MIL-G-45204 Gold Plating: Types, Grades, Classes, and Callouts

Under MIL-G-45204 (now formally MIL-DTL-45204D), gold plating is specified along three independent axes: type for minimum gold purity, grade for hardness, and class for minimum thickness. A complete callout names one of each, and the three together tell a plating shop exactly what deposit the part requires.¹

Types: Minimum Gold Purity

Type is the purity axis. The specification defines three:²

Type I — 99.7% minimum gold
Type II — 99.0% minimum gold
Type III — 99.9% minimum gold

Type I is the workhorse. It covers most general-purpose applications where good conductivity and ductility matter. Type II allows up to 1% alloying elements, typically nickel or cobalt, which makes the deposit harder and more wear-resistant. That tradeoff suits sliding contacts and connectors that endure repeated insertion cycles. Type III is the highest-purity option, reserved for semiconductor die bonding, wire bonding, and applications where even trace impurities can degrade performance.

Note the numbering. Type III is purer than Type I, not less pure. The types are not ordered by purity, so reading them intuitively will mislead you.

Grades: Hardness on the Knoop Scale

Hardness is measured on the Knoop scale at a 25-gram load (HK25) and split into four bands:²

Grade A — 90 HK25 maximum (soft gold)
Grade B — 91 to 129 HK25
Grade C — 130 to 200 HK25
Grade D — 201 HK25 and above

Grade A is pure soft gold. It wire-bonds and solders easily but wears quickly under mechanical contact. Hardness increases from B through D through controlled additions of alloying elements. Grade D gives the most abrasion resistance and is designed for parts that endure frequent mechanical engagement, such as heavy-duty switching contacts. Grade windows are enforced at quality verification, so a part that measures outside its specified band will be rejected regardless of how well it performs on other tests.

Which Type and Grade Combinations Are Allowed

Not every type pairs with every grade. The specification limits combinations to what is metallurgically possible:²

Type I (99.7% gold) — Grades A, B, or C
Type II (99.0% gold) — Grades B, C, or D
Type III (99.9% gold) — Grade A only

The logic is straightforward. Reaching Grade D hardness requires enough alloying content that you cannot hold 99.7% or higher purity, which rules out Types I and III. Type III’s 99.9% minimum leaves no room for enough hardening elements, so its deposit will always be soft, which means Grade A. This is where specification errors show up most often on drawings. A callout like “Type III, Grade C” is impossible, and catching that before the part reaches the plating shop saves weeks of delay.

Classes: Minimum Thickness

Class sets the minimum deposit depth. There are eight, running from very thin flash to heavy overlays:²

Class 00 — 20 microinches (0.5 µm)
Class 0 — 30 microinches (0.8 µm)
Class 1 — 50 microinches (1.3 µm)
Class 2 — 100 microinches (2.5 µm)
Class 3 — 200 microinches (5.1 µm)
Class 4 — 300 microinches (7.6 µm)
Class 5 — 500 microinches (12.7 µm)
Class 6 — 1,500 microinches (38.1 µm)

The thinner classes, 00 and 0, cover low-wear applications or act as flash coatings for solderability. Mid-range Classes 1 through 3 handle most connector and contact work in defense electronics. Classes 4 through 6 apply where long service life in harsh environments justifies the higher gold cost. Class 6 at 1,500 microinches is uncommon; parts calling for it are usually headed somewhere extreme, like deep-space or high-temperature engine environments.

Thickness is verified by measurement, most commonly X-ray fluorescence, which resolves to roughly one microinch and requires calibration standards matched to the plating-and-substrate combination. Beta backscatter is an alternative. The measured value must meet or exceed the minimum for the specified class.

Writing a Complete Callout

A callout has to name all three axes. “MIL-DTL-45204D, Type II, Grade C, Class 2” specifies a 99.0% gold deposit, hardened to between 130 and 200 HK25, at a minimum of 100 microinches thick. Any of the three missing, and the shop has to come back with questions. Any of the three inconsistent with the type-grade table above, and the part cannot be plated as drawn.

Because Type III locks to Grade A, some drawings compress that to “Type III” alone; the shop will read it as Grade A regardless. That shortcut still needs a class.

How MIL-DTL-45204 Compares to ASTM B488 and AMS 2422

Three specifications dominate aerospace and defense gold plating: MIL-DTL-45204D, ASTM B488, and AMS 2422. Understanding how they relate matters because a part built to one is not automatically compliant with another.

ASTM B488 currently aligns its type and grade numbering with MIL-DTL-45204. Type I matches Type I (99.7%), Type II matches Type II (99.0%), and Type III matches Type III (99.9%), and Grades A through D correspond directly. That alignment is recent. Older revisions of ASTM B488 used a different numbering where Type 1 was the highest purity, which is now Type III. The current edition includes a cross-reference table showing old and new designations to prevent errors when working from legacy drawings.³

AMS 2422 is the most permissive of the three. It requires only 99.0% minimum gold purity across the board. A part plated to AMS 2422 could comply with that specification while failing the purity requirements of MIL-DTL-45204 Type I or Type III. Substituting one specification for another without engineering approval will get parts rejected.

MIL-G-45204C vs. MIL-DTL-45204D

Drawings and purchase orders still reference the old MIL-G-45204 name, and often the C revision specifically. MIL-G-45204C was published in June 1983 and was the working version for over a decade.⁴ In 1998, the Department of Defense canceled it and directed contractors to use AMS 2422 or ASTM B488 instead. The commercial standards did not perfectly align with the military type and grade system, and the mismatch caused enough confusion on the shop floor that the specification was reinstated in June 2007 as MIL-DTL-45204D.⁵

The “DTL” prefix (detail specification) replaced the older “G” prefix. Many engineers and plating shops still call it MIL-G-45204 out of habit. The D revision updated sampling from MIL-STD-105 to MIL-STD-1916 and revised restrictions on strikes and underplating. A drawing that still references MIL-G-45204C is generally read as calling for the current D revision, but updating the callout on any new design removes the ambiguity.

1
ASSIST-QuickSearch. MIL-DTL-45204 Gold Plating, Electrodeposited
2
Advanced Plating Technologies. MIL-DTL-45204D Gold Plating, Electrodeposited
3
ASTM International. ASTM B488 Electrodeposited Coatings of Gold for Engineering Uses
4
EverySpec. MIL-G-45204C Gold Plating, Electrodeposited
5
EverySpec. MIL-DTL-45204D Gold Plating, Electrodeposited