The circuit board surface is responsible for raising the wetting ability of the contact areas for soldering; non-chemical surfaces serve simultaneously as protection against oxidation of the covered copper and improve shelf life characteristics.
The circuit board surfaces vary in shelf life, typical application field and price, but are normally specified by the process, product or desired location for the application.
Available surfaces
HAL lead-free
HAL (Hot Air Levelling) is a well-proven method to apply tin as circuit board surface. Every assembler should be able to process circuit boards with HAL surfaces without any problem. HAL is suitable for standard SMD, but exhibits some weaknesses for fine-pitch applications, for which a chemical surface should be preferred (see also HAL vs. immersion gold).
Advantages:
■Good solderability
■Good reflow properties
Disadvantages:
■pads are non-planar
Immersion tin
This is a very popular circuit board surface, which competent assembly companies have no trouble in processing, because its properties are very close to those of HAL. Due to the quick oxidation of the copper, the storability depends on temperature and packaging. It is imperative to avoid fingerprints on the surface.
Advantages:
■Absolutely flat
■Good for fine pitch
■Good for SMD
■Good for press-fit
Disadvantages:
■Extra charge
■Critical manufacturing process
■Faster oxidation, thus limited storability
Immersion silver
This circuit board surface is very similar to immersion tin, with better properties for oxidation and contact applications.
Advantages:
■Absolutely flat
■Good for fine pitch
■Good for SMD
■Good for press-fit
■Better storability than immersion tin
Disadvantages:
■Extra charge
Immersion gold
Just as with immersion tin and immersion silver, immersion gold is an extremely flat circuit board surface, and has very good oxidation properties. It is perfectly suited for fine pitch and bonding (aluminium wire) applications.
Advantages:
■Absolutely flat
■Very good for fine pitch
■Very good for SMD
■Very good wetting characteristics
■Good for press-fit
■Good storability
Disadvantages:
■Extra charge
ENEPIG
ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold) is an extremely flat circuit board surface, and has very good oxidation properties. It is perfectly suited for soldering applications and aluminium-wire- / 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
