Kovar Plating Options: The Complete Guide to Finishes, Processes & Applications
If you’ve spent any time in hermetic packaging, aerospace electronics, or high-reliability semiconductor work, you already know that Kovar is a material that demands respect. It machines beautifully when handled correctly, seals to glass and ceramics like nothing else, and holds tolerances through thermal cycling that would destroy lesser alloys.
But here’s the catch: bare Kovar is almost never the right final surface. To make it solderable, wire-bondable, corrosion-resistant, and electrically stable, you need the right plating—applied the right way, on a surface that’s been prepared by someone who understands what they’re working with.
At KELTRYN, we machine Kovar and Fe-Ni-Co alloy components day in and day out. We’ve seen what happens when plating goes wrong, and we’ve seen what happens when it goes right. This guide is the resource I wish existed when I first started specifying finishes for glass-to-metal seal housings and RF packages. Let’s dig into everything you need to know about Kovar plating options—from pre-treatment chemistry to industry specifications, and from common failure modes to practical supplier selection.
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What Is Kovar and Why Does It Need Specialized Plating?
Before we talk about finishes, let’s make sure we’re all speaking the same language about the base material.
Kovar is a controlled-expansion alloy composed primarily of iron, nickel (approximately 29%), and cobalt (approximately 17%). Its defining characteristic is a coefficient of thermal expansion (CTE) that closely matches borosilicate glass and certain alumina ceramics across a broad temperature range—typically from room temperature up to around 400–450°C. This property makes it the default choice for hermetic glass-to-metal seals and ceramic-to-metal assemblies where mismatched expansion would crack or delaminate the joint.
Let me answer a question we hear frequently: What type of metal is Kovar? It’s a nickel-cobalt ferrous alloy, but it is not a superalloy. Superalloys like Inconel or Hastelloy are designed for high-temperature strength and oxidation resistance. Kovar is designed for dimensional stability and sealability. The distinction matters because the plating challenges are entirely different. Kovar’s surface chemistry—specifically the tenacious chromium and nickel oxide layer that forms almost instantly when it’s exposed to air or heat—makes adhesion a genuine engineering problem.
Here’s the core issue: if you take a machined Kovar part and drop it straight into a standard copper or nickel plating bath, the deposit will likely blister, peel, or fail adhesion testing. Why? Because the oxide layer on Kovar is not removed by the mild acid cleaners typically used for steel or brass. You need a deliberate, multi-step pre-treatment sequence to get down to a chemically active surface that will actually bond with the first layer of electrodeposited metal.
This is why Kovar plating is a specialization, not a commodity service. And it’s why the pre-treatment phase deserves as much attention as the plating bath itself.
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Pre-Treatment: The Foundation of a Durable Kovar Plating
I’ll say it plainly: poor adhesion is the number one cause of Kovar plating failure. It’s not the gold bath, it’s not the nickel chemistry, and it’s not the thickness spec. It’s what happened (or didn’t happen) in the five minutes before the part ever saw current.
A proper Kovar pre-treatment cycle typically includes several stages:
Alkaline degreasing removes machining oils, coolants, and handling contamination. This is straightforward but critical—if the surface isn’t clean, nothing downstream works.
Acid pickling follows. For Kovar, this usually involves a hydrochloric or sulfuric acid solution designed to attack the oxide layer. But here’s the nuance: straight pickling can leave a smut or preferentially etch certain alloy constituents. Many experienced platers use a two-step approach with intermediate rinsing.
Deoxidizing is the stage that separates the professionals from the amateurs. Kovar’s chromium oxide is notoriously stubborn. Proprietary deoxidizing solutions or carefully controlled acid mixtures are used to ensure complete removal without over-etching the base metal.
After the surface is clean and oxide-free, the clock is ticking. Bare Kovar will re-oxidize quickly in air, so the next step—activation—needs to happen without delay.
Activation strikes are the safety net that ensures adhesion. The industry standard is a Wood’s nickel strike: a thin, highly adherent layer of nickel deposited from a chloride-based bath at high current density. Some shops use a sulfamate nickel strike instead. The strike does two things: it bonds chemically to the activated Kovar surface, and it provides a clean, uniform substrate for the main plating layer.
One additional pre-treatment consideration: stress relief annealing. If your Kovar part has been heavily machined or cold-worked, residual stress can cause dimensional instability during plating or later thermal cycling. A stress relief anneal (typically around 900°C in a controlled atmosphere) before final machining—or at least before plating—can prevent warping and reduce the risk of hydrogen embrittlement during subsequent electroplating.
