Case Study: Implantable Cardiac Device Hybrid Assembly

1. Customer Challenge

The engineering team at a Tier-1 implantable cardiac device manufacturer faced a demanding development challenge: design the signal conditioning and RF telemetry module for a next-generation insertable cardiac monitor (ICM) that would be powered by a miniaturised battery and operate reliably inside a human body for a minimum of 10 years.

The existing ICM platform relied on a multi-chip module assembled on a rigid ceramic substrate with discrete off-the-shelf components. It was bulkier than the design target and consumed more power than the new ASIC roadmap allowed. The programme mandate was clear: reduce the hybrid module footprint by 40%, cut quiescent power draw by 60%, and pass every FDA scrutiny item with zero deviations.

Programme Specifications

Beyond the electrical and mechanical requirements, the programme was governed by a comprehensive set of biocompatibility, hermeticity, and software documentation requirements. Failure was not an option — any field failure of a permanently implanted cardiac monitor carries a Class I recall risk and potential patient harm.

2. Solution: Medical-Grade Thin Film Hybrid

The assembler selected for this programme was an established medical device contract manufacturer with an ISO 13485-certified hybrid assembly line and a documented track record of FDA PMA submissions for active implantable devices. After a technical interchange and two design-for-manufacturing (DFM) sessions, the team converged on a thin film hybrid architecture on a high-purity alumina substrate.

Substrate and Metallisation

The substrate was 99.6% pure alumina (Al₂O₃) at 10 mil (254 μm) thickness — chosen for its excellent dielectric properties, thermal conductivity (≈ 24 W/m·K), and long history of use in implantable electronics. The thin film metallisation stack was titanium–platinum–gold (Ti/Pt/Au = 300 Å / 1000 Å / 3000 Å), deposited by sputtering and patterned using standard photolithography and etch. This stack provides superior adhesion to alumina, a reliable wire-bondable surface, and excellent corrosion resistance in body-fluid environments.

Active and Passive Components

The module integrates three key semiconductor devices:

• Custom mixed-signal ASIC — fabricated in CMOS 180 nm, handling ECG signal acquisition, amplification, filtering, and ADC conversion. The die was thinned to 150 μm to minimise the Z-height of the assembled module.
• Bluetooth Low Energy SoC — a qualified medical-grade BLE transceiver handling the 2.4 GHz RF telemetry link to the external patient handheld device. The SoC implements the full BLE 5.0 protocol stack.
• 0402-format chip components — precision thin-film chip resistors and multilayer ceramic capacitors in the 0402 (1005 metric) footprint for decoupling, matching, and DC bias networks.

Interconnect and Underfill

All silicon dice were interconnected using 1.25-mil (≈32 μm) gold wire (99.99% pure gold) with a thermosonic ball-wedge bonding process optimised for low-loop, short-wire configurations to reduce parasitic inductance. After wire bonding, each die was underfilled with a medical-grade epoxy specifically tested and certified to ISO 10993-5 (cytotoxicity) and ISO 10993-10 (sensitisation and irritation). The underfill minimises mechanical stress on the dice during thermal cycling and provides an additional moisture barrier.

3. Biocompatible Packaging Requirements

Any material that contacts tissue or body fluids in a permanently implanted device must pass a rigorous battery of biocompatibility tests defined in ISO 10993-1, the biological evaluation of medical devices. The test battery for this programme spanned six ISO 10993 parts:

Material selection extended to every surface in contact with tissue. The primary enclosure was a medical-grade titanium case (Ti-6Al-4V ELI — Extra Low Interstitial) with a history of use in orthopaedic and cardiovascular implants. The hybrid assembly inside the case was gold-plated to prevent corrosion, and all internal polymers (underfill, glob-top coating on the BLE SoC) were verified as ISO 10993-compatible before production release.

Long-Term Implant Testing

Demonstrating a 10-year service life through accelerated aging is a standard FDA expectation for permanently implanted devices. The programme used the Arrhenius equation to correlate accelerated test conditions with real-time shelf and implant life. The validation protocol set 70 °C storage for 2 years as equivalent to 10 years at 37 °C (assuming an activation energy of 0.7 eV — standard for epoxy and polymer systems). The hybrid module, fully sealed in its titanium case, was subjected to:

• 2,000 hours at 70 °C / 85% RH (pressure cooker test — PCT)
• 500 thermal cycles from −40 °C to +85 °C (simulating transport/storage extremes)
• 3× EtO sterilisation cycles (pre-implant device sterilisation validation)
• Helium fine leak test and gross leak test after each stress condition

4. Hermeticity Requirements

The internal cavity of the hybrid module must remain hermetically sealed from body fluids for the full 10-year service life. Moisture ingress through the seal or feedthroughs would cause galvanic corrosion of the gold metallisation and eventual electrical open or short circuits. The design target leak rate was <1 × 10⁻⁹ atm·cm³/s — the standard definition of a hermetic seal in implantable electronics.

Seal Architecture

The titanium case uses a resistance seam seal to join the Kovar lid (gold-plated) to the titanium header. Resistance seam sealing is a continuous welding process where two electrode wheels roll along the case flange, creating overlapping weld nuggets that form a gas-tight seam. This process produces consistent, inspectable seals without the heat input of laser welding that could thermally stress the internal hybrid.

Electrical signals pass through the titanium header via glass-to-metal seals (GTMS) on 0.017-inch diameter Kovar pins. The glass used is a hermetic borosilicate glass (Corning 7052 or equivalent) with a thermal expansion coefficient matched to Kovar to minimise residual stress after firing. Each feedthrough pin was 100% tested for hermeticity before header assembly.

