Nickel Plating on Kovar: Process, Benefits, and Best Practices

If you work with hermetic packaging, glass-to-metal seals, or high-reliability electronic components, you already know Kovar. What you might not know is how much the success of your final product depends on getting the surface finish right. Nickel plating on Kovar is one of the most common—and most misunderstood—processes in our industry.

At KELTRYN, we machine Kovar components daily: housings, lids, sealing rings, flanges, and frames destined for aerospace, medical, semiconductor, and photonics applications. Over the years, we've seen what happens when plating goes wrong—and what happens when it goes right. This article walks through the entire nickel plating process on Kovar, the challenges you'll face, and the best practices that separate reliable components from field failures.

What Is Kovar and Why Plate It with Nickel?

Kovar is a nickel-iron-cobalt alloy engineered for one remarkable property: it expands and contracts at nearly the same rate as borosilicate glass and certain ceramics when heated. This thermal expansion match—around 5 ppm/°C from room temperature to 400°C—makes Kovar the go-to material for creating hermetic glass-to-metal seals that survive thermal cycling without cracking or leaking.

You'll find Kovar in transistor housings, diode packages, feedthroughs, connectors, vacuum tubes, and sensor housings across aerospace, defense, medical, and scientific instrumentation. It's the backbone of hermetic packaging.

But bare Kovar has a problem. It oxidizes readily, especially at elevated temperatures, and that oxide layer interferes with soldering, brazing, and wire bonding. The surface also lacks corrosion resistance in many environments. Nickel plating solves both issues.

Nickel plating on Kovar provides:

• Corrosion resistance that bare Kovar simply doesn't have
• Improved solderability and wetting characteristics
• A clean, bondable surface for wire bonding and die attach
• A diffusion barrier between Kovar and subsequent finishes

Most importantly, nickel plating is almost always the foundation layer. When you see gold-plated Kovar leads or housings, the gold is typically sitting on top of a nickel underplate. The nickel does the heavy lifting; the gold is there for oxidation resistance and bondability.

What Type of Metal Is Kovar? (Material Composition and Properties)

Before we dive into plating, let's make sure we're speaking the same language about the substrate.

Kovar is a controlled-expansion alloy with a typical composition of approximately 29% nickel, 17% cobalt, and the balance iron. It's specified under ASTM F15 and carries the UNS designation K94610. You'll also see it referred to as Fe-Ni-Co alloy or by trade names like Dilver P or Nilo K.

Its defining physical properties include:

• Coefficient of thermal expansion: approximately 5 ppm/°C from room temperature to 400°C
• Curie temperature: approximately 430°C, above which it loses its magnetic properties
• Tensile strength: roughly 517 MPa (75 ksi) in the annealed condition
• Thermal conductivity: around 17 W/m·K

Kovar belongs to a family of glass-sealing alloys that includes Alloy 42 (42% nickel, balance iron) and Invar (36% nickel, balance iron). Alloy 42 matches alumina ceramics better at certain temperature ranges, while Invar offers even lower expansion but poor glass-sealing characteristics. Kovar sits in the sweet spot for borosilicate glass and most ceramic-to-metal sealing applications.

Here's the critical thing for platers: Kovar's surface oxidizes quickly. The oxide layer that forms is a mix of iron and nickel oxides (with some cobalt oxide), and it's tenacious. If you try to plate over it, you get adhesion failure. That oxide must be completely removed and the surface activated before any nickel goes down.

The Nickel Plating Process on Kovar: Step-by-Step

Getting nickel plating right on Kovar isn't complicated, but it demands discipline. Every step matters, and skipping or rushing any of them leads to scrap.

Surface Preparation and Cleaning

Adhesion failure is the number one problem with Kovar plating. If you spend any time on plating forums, you'll see the same story repeated: "I plated my Kovar parts and the nickel is peeling." In nearly every case, the root cause traces back to surface preparation.

The process starts with cleaning. Kovar parts typically arrive from machining with residual coolants, lubricants, and handling oils. These must be removed before any chemical processing.