The takeaway: if your plating supplier can’t articulate their pre-treatment sequence in detail, that’s a red flag. You want a partner who treats oxide removal as seriously as they treat the gold bath chemistry.
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Common Kovar Plating Finishes — Options Compared
Now we get to the heart of the matter. What can you actually put on Kovar, and how do you choose? Here are the most common Kovar plating options, compared across performance, cost, and application fit.
Nickel Plating (Electrolytic & Electroless)
Nickel is the workhorse of Kovar plating. In most cases, it’s not the final finish—it’s the foundation upon which everything else is built. But it can also serve as a final finish in its own right for certain applications.
Electrolytic nickel is typically deposited from a sulfamate or Watts bath. Sulfamate nickel is preferred for Kovar because it produces lower internal stress, which is critical for parts that will undergo thermal cycling. Typical thicknesses range from 2 to 8 micrometers. Electrolytic nickel provides excellent adhesion when applied over a proper strike, good solderability, and a solid diffusion barrier between the Kovar substrate and any subsequent gold or silver layer.
Electroless nickel-phosphorus is a different beast. Because it’s autocatalytic rather than electrolytic, it deposits uniformly on all surfaces—including internal bores, blind holes, and complex geometries where current distribution would cause thickness variation in an electrolytic bath. The phosphorus content (typically 7–12%) gives electroless nickel superior corrosion resistance compared to electrolytic nickel. The trade-off is that electroless nickel deposits are harder and more brittle, and they contain phosphorus, which can affect solderability and wire bonding if not managed correctly.
So when do you choose which? If you have a simple geometry, need maximum ductility, and are plating over a strike anyway, electrolytic sulfamate nickel is a solid choice. If you have complex internal features, need uniform thickness on all surfaces, or require maximum corrosion protection, electroless nickel is worth the extra cost.
Gold Plating (High-Reliability Finish)
Gold over nickel is the gold standard—pun intended—for Kovar hermetic packages and wire-bondable surfaces. The nickel layer provides corrosion protection and acts as a diffusion barrier, preventing copper or iron from migrating to the surface. The gold layer provides a noble, oxidation-resistant surface that’s ideal for soldering, wire bonding, and low-contact-resistance applications.
Within gold plating, you have two main choices:
Hard gold is alloyed with cobalt or nickel (typically 0.1–0.5%) to increase hardness and wear resistance. It’s specified for contact surfaces, edge connectors, and applications where the part will see repeated mechanical cycling.
Soft gold (pure gold, 99.99%) is used for wire bonding and die attachment. It’s more ductile and produces more reliable thermosonic bonds. For hermetic sealing applications, soft gold over sulfamate nickel is the industry default.
Typical gold thicknesses range from 1 to 5 micrometers. Controlling porosity is critical—porous gold can allow corrosion of the underlying nickel, which leads to adhesion failure or degraded electrical performance. Thicker gold deposits reduce porosity, but they also cost more and can create issues with solder joint embrittlement if the gold content in the solder exceeds about 3–5%.
Silver Plating (High Conductivity at Lower Cost)
Silver offers the highest electrical and thermal conductivity of any platable metal. It’s significantly less expensive than gold, making it attractive for high-current applications, RF shielding, and microwave components.
The catch is tarnishing. Silver reacts with sulfur compounds in the atmosphere to form silver sulfide, which degrades solderability and increases contact resistance. If your application requires long shelf life or operates in a sulfur-rich environment, you’ll need either a protective topcoat (often a thin layer of gold or a chromate conversion) or an anti-tarnish treatment.
Silver plating on Kovar typically uses a strike layer first—often silver or copper—to ensure adhesion. Thicknesses vary widely depending on current-carrying requirements, from 2 micrometers for light-duty applications to 20 micrometers or more for high-current bus bars.
Tin & Tin-Lead Plating (Solderable, Cost-Effective)
For applications where solderability is the primary requirement and the operating environment is benign, tin or tin-lead plating offers a cost-effective alternative to gold or silver.
Matte tin is lead-free and provides excellent solderability. The concern with pure tin is whisker growth—spontaneous, hair-like crystals that can cause electrical shorts. Whisker mitigation strategies include using matte tin rather than bright tin, applying a nickel underlayer, and controlling plating thickness and stress.
Tin-lead (typically 60/40 or 63/37) is still specified in military and aerospace applications where lead-free requirements don’t apply. Tin-lead is more forgiving than pure tin, with reduced whisker risk and better wetting characteristics.
Reflowed tin—where the plated part is heated above the tin melting point—produces a smoother, more solderable surface. This is sometimes specified for Kovar lids and caps in hermetic packages.
