Types of Gold Plating — Hard, Soft, Karat, Methods
"Gold plating" is not one product. Deposits differ in purity, hardness, thickness, color, and deposition method, and choosing the wrong type is the most common reason plated parts underperform. This guide lays out the complete taxonomy so you can specify the right deposit the first time.
What are the main types of gold plating?
Gold plating divides along four independent axes: purity (karat or Type), hardness (soft vs hard), thickness (flash to heavy build), and method (electrolytic, electroless, immersion). A full specification names all four — for example, "Type II hard gold, 50 microinches, electrolytic, over nickel."
Because the axes are independent, "24K" tells you purity but not hardness or thickness, and "hard gold" tells you hardness but not how thick. Treating any single label as a complete spec invites mismatched expectations between buyer and plater.
What is the difference between hard gold and soft gold?
Soft gold is high-purity gold (99.9 percent or better) with a Knoop hardness of 90 or less; hard gold is alloyed with trace cobalt, nickel, or iron to reach 130 to 200 Knoop. The alloying addition is under one percent by weight, but it transforms wear behavior.
- Soft gold excels where purity matters more than wear: gold wire bonding, die attach, high-temperature service, medical and semiconductor applications. It solders beautifully and stays ductile.
- Hard gold is the standard for anything that slides or mates repeatedly: connector contacts, edge fingers, test points, and switch surfaces. A hard gold contact can survive hundreds to thousands of mating cycles that would wipe through soft gold quickly.
ASTM B488 captures this with hardness codes: Code A is 90 Knoop maximum, Code B is 91 to 129, Code C is 130 to 200, and Code D is 201 and above.
How do Type I, Type II, and Type III gold classifications work?
Under MIL-G-45204 and ASTM B488, Type refers to minimum gold purity: Type I is 99.7 percent, Type II is 99.0 percent, and Type III is 99.9 percent. Type is about chemistry, not thickness — thickness is specified separately by Class.
The pairings follow function. Type I covers general-purpose high-purity deposits; Type II permits the alloying additions that create hard gold, so most connector gold is Type II Code C; Type III is the highest-purity grade demanded by semiconductor bonding and certain space and medical programs. Our MIL-SPEC gold plating standards guide covers the full Type, Code, and Class matrix.
What do 24K, 18K, and 14K mean in plating?
Karat expresses gold content in twenty-fourths: 24K is essentially pure gold, 18K is 75 percent gold, and 14K is about 58 percent. In plating, karat labels are used mainly for decorative deposits where color is the point.
- 24K gold plating gives the rich, warm yellow of pure gold and maximum corrosion resistance and biocompatibility — the default for jewelry restoration, awards, religious items, and electronics.
- 18K gold plating is alloyed for a slightly paler, more contemporary color and a harder surface that resists scratching in handled items such as watch cases and hardware.
- Lower-karat and rose or white gold deposits alloy in copper or other metals to shift color further.
Note that a decorative "18K" bath and an engineering "hard gold" bath are different tools for different jobs, even though both are alloyed golds.
How do flash plating and heavy gold plating differ?
Flash (or wash) plating is an extremely thin decorative deposit, typically 1 to 7 microinches; heavy gold plating builds 100 microinches or more for wear or engineering service. Between them sits the standard commercial range of roughly 20 to 50 microinches.
Thickness drives both durability and cost more than any other variable. A flash deposit can look identical to a heavy deposit on day one and wear through in weeks. U.S. FTC guidance for jewelry marking reflects this: an item described as "gold plated" should carry at least 0.5 micron (about 20 microinches) of gold, while "heavy gold plated" requires 2.5 microns (about 100 microinches). The full breakdown lives in our gold plating thickness guide.
What deposition methods are used for gold?
Three methods dominate: electrolytic plating (rectifier-driven, the industrial workhorse), electroless plating (autocatalytic chemical deposition), and immersion gold (a self-limiting displacement reaction). Each has a distinct role.
| Method | Typical thickness | Strengths | Limits | | --- | --- | --- | --- | | Electrolytic | 5 to 1,500 microinches | Fast, economical, any thickness | Requires electrical contact; thickness varies with geometry | | Electroless | 4 to 40 microinches | Uniform on complex shapes | Slower, costlier chemistry | | Immersion | 2 to 8 microinches | No current needed; basis of ENIG | Self-limiting, thin only |
Vapor-phase alternatives such as PVD produce gold-colored ceramic coatings rather than true gold layers in most cases; see electroplating vs PVD for that comparison.
Which type of gold plating should you choose?
Match the deposit to the dominant failure mode of your application: wear means hard gold, bonding or biocompatibility means soft high-purity gold, appearance means karat-matched decorative gold at adequate thickness. When in doubt, specify by standard (ASTM B488 Type, Code, and Class) rather than by marketing terms.
A quick selection guide:
- Connector contacts and sliding surfaces: Type II hard gold, 30 to 50 microinches over nickel
- Wire bonding, semiconductors: Type III soft gold, 50 microinches or more
- Jewelry and decorative restoration: 24K or 18K, 20 to 40 microinches
- Marine, medical, or corrosive service: high-purity gold at 50 microinches or more over a robust nickel barrier
Still not sure which deposit fits your parts? Send photos to our Vista, CA lab and we will recommend the type, thickness, and underplate — then quote it flat at $100 per square inch with a $500 minimum. Get your free quote or call (760) 458-3299.