Our recommended sequence:

1. Solvent degreasing to remove bulk oils and greases
2. Alkaline cleaning to remove remaining organic contamination
3. Thorough rinsing between every step—drag-in contamination ruins baths

The alkaline cleaner should be operated at the manufacturer's recommended temperature and concentration. Parts should be cleaned until they exhibit a water-break-free surface—meaning water sheets off evenly rather than beading up.

Oxide Removal and Activation

Once clean, the oxide layer must go. Kovar's native oxide doesn't dissolve easily, so aggressive acid treatment is required.

Typical approaches include:

• Hydrochloric acid pickling (10–30% by volume) at room temperature or slightly elevated
• Sulfuric acid solutions (10–20%) for heavier oxide removal
• Proprietary etchants designed for glass-sealing alloys

The goal is to remove all visible oxide without over-etching the base metal. Over-etching leaves a roughened, smutty surface that also causes adhesion problems.

After pickling, some platers use a short dip in a dilute acid or cyanide-based solution to neutralize and activate the surface immediately before plating. The key word here is immediately. Kovar re-oxidizes fast—within minutes in some environments. There should be minimal delay between activation and immersion in the nickel bath.

Commercial platers who specialize in glass-sealing alloys often use proprietary pre-plate cycles developed specifically for Kovar. These cycles are designed to produce a consistent, active surface that maximizes nickel-to-substrate adhesion.

Electrolytic Nickel Plating (Sulfamate vs. Watts Baths)

With the surface prepared, you have two main electrolytic options: sulfamate nickel or Watts nickel.

Sulfamate nickel baths are the preferred choice for precision Kovar components. They produce deposits with significantly lower internal stress than Watts baths—an important consideration for thin-wall Kovar parts that could distort. Sulfamate baths also tolerate higher current densities and produce smoother, more ductile deposits.

Watts nickel baths deposit faster and are less expensive to operate, but the resulting deposit has higher internal stress. For Kovar parts that are later subjected to thermal cycling in hermetic seals, that stress can contribute to adhesion failure or distortion.

Typical bath parameters for sulfamate nickel:

| Parameter | Typical Range |

|-----------|---------------|

| Nickel concentration | 75–110 g/L |

| Nickel sulfamate | 300–450 g/L |

| Boric acid | 30–45 g/L |

| pH | 3.5–4.5 |

| Temperature | 50–60°C |

| Current density | 2–10 A/dm² |

A common question we see is: "What is the best voltage for nickel plating?" The honest answer is that voltage is a consequence, not a control parameter. What matters is current density—the current per unit area of part surface. Voltage adjusts automatically based on bath conductivity, anode arrangement, and part geometry. Trying to control plating by voltage alone leads to inconsistent thickness and poor deposit quality. Work with your plater to establish the correct current density for your specific bath and part configuration.

Electroless Nickel Plating as an Alternative

Electroless nickel (EN) plating offers a different route. Instead of using an external current source, EN deposits nickel via an autocatalytic chemical reaction, typically using sodium hypophosphite as the reducing agent.

The advantages are compelling:

• Uniform thickness on complex geometries, including internal bores and blind holes
• Excellent wear resistance (especially with composite or high-phosphorus deposits)
• Consistent hardness across the entire surface

For Kovar parts with intricate features—like the inside of hermetic housings—EN ensures you get coverage in areas that would be difficult to plate uniformly with electrolytic processes.

However, EN on Kovar requires careful attention to adhesion. The same surface preparation and activation steps apply. Some platers also recommend a post-plate heat treatment to improve adhesion and relieve hydrogen. Typical schedules run at 180–200°C for 1–4 hours, depending on the phosphorus content of the deposit.

Post-Plating Treatment and Quality Checks

After nickel deposition, several steps remain before parts are ready for use.

Hydrogen embrittlement relief is non-negotiable for Kovar. During electroplating, atomic hydrogen can diffuse into the substrate, particularly in high-strength or thin-section parts. This hydrogen can cause delayed cracking under stress. The standard remedy is baking at 190–230°C for 2–4 hours, per ASTM B850 or similar specifications. This allows hydrogen to diffuse back out before the part is placed in service.