Other Finishes (Passivation, Palladium, and Specialty Coatings)
Beyond the big four, several specialty finishes deserve mention:
Passivation of bare Kovar (using citric or nitric acid) provides mild corrosion resistance without adding a metallic layer. This is useful when you need to keep the Kovar surface exposed for subsequent welding or brazing operations.
Palladium and palladium-nickel finishes are gaining traction as gold alternatives in advanced packaging. Palladium offers good wire bondability and corrosion resistance. Multilayer stacks like Ni-Pd-Au are used in some high-reliability applications to reduce gold thickness while maintaining performance.
Rhodium provides exceptional hardness and wear resistance, though it’s expensive and primarily used for specialty contact applications.
Chromate conversion coatings can be applied over silver or tin to improve corrosion resistance. PTFE-impregnated coatings are used for anti-galling applications where Kovar parts thread together or slide against each other.
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Plating Process Options: Rack, Barrel, and Selective Plating
The finish is only half the story. How the plating is applied matters just as much for quality, cost, and consistency.
Rack plating involves fixturing individual parts on conductive racks and immersing them in the plating bath. This is the method of choice for large parts, delicate components, precision-machined housings, and any part where thickness uniformity and surface quality are critical. Rack plating allows for precise control of current density and provides good access for solution agitation. The trade-off is cost—rack plating is labor-intensive and has lower throughput than barrel plating.
Barrel plating places small parts in a rotating basket that tumbles them through the solution. It’s efficient and cost-effective for high volumes of small stamped or machined components. The downside is that parts can touch each other, causing plating defects, and the tumbling action can damage delicate features or thin walls. Barrel plating is generally not suitable for precision Kovar components with tight tolerances or fragile geometries.
Selective or brush plating is used for repair, rework, or plating only specific areas of a part. If you need to plate a seal surface on an otherwise finished Kovar housing, brush plating allows you to apply the deposit locally without masking the entire part. This is also valuable for touch-up work after machining operations.
Masking is essential for Kovar parts that must remain unplated in critical tolerance zones. Common masking materials include platers tape, wax, and specialized lacquers. The challenge with Kovar is that the masking must survive the aggressive pre-treatment chemistry—if your mask lifts during deoxidizing, you get plating where you don’t want it.
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Key Plating Specifications and Industry Standards
If you’re working in aerospace, defense, medical, or other regulated industries, you’ll need to specify plating against recognized standards. Here are the ones we see most frequently:
| Finish | Common Specifications |
|---|---|
| Gold | MIL-DTL-45204, ASTM B488 |
| Nickel (electrolytic) | QQ-N-290, MIL-P-27418, ASTM B689 |
| Nickel (electroless) | MIL-C-26074, ASTM B733 |
| Silver | AMS-QQ-S-365, ASTM B700 |
| Tin | ASTM B545 |
| Tin-Lead | MIL-P-81728, ASTM B579 |
The base material specification for Kovar is ASTM F15 (also referenced as SAE AMS-I-23011). This specification defines the composition, CTE, and mechanical properties of the alloy. When you’re specifying plating on Kovar, both the base material spec and the plating spec need to be called out on your drawing.
Acceptance testing for Kovar plating typically includes:
Adhesion testing via bend tests or heat-quench methods (heating the part and quenching in water to check for blistering or peeling).
Solderability testing to verify that the plated surface wets properly with the specified solder alloy.
Porosity testing using nitric acid vapor exposure, which reveals pores in gold or silver deposits by corroding the underlying nickel.
Thickness verification using X-ray fluorescence (XRF) or beta backscatter techniques. XRF is preferred for Kovar because it can measure multiple layers non-destructively and provides mapping capabilities to check thickness uniformity across the part.
One question we often hear is: “What are the different types of metal plating?” In the context of Kovar, the answer comes down to three process categories. Electrolytic plating uses an external current source to reduce metal ions onto the part. Electroless plating uses a chemical reducing agent in the solution, depositing metal autocatalytically without electricity. Immersion plating is a displacement reaction where the substrate metal dissolves and the plating metal deposits—this is rarely used for Kovar because it produces thin, poorly adherent coatings. Understanding which process your supplier uses matters because it directly affects thickness distribution, adhesion, and cost.
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Benefits of Plating Kovar for Real-World Applications
Why go through all this trouble? Because the benefits of proper Kovar plating are substantial and well-documented. The established advantages of electrodeposited finishes on Kovar include:
Improved solderability. Bare Kovar forms a tenacious oxide that resists wetting. A properly plated surface—whether nickel, gold, or tin—provides a clean, solderable interface for hermetic sealing and component attachment.
