PCB surface finish- HASL, ENIG, OSP

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Now the commonly used circuit board surface finish are HASL, ENIG, OSP. The following are the advantages and disadvantages of the three different surface finish.

Surface finishes for printed circuit boards (PCBs) can be made of either organic or metallic materials. However, choosing the right surface finish for your PCB can be challenging as each type has its own benefits, drawbacks, and technical considerations.

To help you select the right finishing technique, this article discusses three common types of surface finishes: Hot Air Solder Leveling (HASL), Organic Solderability Preservative (OSP), and Electroless Nickel Immersion Gold (ENIG).

1. HASL

The advantages of Lead HASL: lower price, excellent welding performance, better mechanical strength.

Disadvantages of Lead HASL: Lead HASL cannot pass ROHS and other environmental protection evaluations.

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

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

The common disadvantage of HASL: it is not suitable for high precision chips.

2. ENIG

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 high precision components. Reflow can be repeated many times without reducing its solderability. Disadvantages of ENIG: high cost, poor welding strength. The nickel layer oxidizes over time, and long-term reliability is an issue.

3. OSP

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; this process is suitable for low-tech circuit boards and high-tech circuits. For boards, if there are no surface connection functional requirements or shelf life restrictions, the 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.

It is crucial to choose the appropriate surface finish for your project by considering various options while factoring in performance requirements and material costs.

For instance, Tin-Lead HASL may seem like a good choice if you are looking for the lowest cost, but it is not suitable for RoHS-compliant products. If your product requires RoHS compliance, you may consider using lead-free HASL. However, if your design uses fine pitch components, LFHASL cannot be applied perfectly flat. In such cases, you will need to use a flat, lead-free finish, such as Immersion Silver or ENIG, to ensure RoHS compliance. But, keep in mind that choosing such finishes will require the use of more costly high-temperature laminate.