Adhesion testing confirms the plating process worked. Common methods include:

• Bend testing on representative coupons
• Tape testing per ASTM D3359
• Thermal shock testing—heating and quenching to expose adhesion failures

Thickness verification typically uses X-ray fluorescence (XRF) for non-destructive measurement or metallographic cross-sectioning for destructive analysis.

Optional topcoats are common. Gold over nickel is the standard for Kovar leads and packages requiring wire bonding. Silver is used for high-conductivity applications, and tin for solderability. Each topcoat has its own requirements for the nickel layer beneath it—typically 2–5 microns of nickel before gold, and sometimes more for demanding applications.

Key Challenges and Solutions in Kovar Nickel Plating

Even with a solid process, Kovar plating presents recurring challenges. Here's how to identify and solve them.

Poor Adhesion and Blistering

If your nickel is peeling, blistering, or flaking, the cause is almost always one of three things:

1. Residual oxides not fully removed during pickling
2. Insufficient activation before plating
3. Contaminated surfaces from improper cleaning or handling

The fixes are straightforward but require discipline:

• Use a properly formulated acid pickle and check that it's within its working life
• Consider a Wood's nickel strike—a chloride-based, high-acid nickel bath used at low current density to deposit a thin, highly adherent strike layer before the main nickel build
• Plate immediately after activation—don't let parts sit
• Maintain clean rinses and avoid touching prepared surfaces with bare hands

Industry forums are full of adhesion troubleshooting threads, and the pattern is always the same: someone skipped a step, diluted a bath past its limits, or let parts sit too long between processes. Don't be that person.

Hydrogen Embrittlement

Kovar, especially in thin sections or after cold working, is susceptible to hydrogen embrittlement from electroplating. The hydrogen generated at the cathode during plating can diffuse into the metal and cause cracks under tensile stress.

Prevention and mitigation:

• Bake promptly after plating—190–230°C for 2–4 hours is the common schedule
• Minimize plating time by specifying only the thickness you actually need
• Consider electroless nickel for thin sections, as EN generally produces less hydrogen pickup
• Follow ASTM B850 or your contract's specified embrittlement relief process

Thermal Expansion Mismatch During Plating

Kovar's entire reason for existing is its thermal expansion match with glass. But that match only holds within a specific temperature range. If you heat Kovar above its Curie temperature (~430°C) during processing, its expansion behavior changes—and it may not return to its original characteristics.

This matters during plating because some processes involve elevated temperatures. Electroless nickel baths run at 85–95°C, which is well below the Curie temperature. However, if you're doing post-plate heat treatments or brazing operations, keep the temperature in mind. Precision Kovar components can distort if subjected to temperatures that alter their metallurgical state.

Control bath temperatures carefully, and verify that any post-plating thermal processes stay within the material's safe operating range.

Removing Nickel Plate from Kovar (Selective Stripping)

Sometimes plating goes wrong, or a part needs rework. Stripping nickel from Kovar without damaging the substrate requires care.

Chemical strippers for nickel typically use nitric acid-based solutions. These dissolve nickel rapidly but can also attack the Kovar substrate if left too long. Proprietary strippers are available that are formulated to be more selective.

Electrochemical stripping uses an anodic current in a suitable electrolyte to dissolve the nickel. This offers more control but requires proper fixturing and process knowledge.

Cautions:

• Monitor stripping time closely—over-etching weakens the Kovar and ruins dimensional tolerances
• Inspect after stripping for pitting or roughening
• Re-passivate or re-plate promptly after stripping, as the exposed Kovar surface is highly reactive

Benefits of Nickel Plating on Kovar for Industrial Applications

Why go to all this trouble? Because nickel plating transforms Kovar from a difficult-to-work-with alloy into a manufacturable, reliable component material.

The functional benefits include:

Corrosion protection. Bare Kovar rusts. Nickel plating provides a barrier that protects the substrate in humid, saline, or chemically aggressive environments. This is essential for aerospace, defense, and marine applications.

Improved solderability and weldability. Nickel wets well with most solders and provides a clean surface for welding operations. Bare Kovar's oxide layer makes soldering unreliable.

Wear resistance. Nickel, especially electroless nickel, offers significant hardness that protects Kovar surfaces from abrasion and fretting.

