced, but usually only once.

Disadvantages of OSP: susceptible to acid and humidity. When used for secondary reflow soldering, it needs to be completed within a certain period of time. Usually, the effect of the second reflow soldering will be poor. If the storage time exceeds three months, it must be resurfaced. Use within 24 hours after opening the package. OSP is an insulating layer, so the test point must be printed with solder paste to remove the original OSP layer to contact the pin point for electrical testing. The assembly process requires major changes, probing raw copper surfaces is detrimental to ICT, over-tipped ICT probes can damage the PCB, require manual precautions, limit ICT testing and reduce test repeatability.

The above is the analysis of HASL, ENIG, OSP circuit board surface treatment process, any PCB demands, please feel free to contact me.