Electronics Guide

Defining Hermeticity

True hermeticity implies zero permeation of gases or liquids through package walls and seals. In practice, all materials exhibit some permeability, but hermetic packages reduce leak rates to levels where the internal atmosphere remains stable over the required lifetime. Industry standards typically define hermeticity through leak rate specifications rather than absolute impermeability.

The standard metric for hermeticity is leak rate expressed in atmospheric cubic centimeters per second (atm-cc/s) of a tracer gas, usually helium due to its small atomic size and detectability. Hermetic packages typically achieve leak rates below 10^-8 atm-cc/s of helium, while highly demanding applications may require 10^-9 atm-cc/s or better.

Permeation Mechanisms

Gas and moisture can enter packages through several mechanisms:

• Bulk permeation: Diffusion through solid materials, significant in polymers but negligible in metals, ceramics, and glasses
• Leak paths: Physical channels through seals or package walls, from gross cracks to molecular-scale pathways
• Virtual leaks: Outgassing from internal materials that appears as seal leakage but originates from absorbed or entrapped gases

Hermetic sealing addresses primarily leak path mechanisms. Polymeric materials cannot provide true hermeticity due to bulk permeation; only metals, ceramics, glasses, and certain crystalline materials are inherently hermetic.

Why Hermeticity Matters

Moisture within electronic packages causes numerous failure mechanisms:

• Corrosion: Electrochemical attack on metallization, bond wires, and leads
• Dendrite formation: Metal migration between biased conductors creating short circuits
• Parametric drift: Changes in device characteristics due to surface effects
• Die attach degradation: Moisture weakening adhesion and thermal interfaces
• Popcorning: Steam pressure during thermal excursions causing package damage

For long-lifetime applications, even extremely low moisture ingress rates accumulate to significant internal humidity over years or decades of operation. Hermetic sealing prevents this accumulation entirely.

Glass-to-Metal Seals

Glass-to-metal seals form hermetic bonds between glass insulators and metal conductors, providing electrical feedthroughs while maintaining hermeticity. The technology relies on chemical bonding between glass and oxidized metal surfaces, combined with mechanical compression from differential thermal contraction.

Two primary seal types exist:

• Compression seals: The metal contracts more than the glass during cooling, placing the glass in compression. Common combinations include steel with soda-lime or borosilicate glass. Robust and tolerant of process variations
• Matched seals: Metal and glass have similar thermal expansion coefficients, minimizing stress. Kovar (iron-nickel-cobalt alloy) with borosilicate glass is the classic combination. More delicate but allows larger seals

Glass-to-metal seal quality depends on glass-metal oxide wetting, absence of voids and inclusions, proper thermal expansion matching, and controlled cooling to avoid thermal shock.

Ceramic-to-Metal Seals

Ceramic-to-metal seals bond ceramic insulators to metal structures through metallization processes that enable brazing or soldering. Alumina (Al2O3) is the most common ceramic, metallized by molybdenum-manganese processes or by plating onto pre-metallized surfaces.

The sealing process typically involves applying metallization paste to ceramic and firing at high temperature, nickel plating the metallized surface, brazing to metal components using silver-copper or other active brazes, and inspection and testing of completed seals.

Seam Welding

Seam welding creates hermetic closures by resistance welding metal lids to metal package frames. The process uses wheel electrodes rolling along the lid perimeter, applying current pulses that create overlapping weld nuggets forming a continuous hermetic seal.

Seam welding advantages include rapid processing (seconds per package), no flux or filler materials, process compatibility with many package designs, and good reliability when properly executed. However, the process generates heat that must not damage internal components, and requires precise control of welding parameters.

Laser Welding

Laser welding uses focused laser energy to fuse metal lids to package bodies. Compared to seam welding, laser welding offers lower heat input (protecting sensitive devices), ability to weld close to internal components, more flexible weld path geometry, and suitability for smaller packages.

Laser welding requires clean, well-fitted parts and careful parameter control. Common laser types include Nd:YAG and fiber lasers, operating in pulsed or continuous-wave modes depending on application requirements.

Solder Sealing

Solder sealing forms hermetic joints by melting solder preforms between lid and package body. The process operates at lower temperatures than welding, making it suitable for temperature-sensitive devices. Gold-tin (80Au-20Sn) solder is preferred for high-reliability applications due to its fluxless processing capability, resistance to intermetallic formation, and excellent hermeticity.

Solder sealing typically occurs in controlled atmosphere furnaces, with packages passing through preheat, reflow, and cooling zones. Proper surface preparation and solder volume control are critical for reliable seals.

Brazing

Brazing creates hermetic joints using filler metals with melting points above 450 degrees Celsius. Silver-copper eutectic and gold-based brazes are common choices. Brazing provides strong, high-temperature-capable joints suitable for packages that will experience elevated operating temperatures or aggressive thermal cycling.