Diffusion barrier. Nickel prevents interdiffusion between Kovar and subsequent finishes like gold or silver. Without a nickel barrier, gold can diffuse into Kovar over time, degrading the surface and compromising bondability.

Enhanced hermetic seal performance. In glass-to-metal seals, nickel plating on the Kovar side improves the reliability of the seal by preventing oxidation at the interface and providing a clean surface for subsequent assembly steps.

Cost-effectiveness. Nickel is dramatically less expensive than gold, silver, or palladium. Using nickel as the primary finish—or as an underplate for a thin precious metal topcoat—reduces material costs while still achieving the required performance.

Now, the honest answer to the question "What are the disadvantages of nickel plating?" Nickel plating has real limitations:

• Porosity in thin deposits can compromise corrosion protection
• Hydrogen embrittlement risk exists with electrolytic processes
• Process control demands are high—poor control leads to adhesion failures, thickness variation, and contamination
• Magnetic properties of nickel may matter in some applications, though thin deposits have minimal effect

These disadvantages are manageable with proper process control, but they're real. Don't specify nickel plating without understanding what it can and cannot do.

Nickel Plating Specifications and Standards for Kovar

If you're specifying nickel plating on Kovar, you need to reference the right standards. The key documents include:

| Standard | Scope |

|----------|-------|

| ASTM B689 | Electrolytic nickel plating |

| ASTM B733 | Electroless nickel plating |

| MIL-DTL-32119 | Military specification for nickel plating |

| AMS 2404 | Electrolytic nickel plating |

| AMS 2405 | Electroless nickel plating |

| ASTM B850 | Post-coating treatment of steel for hydrogen embrittlement relief |

When writing your specification, define:

• Thickness range—typically 2–10 microns (80–400 microinches) for Kovar, depending on application
• Hardness requirements—if wear resistance matters
• Adhesion requirements—and the test method to verify them
• Post-plate baking—schedule and temperature
• Acceptance criteria—visual, dimensional, and functional

Quality control methods include XRF thickness testing, salt spray testing for corrosion resistance, and metallographic cross-sectioning for deposit structure and adhesion evaluation.

Applications of Nickel-Plated Kovar Across Industries

Nickel-plated Kovar appears in more places than most people realize.

Electronics and Semiconductor Packaging

Kovar lead frames, transistor housings, and integrated circuit packages are routinely nickel-plated, then gold-plated, to provide wire-bondable surfaces. The nickel layer ensures adhesion and acts as a diffusion barrier between the Kovar and gold. Without it, gold migrates into the Kovar and the bondability degrades over time.

Aerospace and Defense

Hermetic connectors, feedthroughs, and sensor housings in aerospace and defense applications demand corrosion resistance and thermal stability. Nickel-plated Kovar survives the temperature extremes, vibration, and atmospheric exposure these components face. Military specifications often require specific plating thicknesses and adhesion test protocols.

Medical Devices

Implantable device casings and surgical instruments use nickel-plated Kovar where biocompatibility and reliability are essential. The nickel layer provides a clean, corrosion-resistant surface that can be further finished for specific medical requirements.

Optical and Laser Components

Glass-to-metal seals in laser diodes, photodetectors, and vacuum tubes depend on nickel-plated Kovar. The plating ensures that the seal area remains clean and solderable during assembly, contributing to the hermetic integrity of the final device.

Cost Considerations for Kovar Nickel Plating

"What is the cost of nickel plating?" It's a common question, and the answer depends on several factors.

Primary cost drivers:

• Part size and surface area—more area means more nickel and more process time
• Plating thickness—cost scales roughly linearly with thickness
• Batch size—larger batches amortize setup and processing costs
• Finishing requirements—additional topcoats or specialized processes add cost
• Quality requirements—more testing and documentation means more expense

Electrolytic vs. electroless nickel costs: Electrolytic nickel is generally less expensive per unit of thickness because deposition rates are higher and bath chemistry is simpler. Electroless nickel costs more due to the cost of reducing agents and tighter process control, but it offers uniform coverage that can reduce total nickel usage on complex parts.

