MIL-SPEC Gold Plating Standards — MIL-G-45204, B488

When a drawing calls out "Gold plate per MIL-G-45204, Type II, Class 1," every word carries a precise, testable meaning. This guide decodes the three standards that govern engineering gold deposits in the United States — MIL-G-45204, ASTM B488, and AMS 2422 — so engineers, buyers, and quality managers can read, write, and verify a gold plating callout with confidence.

What is MIL-G-45204 and is it still used?

MIL-G-45204 is the U.S. military specification for electrodeposited gold plating, defining purity Types, hardness Codes, and thickness Classes. Although formally inactivated for new design, it remains cited on thousands of active drawings, and ASTM B488 is its designated modern replacement with an equivalent structure.

In practice, platers treat the two as a family: a lab that plates to B488 can satisfy legacy 45204 callouts, and certifications typically reference whichever document the drawing names. SAE AMS 2422 covers gold plating for aerospace applications from the SAE side, with additional process-control emphasis. If your drawing cites any of the three, the questions to answer are the same: what purity, what hardness, what thickness, where measured, and what testing.

How do Type, Code, and Class work in a gold plating callout?

The callout is a three-axis coordinate: Type sets minimum gold purity, Code sets hardness range, and Class sets minimum thickness. All three are independent, and a complete specification states all three.

Type — purity:

Type I: 99.7 percent gold minimum
Type II: 99.0 percent gold minimum (allows hardening alloys)
Type III: 99.9 percent gold minimum

Code — Knoop hardness:

Code A: 90 Knoop maximum (soft)
Code B: 91 to 129 Knoop
Code C: 130 to 200 Knoop (hard gold)
Code D: over 200 Knoop

Class — minimum thickness:

Class 00: 20 microinches — Class 0: 30 — Class 1: 50 — Class 2: 100 — Class 3: 200 — Class 4: 300 — Class 5: 500 — Class 6: 1,500

Some combinations are impossible by chemistry: Type III (99.9 percent pure) can only be Code A, because hardening requires alloy additions that would violate the purity floor. Likewise Code C hardness implies Type II purity. A callout like "Type II, Code C, Class 1" reads: hard gold, at least 99.0 percent pure, 130 to 200 Knoop, minimum 50 microinches.

Which Type and Class should an engineer specify?

Specify by function: Type II Code C for sliding electrical contacts, Type III Code A for wire bonding and semiconductor work, Type I for general high-purity protection, with Class chosen from expected wear and environment. The most common engineering callouts are Type II Class 1 for connectors and Type III Class 1 or higher for bonding surfaces.

Guidance by application:

Separable connectors, contact fingers: Type II, Code C, Class 1 (50 microinches) — the workhorse of aerospace and defense electronics
Gold wire or ribbon bonding: Type III, Code A, Class 1 or 2 — alloy hardeners poison bondability
High-temperature service: Type III soft gold; cobalt-hardened gold degrades above roughly 125 degrees C
Corrosion protection, reflectors: Type I or III, Class set by environment
Medical and implantable hardware: high-purity Type III over nickel, per device master file — see our medical device plating capabilities

Overspecifying costs real money: every additional Class step adds gold mass, so a Class 2 callout where Class 1 suffices roughly doubles the gold cost on the significant surface.

What does an underplate requirement mean in these specs?

Engineering gold standards presume — and drawings usually require — a nickel underplate beneath the gold, typically 50 to 200 microinches, serving as a diffusion barrier and mechanical support. The gold Class thickness is measured above the nickel, never combined with it.

Nickel stops copper and zinc migration from the substrate, hardens the contact stack so thin gold survives mating cycles, and limits corrosion creep at pores. Low-stress sulfamate nickel is standard under MIL-type gold work. Drawings may cite the nickel to its own spec (such as AMS-QQ-N-290 or SAE AMS 2403). The metallurgy is covered in our nickel underplating resource.

How is compliance tested and certified?

Compliance rests on four verifications: thickness by X-ray fluorescence on the significant surface, purity by bath control and periodic assay, adhesion by bend or tape test, and — where specified — porosity testing. A certificate of conformance ties the lot to the standard, Type, Code, and Class.

Key testing details buyers should know:

Significant surface. The spec applies where the drawing says it applies. Thickness on a corner or recess may legitimately differ; measurement points should be agreed before plating.
XRF measurement is non-destructive and fast, making 100 percent or sampled verification practical even on production volume.
Porosity tests (nitric acid vapor, per ASTM B735 and related methods) matter for Class 00 through Class 1 deposits, where thin gold can have pinholes reaching the underplate.
Hardness verification uses Knoop microindentation on plated test coupons, since production parts are rarely sacrificed.

What should you send a plater for a MIL-SPEC job?

Send the drawing callout verbatim, the substrate alloy, the significant surface definition, any masking requirements, and the certification paperwork you need — that is everything a lab requires to quote accurately. Ambiguity on any of the five is where spec jobs go wrong.

If the drawing only says "gold plate," push the designer for Type, Code, and Class, or ask the plating lab to recommend a callout based on function; a good lab will put its recommendation in writing. Thickness questions — including how Class interacts with the FTC-style commercial designations — are unpacked in gold plating thickness.

Our Vista, CA lab plates to MIL-G-45204, ASTM B488, and AMS 2422 callouts with XRF verification on every lot, serving defense, aerospace, and medical customers from San Diego to Los Angeles with fast turnaround and high-volume capacity. Send your drawing for a spec review — request a quote or call (760) 458-3299. $500 minimum; pricing from $100 per square inch.