Hermeticity Testing Protocol

Every production unit was subjected to a two-stage leak test per MIL-STD-883 Method 1014:

1. Fine leak test (HeBomb test): The sealed assembly is pressurised in helium at 60 psig for 2 hours. A helium mass spectrometer leak detector then measures the helium leak rate from the cavity. Pass criterion: R₁ < 1 × 10⁻⁸ atm·cm³/s (Method 1014, Test Condition A).
2. Gross leak test (fluorocarbon extraction): The assembly is immersed in PFID fluorocarbon fluid under vacuum, then pressurised. Emerging bubbles indicate gross leak (bubble test). Units with gross leak are rejected andFail analysed.

For the qualification build, a supplementary helium accumulation test was performed to verify sub-10⁻⁹ atm·cm³/s leak rates — a more sensitive method where the test device is helium-charged and the accumulated helium in a sealed chamber is measured over time.

The header-to-case seal used a gold-tin (AuSn 80/20) solder preform, reflowed in a nitrogen atmosphere. AuSn solder provides excellent wetting to gold-plated surfaces, a high joint strength (≈ 70 MPa shear), and resistance to creep and fatigue over thermal cycles. The solder seal also acts as a secondary corrosion barrier at the header interface.

5. FDA Submission Support

The assembler played a central role in assembling the technical documentation required for the FDA Premarket Approval (PMA) submission. For a Class III permanently implanted device with an RF telemetry link, the PMA requires a comprehensive Design History File (DHF), Device Master Record (DMR), and Process Validation package.

Design Controls (21 CFR 820.30)

The programme implemented the full FDA Quality System Regulation (21 CFR Part 820) design controls, including:

• Design Input: All electrical, mechanical, biocompatibility, and reliability specifications were formally documented, reviewed, and signed off by the cross-functional team before any detailed design work began.
• Design Output: Every deliverable — drawings, schematics, bill of materials, assembly traveller — was reviewed against the design inputs and formally approved before release.
• Design Verification: Each specification was verified by test or analysis, with formal test reports referencing the specific test method, equipment, and acceptance criteria.
• Design Validation: Functional prototypes were validated in an simulated-use animal study and a human clinical trial under an Investigational Device Exemption (IDE).

Risk Analysis and Process Validation

A formal Risk Analysis was conducted per ISO 14971 (Medical Devices — Application of risk management to medical devices), identifying 47 failure modes across the hybrid module, sealing process, and software stack. Each failure mode was assessed for severity, probability of occurrence, and detectability, and mitigation actions were implemented for all High risk items before design freeze.

Process Validation for the sealing operation followed the Installation Qualification / Operational Qualification / Performance Qualification (IQ/OQ/PQ) framework. The IQ documented that the seam sealer was installed correctly with calibrated tooling. The OQ demonstrated that the process parameters (weld current, weld speed, electrode force) produced consistent seals within specification across the qualification temperature range. The PQ validated that three consecutive production lots of 30 units each met all mechanical and hermeticity acceptance criteria.

Software and Sterilisation Documentation

The BLE SoC runs a firmware stack that implements the Bluetooth 5.0 protocol, patient data encryption, and the proprietary telemetry protocol. Software documentation was prepared to IEC 62304 (Medical device software — Software life cycle processes), with the software classified as Class B (non-serious injury possible) and a full software development plan, requirements traceability matrix, and unit/module/system-level testing records.

Sterilisation validation confirmed that the hybrid module, inside its sealed titanium case, withstood three EtO (ethylene oxide) sterilisation cycles (each cycle: 600 mg/L EtO, 55 °C, 60% RH, 6-hour exposure, followed by 12-hour aeration) without degradation of seals, electrical performance, or wire bond integrity.

6. Qualification and Production Results

The qualification build comprised three production lots of 30 units each, assembled and tested to IPC Class 3 workmanship standards with enhanced inspection requirements for medical applications. All units passed 100% visual inspection at 20× magnification, and all critical bond and seal parameters were verified on every unit.

Mechanical Test Results

The production yield at release was 96.8% first-pass yield, with the three fallout units attributed to wire bond parameter drift on a single eutectic die attach station — identified and corrected during the production lot. After process containment, the subsequent three production lots achieved 100% first-pass yield.

Field Reliability

At three years post-launch (representing the first 3 years of the 10-year expected service life), field data from over 42,000 implanted units shows a DPM of 0 — zero hybrid module-related field failures attributed to the assembly process. The overall device survival rate is 99.97% (excluding battery depletion events unrelated to the hybrid module).

Programme Timeline

The total regulated programme duration — from project kickoff to FDA PMA approval — was 18 months. This included a three-month DFM phase, a six-month design and qualification build, a four-month FDA submission preparation, and a five-month FDA review cycle.

7. Conclusion

This case study demonstrates that medical-grade thin film hybrid assembly is a mature, reliable, and regulatory-compatible technology for the most demanding implantable medical device applications. The combination of thin film metallisation on high-purity alumina, precision gold wire bonding, medical-grade epoxy underfill, and a hermetically sealed titanium package delivered a module that exceeds all mechanical, electrical, biocompatibility, and reliability requirements.

The keys to success were a rigorous Design for Manufacturability process at the outset, strict adherence to IPC Class 3 workmanship standards, exhaustive qualification testing aligned with FDA expectations, and a well-documented Quality System that generated the DHF and DMR packages required for PMA submission. At three years and 42,000 units in the field with zero hybrid-related failures, the programme stands as a benchmark for medical hybrid reliability.

For engineering teams developing next-generation permanently implanted devices — whether cardiac monitors, neurostimulators, implantable sensors, or drug delivery systems — selecting an assembler with documented FDA PMA experience, ISO 13485 certification, and a mature biocompatibility and hermeticity testing capability is as important as the component-level design decisions.