Wire bondability. Gold-plated Kovar surfaces produce reliable thermosonic and ultrasonic wire bonds, which are essential for semiconductor and optoelectronic packaging.
Corrosion protection. Kovar is not inherently corrosion-resistant. Plating provides a barrier against environmental attack, extending the service life of components in humid, salty, or chemically aggressive environments.
Wear resistance. Hard gold, nickel, or rhodium finishes protect Kovar surfaces from mechanical wear in contact applications.
Electrical performance. Silver and gold plating reduce contact resistance and improve RF performance in microwave and high-frequency applications.
Enhanced hermeticity. The right plating stack—typically sulfamate nickel followed by soft gold—improves the reliability of glass-to-metal and ceramic-to-metal seals by providing a clean, oxide-free surface for the sealing process.
Here’s a quick application guide:
| Application | Recommended Finish |
|---|---|
| Hermetic glass-to-metal seals | Sulfamate nickel + soft gold (1–3 µm) |
| RF connectors & microwave components | Nickel + silver or gold |
| Power semiconductor packages | Nickel + silver (high current) |
| Medical implant feedthroughs | Nickel + gold (biocompatible, corrosion-resistant) |
| Aerospace lids and caps | Nickel + gold or tin-lead |
| Sensor housings | Electroless nickel (uniform coverage) |
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Common Kovar Plating Problems and How to Avoid Them
Let’s talk about what goes wrong. If you search for “Kovar plating problems,” you’ll find a consistent list of issues: poor adhesion, blistering, peeling, and porosity. Here’s what causes them and how to prevent them:
Poor adhesion is almost always traced back to inadequate oxide removal or a skipped strike layer. If your supplier isn’t using a nickel strike before the main deposit, you’re playing with fire. The fix is rigorous pre-treatment validation—test coupons processed alongside production parts to verify adhesion before committing to a full batch.
Blistering and peeling typically occur during or after thermal processing. If the plating bath chemistry is contaminated, or if current density is too high, the deposit can incorporate impurities or develop excessive internal stress. When the part is later heated for soldering or brazing, that stressed deposit delaminates. Solution: maintain tight bath control, monitor current density, and perform heat-quench adhesion tests on a sampling basis.
Porosity in gold or silver deposits allows corrosion of the underlying nickel, which shows up as dark spots or blisters weeks or months after plating. The fix is thicker deposits, pulse plating to refine grain structure, or the use of a barrier layer like electroless nickel, which is less susceptible to pinhole porosity.
Hydrogen embrittlement is a concern for any high-strength alloy, and while Kovar is less susceptible than martensitic steels, it’s still a risk—especially for thin-wall parts or components under residual stress. Baking after plating (typically 190–230°C for several hours) drives out absorbed hydrogen.
Dimensional changes from plating buildup can push precision features out of tolerance. If your Kovar part has critical sealing surfaces, remember that plating adds material on all exposed surfaces. Design your machined dimensions to account for the final plating thickness, or specify masking for features that must remain at their machined size.
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Kovar vs. Invar vs. Superalloys — Material Comparison for Platers
If you’re evaluating materials for a hermetic or high-reliability application, you’ve likely come across Invar and various superalloys. Here’s how they compare:
Kovar vs. Invar. Invar is an iron-nickel alloy (approximately 36% nickel) with an extremely low CTE at room temperature—even lower than Kovar. However, Invar’s CTE rises sharply above its Curie temperature (around 230°C), making it less suitable for glass-to-metal sealing at high temperatures. Kovar maintains its matched expansion to higher temperatures, which is why it’s preferred for hermetic seals. Magnetically, Kovar is ferromagnetic (like Invar), while some other sealing alloys are not. For plating, Invar presents similar oxide removal challenges to Kovar, though the absence of cobalt changes the surface chemistry slightly.
Kovar vs. Superalloys. Superalloys like Inconel and Hastelloy are designed for high-temperature strength and oxidation resistance. They have much higher CTEs than Kovar and are not suitable for glass-to-metal sealing. Plating superalloys requires different pre-treatment chemistry because their oxide layers are even more tenacious than Kovar’s. If you’re working with Inconel, expect your plater to use more aggressive activation steps.
What is the melting point of Kovar? Kovar melts at approximately 1450°C (2642°F). This matters because it sets the upper limit for brazing and annealing operations. Kovar can be brazed at temperatures up to about 1000–1100°C using silver-copper or gold-copper filler metals, but the base material must not approach its melting point. For plating, the melting point matters less than the CTE—thermal cycling during soldering or brazing will stress the plating, which is why adhesion and ductility are so important.
