Chemical Properties:

Symbol: Au - from the Latin word Aurum (shining dawn)

Atomic Number:  79

Atomic weight:  196.967

Melting Point:  1063.0°C (1945.4°F)

Boiling Point: 2966.0° (5370.8°F)

Specific Gravity:  19.32

Guidelines:

Gold is usually specified as soft (pure) or hard gold plate. Using the MIL spec terminology, soft gold would be Type III, Grade A, while hard gold would be Type I or II, Grade C. Grades B and D are seldom used.

Hard gold (Type II, Grade C) is useful in electronic applications where wear (mating and actuation) is required. Hardness is achieved by alloying the gold with approximately 0.1-0.2% nickel or cobalt. A small amount of carbon (as an organic compound) is also co-deposited that contributes to hard gold’s good wear characteristics. Due to the co-deposited materials, hard gold will discolor when heated to about 400ºF or higher.

Solderability of hard gold is very good although the gold thickness should be limited to 50 microinches (1.25 µm).

Soft (pure) gold (Type III, Grade A) is useful in electronic applications where bondability or heat resistance is required. Soft gold is used for wire bonding in semiconductor applications and will retain its color in excess of 1000ºF. Due to its purity and softness, soft gold galls and should not be used in wear applications.

Solderability of soft gold is excellent although the gold thickness should be limited to 100 microinches (2.5 µm) due to the possible formation of brittle gold-tin intermetallics.

The standard thickness callout for connector applications is 50-microinch (Class 1) thickness, where thousands of mating or actuation cycles are required. High reliability applications may require 100 microinch thickness (Class 2). Thicknesses greater than 100 microinches minimum are usually required for very specialized applications.

Common Specifications:

MIL-G-45204

Types: (Purity)

Type I              99.7 percent minimum gold

Type II             99.0 percent minimum gold

Type III            99.9 percent minimum gold

Grades: (Hardness)

Grade A*           Knoop hardness 90 max

Grade B            Knoop hardness 91-129 inclusive

Grade C*           Knoop hardness 130-200 inclusive

Grade D            Knoop hardness 201 and over

(*Please note: We offer Grade A and Grade C only)

                        Purity (types) and Hardness (grade) relationship

                        Purity               Hardness

                        Type I              A*, B, or C*

                        Type II             B, C*, or D

                        Type III            A* only

Classes: (Thickness)

Class 00            0.00002 inch thick minimum (20 microinches / 0.5 µm)

Class 0             0.00003 inch thick minimum (30 microinches / 0.75 µm)

Class 1             0.00005 inch thick minimum (50 microinches / 1.25 µm)

Class 2             0.00010 inch thick minimum (100 microinches / 2.5 µm)

Class 3             0.00020 inch thick minimum (200 microinches / 5.0 µm)

Class 4             0.00030 inch thick minimum (300 microinches / 7.5 µm)

Class 5             0.00050 inch thick minimum (500 microinches / 12.5 µm)

Class 6             0.00150 inch thick minimum (1500 microinches / 37.5 µm)

ASTM B488

Purity and Hardness designations are the same as the MIL-G-45204.

Class: (Thickness)

0.25                 0.25 µm (0.000010 inch)

0.50                 0.50 µm (0.000020 inch)

0.75                 0.75 µm (0.000030 inch)

1.0                   1.0 µm (0.000040 inch)

1.25                 1.25 µm (0.000050 inch)

2.5                   2.5 µm (0.000100 inch)

5.0                                      5.0 µm (0.000200 inch)

AMS 2422

Unless otherwise specified on the print or purchase order:

 Gold Plate:    Not less than 0.00005 inch (50 microinches / 1.27 µm) over

 Nickel Strike:    Not less than 0.0001 inch (100 microinches / 2.5 µm) over

 Copper Flash or Strike:   Not less than 0.0001 inch (100 microinches / 2.5 µm) 

The minimum gold purity is 99.0% (hard gold)

Note:

Government and/or industry specification(s) should be called out when required by the blue print or per customer requirement. If a specification callout is used as a type of shorthand, then qualify the callout to say that only hardness/thickness certification (as appropriate) is required. A government/industry specification callout invokes additional testing that may not be required and will increase the cost of plating.

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