A Guide to Kovar Plating and Finishing

Kovar is the go-to alloy when you need a metal that expands at almost the same rate as glass in electronic packages. I’ve helped dozens of aerospace and medical device engineers get perfect plating results on Kovar parts — and avoid expensive seal failures.

This guide gives you everything I’ve learned in 15 years: why plating is mandatory, which finishes actually work, and how we machine and plate Kovar daily at Standard Machining.

Understanding Kovar: Properties and Key Characteristics

The Thermal Expansion Advantage

Kovar (ASTM F15) has a coefficient of thermal expansion (CTE) of 5.0–5.5 × 10⁻⁶/°C from room temperature to 500 °C — nearly identical to borosilicate glass (Pyrex). This match creates hermetic glass-to-metal seals that survive thousands of thermal cycles without cracking.

Chemical Composition of Kovar Alloy

• 29% Nickel
• 17% Cobalt
• 53% Iron
• <0.1% Carbon and trace elements

The high nickel and cobalt content gives the low CTE, but it also makes the alloy prone to oxidation.

Why Is Surface Plating Necessary for Kovar?

Enhancing Corrosion Resistance

Bare Kovar forms iron oxide quickly in humid environments. A 5–10 µm nickel layer raises corrosion resistance by 100× and prevents red rust stains inside vacuum packages.

Improving Solderability and Brazing

Raw Kovar does not wet well with common solders. Gold or heavy-electroless nickel (99.9% Ni, 0.1% P) provides a solderable surface that stays active for 12+ months.

Critical Surface Preparation Steps for Kovar

Cleaning and Degreasing Methods

1. Vapor degrease or alkaline soak at 60 °C
2. Ultrasonic clean in neutral detergent
3. DI water cascade rinse

Any oil film left behind will cause plating adhesion failure.

Acid Pickling and Activation Techniques

Standard sequence we use:

• 50% HCl pickle 1–2 min → removes oxide scale
• Wood’s nickel strike (high chloride bath) 2–4 min → 0.5–1 µm strike layer This strike is the single most important step for zero peel-off.

Top Kovar Plating Options and Materials

Electroless and Electrolytic Nickel Plating

• Electroless nickel (EN): 5–25 µm, uniform thickness even in deep holes
• Sulfamate electrolytic nickel: harder (450–550 HV), better for wear parts

We default to high-phosphorus EN (10–12% P) for hermetic packages.

Gold Plating for High Conductivity

• 0.5–2 µm soft gold over 5 µm EN → MIL-G-45204 Type III
• 1–3 µm hard gold for connector pins

Gold thickness > 3 µm risks cohesive failure in thermal cycling.

Silver and Copper Finishing Options

• Silver: excellent RF conductivity, but migrates → only used in non-hermetic microwave packages
• Copper: sometimes used as under-layer to reduce gold cost, but requires immediate gold flash to prevent oxidation

Overcoming Common Kovar Plating Challenges

Dealing with Adhesion Issues

99% of peel failures come from poor activation. Always use fresh Wood’s strike and keep current density 5–10 A/dm².

Managing Porosity and Blistering

Hydrogen embrittlement is real on Kovar. We bake all plated parts 190 °C for 4 hours within 1 hour of plating to drive out hydrogen.

Key Industry Applications for Plated Kovar Parts

• Aerospace: radar feed-throughs
• Medical implant battery headers
• Semiconductor laser diode housings
• Vacuum electron tubes and klystrons

Every hermetic package you see with glass windows almost certainly has plated Kovar inside.

Standard Machining’s Precision Kovar Services

We stock Kovar bar and plate, machine to ±0.0005 in tolerances, and plate in-house (electroless nickel + gold) with full lot traceability and helium leak certification down to 1×10⁻⁹ atm-cc/s. Most customers save 25–40% versus sending parts to three different vendors.

Conclusion

Kovar is unbeatable for glass-to-metal seals, but only if the plating is perfect. Skip the Wood’s strike or hydrogen bake and your package will leak in the field.

Send me your Kovar drawing today — I’ll review it for free and quote machining + plating in 24 hours.

FAQs: Kovar Plating and Machining

Q1: Can Kovar be machined like stainless steel?

Yes, but use sharp carbide, low speeds (80–120 SFM), and plenty of coolant. Kovar work-hardens quickly.

Q2: How thick should electroless nickel be on Kovar?

5–8 µm minimum for hermetic seals; 10–15 µm if the part will see salt spray.

Q3: Does gold plating thickness affect hermeticity?

Gold itself is porous. The nickel under-layer provides the barrier. Gold is only for solderability and oxidation protection.

Q4: Why do some shops refuse to plate Kovar?

Because adhesion failures give plating shops a bad reputation. Proper Wood’s strike and baking eliminate the risk.

Q5: How fast can you deliver plated Kovar parts?

Machined blanks in 5–7 days, plating adds 3–5 days. Expedite available.