The process requires careful joint design, surface preparation, and atmosphere control (vacuum or reducing atmosphere) to achieve oxide-free joints with good filler metal flow.

Electron Beam Welding

Electron beam welding uses a focused beam of electrons in vacuum to create deep, narrow welds with minimal heat input. The process is particularly suited for high-reliability applications requiring very clean welds with no flux or filler materials. The vacuum environment inherently provides clean conditions for the weld.

Metal Can Packages

Metal can packages, including TO-style transistor outlines, represent the simplest hermetic package form. A formed metal body contains the device, with glass-to-metal sealed leads providing electrical feedthroughs. A metal lid welded to the body completes the hermetic enclosure.

Metal cans offer excellent hermeticity, good thermal performance through the metal body, proven reliability over decades of use, and cost-effectiveness for moderate volumes. However, lead count is limited and package size may be larger than modern surface-mount alternatives.

Ceramic Packages

Ceramic packages provide hermetic housing for integrated circuits with higher lead counts and more sophisticated configurations:

• Ceramic dual in-line packages (CERDIP): Two ceramic halves with a glass frit seal, providing hermetic protection with through-hole mounting
• Ceramic flat packs: Low-profile packages with leads on two or four sides for surface mounting
• Ceramic chip carriers: Leadless packages with castellated contacts for direct surface attachment
• Ceramic ball grid arrays (CBGA): Area array packages with solder balls for high-density interconnection
• Ceramic column grid arrays (CCGA): Similar to CBGA but with solder columns providing better thermal cycling reliability

Multilayer Ceramic Packages

High-temperature co-fired ceramic (HTCC) packages integrate multiple ceramic layers with metallized interconnects, creating complex internal routing and cavity structures. HTCC uses alumina-based ceramics co-fired with refractory metallization (tungsten or molybdenum) at temperatures around 1600 degrees Celsius.

Low-temperature co-fired ceramic (LTCC) packages fire at lower temperatures (around 900 degrees Celsius), enabling use of higher-conductivity metallization (silver, gold) and integration of passive components within the ceramic structure. LTCC is particularly suited for RF and microwave applications.

Hybrid Microelectronic Packages

Hybrid packages house multiple die, passive components, and interconnecting substrates within a hermetic enclosure. These packages enable complex multi-chip modules with hermetic protection, typically using metal or ceramic housings with welded or brazed lids.

Custom Hermetic Assemblies

Applications with unique requirements may demand custom hermetic package designs. These range from modified standard packages to completely custom enclosures designed for specific devices or environmental conditions. Custom designs balance performance requirements against development cost and manufacturing complexity.

Fine Leak Testing

Fine leak testing detects small leaks using helium tracer gas. The two primary methods are:

• Helium bomb testing: Packages are pressurized in a helium atmosphere, forcing helium into any leak paths. Packages are then placed in a mass spectrometer leak detector that measures helium escaping from the package. This method detects leaks down to approximately 10^-8 atm-cc/s
• Helium backfill testing: Packages sealed in a helium atmosphere are tested directly without bombing. This approach is faster but requires process control to ensure known internal helium content

Fine leak test interpretation requires understanding that measured leak rate depends on bombing pressure, bomb time, and time between bombing and measurement. Standards specify procedures to ensure consistent, meaningful results.

Gross Leak Testing

Gross leak testing detects larger leaks that fine leak testing may miss due to rapid gas exchange. Methods include:

• Fluorocarbon bubble test: Packages are soaked in a low-boiling-point fluorocarbon liquid under pressure, forcing liquid into gross leaks. Packages are then heated above the liquid boiling point; escaping vapor forms visible bubbles indicating leaks
• Dye penetrant testing: Colored dye is forced into leak paths under pressure, then detected visually or under UV light
• Weight gain testing: Packages are weighed before and after pressure soaking in a liquid; weight gain indicates ingress through gross leaks

Optical Leak Testing

Optical leak testing observes lid deflection under pressure or vacuum conditions. Packages with gross leaks equalize pressure and show no deflection, while hermetic packages deflect in response to external pressure changes. Laser interferometry provides sensitive measurement of lid motion.

Residual Gas Analysis

Residual gas analysis (RGA) opens packages in a mass spectrometer system, analyzing the internal atmosphere. This destructive test verifies that sealed packages contain the intended atmosphere and reveals internal contamination or moisture levels. RGA complements leak testing by confirming seal process effectiveness.

Testing Standards

Major leak testing standards include:

• MIL-STD-883, Method 1014: Seal test for microelectronics
• MIL-STD-750, Method 1071: Seal test for discrete semiconductors
• MIL-STD-202, Method 112: Seal test for relays and other components

These standards define test procedures, acceptance criteria, and reject limits appropriate for different package sizes and application requirements.

