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.
Available surfaces
| Surface | RoHS-compliant | Soldering | Bonding alu-wire | Bonding gold-wire | BGAs | Flex-/ Rigid-Flex | Press-fit | Gold fingers | Sliding contacts | Thickness | Shelf life | Reflow Cycles |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Hal SnPb |  | | >5µm | ca.12 months | 5** | |||||||
| HAL lead-free | | | | acc. to IPC-6012* | ca. 12 months | 5** | ||||||
| Chemical tin | | | | | >1µm | ca. 6 months | 5** | |||||
| Chemical silver | | | | | | 0.12 - 0.3µm | ca. 6 months | 5** | ||||
| Chemical gold ENIG | | | | | | | 3-6µm Ni 0.05-0.1µm Au | ca. 12 months | 5** | |||
| ENEPIG | | | | | | | 3-8µm Ni 0.05-0.15µm Pd 0.05-0.1µm Au | ca. 12 months | - | |||
| Galvanic softgold | | | | 4 - 6µm Ni >1µm Au | ca. 12 months | - | ||||||
| Galvanic hardgold | | | | 4 - 6µm Ni 0.8 - 2µm Au | ca. 12 months on request | - |
HAL SnPb
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 HAL SnPb has been replaced by HAL lead-free.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).
Advantages:
- Good solderability
- Good reflow properties
Disadvantages:
- Pads are non-planar
- Not RoHS-compliant
HAL lead-free
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).
Advantages:
- Good solderability
- Good reflow properties
- Available in Pool-Service
Disadvantages:
- Pads are non-planar
Chemical tin
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.
Advantages:
- Absolutely flat
- Good for fine pitch
- Good for SMD
Disadvantages:
- Extra charge
- Critical manufacturing process, handle with gloves
- Faster oxidation, thus:
- Limited storability
- Better: use chemical gold
Chemical silver
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.
Advantages:
- Particularly suitable for high frequency applications (low-loss HF transmission see skin effect)
- Absolutely flat
- Good for fine pitch
- Good for SMD
- Better shelf life than chemical tin
Disadvantages:
- Extra charge
- Comparatively damageable surface finish, use gloves
- Limited storability
- Surface tarnishes over time and there can be creep corrosion
- Solder mask defined pads should be avoided
- Better: use chemical gold
Chemical gold (ENIG)
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).
Advantages:
- Absolutely flat
- Very good for fine pitch
- Very good for SMD
- Very good wetting characteristics
- Good storability
- Available in Pool-Service
Disadvantages:
- Possible extra charge
ENEPIG
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.
Advantages:
- Absolutely flat
- Very good for alu-wire-bonding
- Very good for gold-wire-bonding
- Very good for fine pitch, SMD
- Very good wetting characteristics
- Good storability
Disadvantages:
- Extra charge