A surface finish can be defined as a coating, either metallic or organic in nature, that is applied to a PCB to ensure the solderability of the metal underneath. Most of the surface treatment dissolves into the solder paste or wave solder during the soldering process, and the solder joint is forming between the solder and the copper. One exception is ENIG/immersion gold, where the solder dissolves the thin layer of gold and forms a joint with the underlying nickel phosphorous alloy.
There are only two different types of surface finishes for PCBs: organic and metal. Here are the six most common finishes on the market:
- HASL leaded
- HASL lead-free
- Organic solderability preservatives (OSP)
- Immersion Sn
- Immersion Ag
OSP is more like a lacquer, which prohibits oxygen from attacking the copper underneath. All others are coating metals, and they may be applied using one of two different methods: electroless or immersion.
Electroless systems work in the same fashion; they use a reducing agent inside the bath itself. This means that the metal thickness increases during the whole period that the PCB is in contact with the solution.
An immersion systems process uses a chemical displacement reaction to deposit a metal layer onto the exposed metal surface of the PCB. The base metal donates the electrons that reduce the positively charged metal ions present in the solution. The immersion layer will continue to grow; however, as the thickness of the deposit increases, the rate of deposition falls. Therefore, the process is self-limiting.
Comparing both types and all available options can quickly demonstrate the relative benefits or drawbacks. Typically, the decisive factors when it comes to selecting the most suitable finish is the end application, the assembly process, and the design of the PCB itself.
For each of the six most common surface finish, these tables illustrate the pros and cons (Tables 1–6).
Table 1: ENIG pros and cons.
Table 2: HASL leaded pros and cons.
Table 3: HASL lead-free pros and cons.
Table 4: OSP pros and cons.
Table 5: Immersion Sn pros and cons.
Table 6: Immersion Ag pros and cons.
There is much more to understand about surface finishes. Each of the six surface finishes I mentioned has a different process for application, handling recommendations, storage conditions and time, baking requirements, thickness requirements, masking limitations and design considerations and concerns. Tables 1–6 are simply an overview. We always recommend discussing surface finishes with your PCB supplier as early as possible in the design stage.
Harry Kennedy is a field application engineer at NCAB Group.