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How to Choose a Kovar Plating Supplier
Not all plating shops are created equal, and Kovar is not a material you want to trust to a generalist. Here’s what to look for:
Accreditations. Nadcap accreditation for chemical processing is a strong signal that a supplier takes quality seriously. Look for certifications to ISO 9001 or AS9100 as well.
In-house analytical lab. A plating shop that can perform bath analysis, thickness verification (XRF), and adhesion testing in-house has the process control necessary for consistent results. If they outsource all their testing, you have no visibility into their quality.
Experience with your specifications. Ask whether they’ve worked with MIL-DTL-45204, QQ-N-290, AMS-QQ-S-365, and ASTM B545. If they look at you blankly, move on.
Questions to ask:
- What strike system do you use for Kovar?
- What is your typical adhesion failure rate?
- Can you provide XRF thickness mapping on complex geometries?
- How do you handle masking for critical tolerance zones?
- What is your turnaround time for small-batch, high-mix runs?
Red flags: No process documentation. No bath control records. Inability to handle small batches. Reluctance to discuss their pre-treatment chemistry. Unwillingness to provide test coupons or certification documentation.
One advantage we offer at KELTRYN is the ability to machine and coordinate plating under one roof—or at least with a tightly managed supply chain. When your machined Kovar parts go to a plater that understands the material, you avoid the classic failure mode where the machinist blames the plater and the plater blames the machinist. We’ve built relationships with qualified plating partners who understand ASTM F15, and we can manage the entire flow from raw bar stock to plated, ready-to-seal components.
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Frequently Asked Questions (FAQ)
What type of metal is Kovar?
Kovar is a nickel-cobalt ferrous alloy (approximately 29% nickel, 17% cobalt, balance iron) designed to match the thermal expansion of borosilicate glass and alumina ceramics. It’s classified as a controlled-expansion alloy, not a superalloy.
What are the key differences between Kovar and Invar?
Invar has a lower CTE at room temperature but loses its low-expansion properties above approximately 230°C. Kovar maintains matched expansion to higher temperatures, making it suitable for glass-to-metal sealing. Kovar also contains cobalt, which affects its magnetic and surface chemistry properties.
What is the melting point of Kovar?
Kovar melts at approximately 1450°C (2642°F). This high melting point allows brazing and annealing operations at temperatures up to about 1000–1100°C without compromising the base material.
Can Kovar be plated without a nickel strike?
Technically yes, but it’s risky. Without a strike layer, the adhesion between the Kovar substrate and the main plating deposit is unpredictable. The strike provides a chemically bonded transition layer that prevents blistering and peeling. Skipping the strike is a false economy.
What is the best plating for Kovar hermetic seals?
The industry standard is sulfamate nickel (2–5 µm) followed by soft gold (1–3 µm). This stack provides excellent adhesion, corrosion protection, solderability, and wire bondability. For less demanding applications, electroless nickel or even tin-lead can be used.
How thick should Kovar plating be?
Typical thickness ranges: nickel 2–8 µm, gold 1–5 µm, silver 2–20 µm, tin/tin-lead 3–10 µm. The exact thickness depends on your application requirements, including solderability, corrosion resistance, and electrical performance. Your plating supplier should recommend a range based on your specifications.
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Conclusion and Next Steps
Choosing the right Kovar plating options isn’t just about picking a finish from a catalog. It’s about understanding your application’s requirements—solderability, wire bondability, corrosion resistance, electrical performance, and cost—and matching those requirements to the right plating stack, applied by a supplier who understands Kovar’s unique challenges.
The key decision factors are:
- End-use environment: Will the part see temperature extremes, humidity, or corrosive chemicals?
- Solderability and bonding requirements: What joining processes will be used?
- Cost constraints: Gold is expensive; silver and tin are more economical.
- Specification compliance: What military, aerospace, or medical standards apply?
My strongest piece of advice: engage with your plating supplier early in the design phase. If you’re designing a Kovar housing or lid, talk to your plater before you finalize your machined dimensions. Plating thickness affects your tolerances, and masking requirements affect your design. A little forethought saves a lot of rework.
At KELTRYN, we specialize in precision-machined Kovar components—housings, lids, caps, sealing rings, flanges, and frames for hermetic and high-reliability applications. We understand how plating interacts with machined features, and we work closely with qualified plating partners to ensure your parts perform exactly as designed.
If you’re working on a Kovar project and need a machining partner who understands the full picture—from raw material to final plated finish—we’d love to help. Contact our applications engineering team for a quote, or send us your drawings and let us provide manufacturability feedback before you commit to a plating specification.
Ready to get started? Reach out to KELTRYN today and let’s make your Kovar components a success—from first article to full production.