The bare copper on the PCB would oxidize and compromise the components soldering. As one of the last working steps in the PCB manufacturing, the bare copper on the PCB is coated with a surface finish.
The surface finish protects the underlying copper against oxidation and therefore increases storability. In addition, the surface finish is responsible for raising the wetting ability of the contact areas for soldering.
The selectable surface finishes vary in shelf life, typical application and price. Yet the surface finish is mostly determined by the following processing steps, the final product or the designated point of use of the final product.
| Surface |  RoHS-compliant |  Soldering |  Bonding aluminium wire |  Bonding gold wire |  BGA |  Flex-/Rigid-Flex |  Pressfit |  PCI contacts |  Sliding contact |  Thickness |  Shelf life |  Reflow Cycles | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Surface | Hal SnPb | RoHS-compliant | Soldering |  | Bonding aluminium wire | Bonding gold wire | BGA | Flex-/Rigid-Flex | Pressfit | | PCI contacts | Sliding contact | Thickness | >5µm | Shelf life | ca.12 months | Reflow Cycles | 5** | |||||||
| Surface | HAL lead-free | RoHS-compliant | | Soldering | | Bonding aluminium wire | Bonding gold wire | BGA | Flex-/Rigid-Flex | Pressfit | | PCI contacts | Sliding contact | Thickness | acc. to IPC-6012* | Shelf life | ca. 12 months | Reflow Cycles | 5** | ||||||
| Surface | Chemical tin | RoHS-compliant | | Soldering | | Bonding aluminium wire | Bonding gold wire | BGA | | Flex-/Rigid-Flex | Pressfit | | PCI contacts | Sliding contact | Thickness | >1µm | Shelf life | ca. 6 months | Reflow Cycles | 5** | |||||
| Surface | Chemical silver | RoHS-compliant | | Soldering | | Bonding aluminium wire | | Bonding gold wire | BGA | | Flex-/Rigid-Flex | Pressfit | | PCI contacts | Sliding contact | Thickness | 0.12 - 0.3µm | Shelf life | ca. 6 months | Reflow Cycles | 5** | ||||
| Surface | Chemical gold ENIG | RoHS-compliant | | Soldering | | Bonding aluminium wire | | Bonding gold wire | BGA | | Flex-/Rigid-Flex | | Pressfit | | PCI contacts | Sliding contact | Thickness | 3-6µm Ni 0.05-0.1µm Au | Shelf life | ca. 12 months | Reflow Cycles | 5** | |||
| Surface | ENEPIG | RoHS-compliant | | Soldering | | Bonding aluminium wire | | Bonding gold wire | | BGA | | Flex-/Rigid-Flex | Pressfit | | PCI contacts | Sliding contact | Thickness | 3-8µm Ni 0.05-0.15µm Pd 0.05-0.1µm Au | Shelf life | ca. 12 months | Reflow Cycles | - | |||
| Surface | Galvanic softgold | RoHS-compliant | | Soldering | | Bonding aluminium wire | Bonding gold wire | | BGA | Flex-/Rigid-Flex | Pressfit | PCI contacts | Sliding contact | Thickness | 4 - 6µm Ni >1µm Au | Shelf life | ca. 12 months | Reflow Cycles | - | ||||||
| Surface | Galvanic hardgold | RoHS-compliant | | Soldering | Bonding aluminium wire | Bonding gold wire | BGA | Flex-/Rigid-Flex | Pressfit | PCI contacts | | Sliding contact | | Thickness | 4 - 6µm Ni 0.8 - 2µm Au | Shelf life | ca. 12 months on request | Reflow Cycles | - |
* IPC-6012: Fully covered and solderable | ** not reliable empirical values
HAL SnPb (lead-tin) is nowadays allowed only in certain technology fields such as medicine and aeronautics. HAL SnPb has been replaced by HAL lead-free. If the surface finish is specified only as HAL or HASL, typically the lead-free version is meant. HAL SnPb is suitable for standard SMD, but shows weaknesses in fine pitch applications. A chemical surface is preferable for finepitch (see also HAL vs. chemical gold).
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HAL (Hot Air Leveling, also known as HASL = Hot Air Solder Leveling) lead-free is a well-proven method to apply tin as circuit board surface. Every assembler should be able to process circuit boards with HAL lead-free surface without any problem. HAL lead-free is suitable for standard SMD, but exhibits some weaknesses for finepitch applications, for which a chemical surface should be preferred (see also HAL vs. chemical gold). Due to popular demand, Multi-CB can offer HAL lead-free in pooling service (cheaper price).
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The surface finish chemical tin (also known as immersion tin) is rarely requested anymore for manufacturing reasons. Due to the comparatively rapid oxidation of the copper, the shelf life is highly dependent on temperature and packaging. Fingerprints on the surface must be avoided.
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Chemical silver (also known as immersion silver) is comparable to the surface finish chemical tin, with better properties concerning oxidation and contact applications. However, also chemical silver is a comparatively damageable surface finish and has therefore a low market share. Chemical silver is particularly suitable for high frequency applications, as the surface allows a low-loss HF transmission because of the skin effect.
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Chemical gold is also known as immersion gold or ENIG (Electroless Nickel Immersion Gold). Just as with chemical tin and chemical silver, chemical gold is an extremely flat circuit board surface, and has very good oxidation properties. It is perfectly suited for finepitch and bonding (aluminium wire) applications. Due to popular demand, Multi-CB can offer chemical gold in pooling service (cheaper price).
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ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold) is extremely planar and has very good oxidation properties. It is ideally suited for soldering applications and aluminum wire bonding / gold wire bonding.
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