In printed circuit board technology, different gold surfaces are used for different applications. Here one distinguishes mainly between:

chemical nickel / gold surface
Application: Aluminum wire bonding and / or multiple soldering with flat pad surface.

Reductive gold surface
Application: Gold wire bonding (single case application)

galvanic nickel / hard gold surface
Application: mechanical contact (wear resistance) with simultaneous solderability.

galvanic nickel / bondgold surface
Application: Gold wire bonding (main application)

In the following list, the gold surfaces mentioned above are to be assigned to the typical fields of application. More detailed information about advantages, properties, etc. can be found in the respective subtopics. Of course, an overlap of application areas can not be ruled out and is required for special applications.

Advantages:
— excellent soldering and bonding properties (aluminum wire bonding)
— flat pad surface
— uniform layer thicknesses
— optimum edge protection

Characteristics:
Nickel:
Nickel / phosphorus with 6 – 9% phosphorus
Hardness: 550 HV = 48 Rockwell
Density: 7.9 g / cm²
Melting point: 890 ° C
Specific resistance: 75 μΩ / cm
Thermal expansion coefficient: 13.8 μm / ° C
Corrosion resistance: approx. 250 h according to ASTM B117
suitable according to MIL-C-26074 E and AM52404A

Gold:
Fine gold: 99.99%
Hardness 60-80 HV
suitable according to MIL-G-45204 B Amendment 2, type III
Chemically nickel / palladium / gold

Advantages:
— excellent soldering and bonding properties (gold wire bonding)
— flat pad surface
— uniform layer thicknesses
— optimum edge protection

The chemical bond gold surface is used as the galvanic process (see also galvanic bond gold) for gold wire bonding. Here, the fine gold layer is deposited in the chemical thick-film late gold process. The layer thickness is significantly lower compared to gold wire bonding in the galvanic process. A process security of the bonding process is a prerequisite here. It is advantageous that a selective application of the gold layer is possible by the chemical method.

Characteristics:
Special suitability for HF printed circuit boards

Nickel:
Nickel / phosphorus with 7 – 10% phosphorus
Hardness: 550 HV = 48 Rockwell
Density: 7.9 g / cm²

Gold:
Fine gold: 99.99%
Hardness 50-60 HV

The galvanic hard gold surface is used mainly where a high wear resistance and in additional good electrical properties with good corrosion resistance are required (sliding contacts / connector strips / test points for frequent contacting / etc.). Here, the hard gold layer of a gold / cobalt alloy with about 0.2% Co has proven. As a diffusion barrier between copper and gold, nickel is used in a layer thickness of > 3 microns.

Characteristics:
Nickel:
Chloride-free nickel sulfamate
low internal tension
high ductility
good soldering properties

Gold:
Gold / cobalt with 0.2% Co
Hardness 160-180 HV
Contact resistance 0.3 milliohms
high corrosion resistance
excellent abrasion resistance
low internal tension
specified according to MIL spec. G-45204 B Amendment 2 Type 1
Grade C-DTD 938

The galvanic bond gold surface is considered the standard surface for gold wire bonding. Here, the fine gold layer is deposited by electroplating. The layer thickness is significantly higher than the gold wire bonding with Dickschichtsudgold (see also reductive gold). This brings a simplification of the bonding process. The galvanic process, however, makes selective application difficult under certain circumstances, since an electrical connection is necessary. An an alternative to the selective method, a complete gold plating of the PCB is possible. Here, however, the cost factor has to be considered.

Characteristics:
Nickel:
Chloride-free nickel sulfamate
low internal tension
high ductility
good soldering properties

Gold:
Fine gold: 99.99%
Hardness 60-80 HV
suitable according to MIL-G-45204 B Amendment 2, type III

Advantages:
— excellent soldering properties or Mehrfachlötbarkeit
— flat pad surface (comparable to chem. Ni / Au)
— uniform layer thicknesses
— optimum edge protection
— low cost in the series

Disadvantages:
— sensitive surface
— Lower storage life compared to HAL surface

Layout requirements:
To ensure that a clean, residue-free finish is created, you must
everything at chem. Sn surface all vias are opened on both sides.

The process based on the copper imidazole chemistry separates controlled from a plane, multi-solderable 0.2 to 0.5 micron thick organometallic copper layer. This is transparent and makes the copper look dull. The ENTEK layer is selectively deposited on the copper.

Advantages:
— flat surface
— good soldering properties or Mehrfachlötbarkeit
— no thermal stress during coating
— low cost in the series

Disadvantage:
— sensitive surface

At ELEKONTA Marek the photoimageable solder resist Peters Elpemer GL2467 SM is used. This is applied by curtain coating.

Characteristics:
— High resolution of fine details
— High resistance to chemical nickel / gold and tin processes
— Non-flammability level according to UL: UL 94 V-0
— good conductor edge coverage