Cost-saving strategies:

• Specify the minimum thickness that meets your performance requirements
• Optimize rack loading to maximize part density per plating run
• Combine nickel plating with other finishes in a single process run
• Work with a plater experienced in Kovar to avoid rework and scrap

Remember: Kovar itself is expensive. A failed plating run on a precision-machined Kovar housing means losing not just the plating cost, but the entire machined part. Quality matters more than price when the alternative is scrapping expensive components.

Choosing the Right Kovar Plating Supplier

Not all plating shops are created equal, and Kovar is not the same as plating stainless steel or copper. When evaluating a plating supplier, ask the right questions:

Experience with glass-sealing alloys. Have they plated Kovar before? Do they understand its oxidation behavior and activation requirements?

In-house analytical lab. Can they verify bath chemistry and maintain process control? A supplier without lab capabilities is guessing.

Pre-plate activation procedures. What specific steps do they use for Kovar? Do they use a Wood's nickel strike or proprietary activation cycle?

Post-plate testing capabilities. Can they perform XRF thickness measurement, adhesion testing, and hydrogen embrittlement relief?

Certifications. Look for ISO 9001, AS9100, Nadcap, or ITAR compliance as your application requires.

Sample parts. Before committing to full production, request sample parts or coupons to validate adhesion and thickness. This is the only way to truly verify the supplier's process.

At KELTRYN, we work with trusted plating partners who understand Kovar's unique requirements. We've seen what happens when components are sent to general-purpose platers who don't appreciate the material's sensitivity—and it's not pretty.

Frequently Asked Questions About Nickel Plating on Kovar

Can you solder to nickel-plated Kovar?

Yes. Nickel-plated Kovar solders reliably with proper flux and solder selection. Nickel improves wetting over bare Kovar, which tends to form oxides that resist solder. For best results, use activated fluxes and lead-free or tin-lead solders appropriate for your application.

How thick should nickel plating be on Kovar?

The typical range is 2–10 microns (80–400 microinches), depending on the application and whether a topcoat follows. For gold-plated Kovar leads, 2–5 microns of nickel is common. For corrosion protection in harsh environments, thicker deposits—up to 10 microns or more—may be specified.

Does nickel plating affect Kovar's magnetic properties?

Nickel is magnetic, but thin deposits have a negligible effect on Kovar's overall magnetic behavior. The volume of nickel in a typical plating is minuscule compared to the Kovar substrate. For most applications, magnetic properties are unaffected.

What is the best activation method before nickel plating Kovar?

A Wood's nickel strike—a chloride-based, high-acid nickel bath operated at low current density—is widely recommended for strong adhesion on Kovar. This strike deposits a thin, adherent layer that provides an excellent base for the main nickel build. Some platers use proprietary activation cycles, but the Wood's strike remains the industry standard.

Can Kovar be nickel plated after glass sealing?

Yes, but it requires masking of the glass areas to prevent plating solution from attacking the seal. Many manufacturers prefer to plate before glass sealing to avoid this complication. If you must plate after sealing, work with a plater experienced in selective plating and masking.

Conclusion and Next Steps

Nickel plating on Kovar is a mature process, but it demands respect. Surface preparation is critical, process control determines success, and the difference between a reliable hermetic component and a field failure often comes down to how well the plating was executed.

The key takeaways:

• Start with a clean, oxide-free surface—everything else follows from this
• Activate properly—a Wood's nickel strike is your friend
• Control the process—current density, bath chemistry, and temperature all matter
• Bake to relieve hydrogen—don't skip this step
• Verify quality—test adhesion and thickness before committing to production

At KELTRYN, we machine Kovar components every day, and we understand that the surface finish is as important as the dimensional accuracy. Whether you're developing a prototype or moving into production, we can help you navigate the machining and finishing challenges that come with this demanding material.

If you're working on a Kovar component that needs precision machining—or if you're evaluating your current supplier's capabilities—we'd welcome the opportunity to discuss your project. Our team provides practical manufacturability feedback, material traceability, and reliable delivery for components destined for hermetic and high-reliability applications.

Contact KELTRYN today to discuss your Kovar machining and plating requirements. Send us your drawings or CAD models, and we'll provide engineering feedback and a quote for your next project.