Material Compatibility

Materials within hermetic packages must be compatible with the sealing process and long-term operation:

• Outgassing: Internal materials must have low outgassing to maintain internal atmosphere purity
• Thermal compatibility: CTE matching prevents stress from thermal cycling
• Chemical compatibility: Materials must not react with each other or the internal atmosphere
• Process compatibility: Internal components must survive sealing temperatures and atmospheres

Internal Atmosphere

The atmosphere sealed within hermetic packages affects device performance and reliability:

• Dry nitrogen: Standard fill gas providing inert environment with minimal moisture
• Nitrogen-helium mixtures: Helium addition enables fine leak testing without additional bombing step
• Vacuum: Required for some sensor types and to minimize convective heat transfer
• Argon or other gases: Specialty fills for specific applications

Moisture control during sealing is critical. Desiccants may be included within packages to absorb moisture released from internal materials or minor leak ingress over time.

Seal Design

Effective seal design considers seal width and area needed for adequate bond strength, surface preparation requirements for chosen sealing method, thermal management during sealing to protect internal components, and inspection and test access for quality verification.

Thermal Management

Hermetic packages may present thermal challenges:

• Limited convection: Sealed internal atmosphere reduces convective cooling
• Conduction paths: Heat must conduct through package materials to external surfaces
• Thermal interface: Die attach and thermal interface materials affect heat transfer

Design approaches include high-thermal-conductivity substrates and package materials, thermal vias and heat spreaders, minimized thermal interface resistance, and external heat sinking attached to package surfaces.

Process Control

Critical process parameters require monitoring and control:

• Sealing atmosphere: Moisture content, oxygen level, and fill gas composition
• Seal temperature profile: Time-temperature history affecting seal formation
• Surface cleanliness: Contamination affecting seal integrity
• Part fit-up: Gap and alignment of mating surfaces

Inspection Methods

Visual and non-destructive inspection verifies package quality:

• Visual inspection: Examination for obvious defects, contamination, and workmanship issues
• X-ray inspection: Internal examination for voids, wire bond integrity, and die attach quality
• Acoustic microscopy: Detection of delamination, voids, and cracks within materials
• Hermeticity testing: Fine and gross leak testing per applicable standards

Reliability Testing

Qualification and lot acceptance testing validates hermetic package reliability:

• Temperature cycling: Repeated thermal excursions stress seals and internal connections
• Thermal shock: Rapid temperature transitions reveal weak seals
• Mechanical shock and vibration: Dynamic stress testing for structural integrity
• HAST/PCT: Accelerated moisture testing (for comparative purposes with non-hermetic packages)
• Salt atmosphere: Corrosion testing for external package surfaces

Failure Modes

Understanding failure modes guides prevention and improvement:

• Seal leaks: Inadequate seal formation, contamination, or stress-induced damage
• Lead seal failures: Cracked glass-to-metal seals from mechanical or thermal stress
• Package body cracks: Ceramic or glass fracture from handling or thermal stress
• Internal degradation: Die attach, wire bond, or internal component failures

Aerospace and Space

Aerospace applications demand hermetic packaging due to long mission lifetimes without possibility of repair, exposure to vacuum and radiation, extreme temperature cycling, and high reliability requirements for mission-critical systems.

Military and Defense

Military electronics require hermetic packaging for operation across extreme environmental conditions, long storage life requirements (often 20+ years), field reliability without maintenance access, and resistance to chemical and biological threats.

Medical Implants

Implantable medical devices require hermeticity to prevent body fluid ingress that would damage electronics, contain potentially harmful materials within the package, ensure long-term reliable operation within the body, and meet stringent biocompatibility requirements.

High-Reliability Industrial

Industrial applications with long lifecycle requirements or severe environments use hermetic packaging for oil and gas exploration equipment, nuclear power instrumentation, transportation and rail systems, and critical infrastructure electronics.

Sensors and MEMS

Many sensors and MEMS devices require specific internal atmospheres or vacuum conditions, making hermetic packaging essential for proper device function as well as environmental protection.

When Hermeticity Is Required

Hermetic packaging is generally necessary when application lifetime exceeds 10 to 20 years in challenging environments, operating conditions include high humidity, corrosive atmospheres, or repeated condensation cycles, device function requires controlled internal atmosphere, failure consequences are severe (safety, mission, or financial impact), and regulatory or specification requirements mandate hermetic packaging.

Alternatives to Consider

Where full hermeticity may not be required, alternatives include near-hermetic packages with very low but non-zero permeation, high-performance conformal coatings and encapsulants, controlled environment operation with external environmental control, and redundant systems that tolerate individual device failures.

Cost Optimization

When hermetic packaging is necessary, cost optimization approaches include standard package designs rather than custom, volume manufacturing where applicable, appropriate test sampling rather than 100 percent testing for all parameters, and specification review to ensure requirements match actual needs.