Implantable Device PCB: Materials, Sealing and Reliability

An implantable device is the most demanding application a circuit board can have. It cannot be repaired, replaced or reset by the user, it operates in a warm saline environment for years, and the consequence of a failure is measured in human terms rather than in warranty cost. Every decision on the board, from the laminate to the cleaning process to the coating, is made with that in mind.

What Makes the Application Different

Three constraints dominate. The board must be electrically reliable to a level that ordinary industrial standards do not describe, it must be physically small enough to fit the implant, and every material that touches the body must be shown to be safe. Those constraints interact: a smaller board means denser routing and thinner materials, and thinner materials are harder to seal hermetically.

The consequence is that the design margin on an implantable device PCB is wider than the application strictly requires. A conductor is sized for more current than it will carry, a via is larger than the routing needs, and a component is derated further than its rating suggests, because the cost of an unexpected failure is not comparable with the cost of the extra area.

Materials and Biocompatibility

Biocompatibility is not a property of the board alone; it is a property of the finished sealed device. Inside a hermetic enclosure the board sees only the controlled internal atmosphere, and the material requirements are then electrical, thermal and mechanical. Where the board or a flexible lead is exposed to tissue, the materials that touch the body have to be selected from those with a documented history of use, and the surface finish must not release ions that the body will react to.

Polyimide is common for flexible interconnects because it is tough, stable and well characterised, and it can be patterned thinly enough for a lead that must flex with the tissue. Silicone encapsulation is used where a soft, compliant interface is required, while a metal or ceramic enclosure is used where a true hermetic barrier is needed.

The surface finish matters as much as the substrate. A finish that releases an ion the body reacts to is unacceptable on an exposed surface, and even inside a sealed enclosure the finish determines how the joints behave over a long service life. Coatings and encapsulants are therefore selected from the small set of materials that have been characterised for this application rather than from the general catalogue, and the supplier is expected to demonstrate the qualification of the materials used rather than to assert it.

The choice drives the rest of the design.

Hermetic Sealing and Feedthroughs

Hermetic sealing means a barrier that limits the ingress of moisture to a level the electronics can tolerate for the design life. A metal can with welded seams and ceramic feedthroughs is the classic solution. The feedthroughs carry the signals and the power through the wall of the enclosure, and they are usually the most expensive and the most carefully specified part of the assembly.

The board inside the can is not sealed in the sense that a potted assembly is sealed. It sits in a controlled atmosphere, and the seals at the feedthroughs, the welds and the lid are what keep it that way. Because the failure of a single seal ends the life of the device, the process controls around welding and leak testing are as important as the electrical design, and a conformal coat inside the enclosure is used to protect against the residual moisture that will inevitably be present. The options for that layer are described in conformal coating for board protection.

Miniaturisation and Interconnection

Space inside an implant is measured in cubic centimetres, so the board is usually a rigid-flex or a flexible circuit that folds into the available volume. Folding brings its own risks, because a bend that is formed once during assembly can still crack a conductor if the radius is too tight or the copper too thick, and the flexing that follows a lead in the body is dynamic and repeated.

The components are the smallest available, which means fine pitch packages and bare die, and the interconnections are made by wire bonding or by laser welding rather than by conventional soldering where the thermal budget is limited. Pad geometry, bond placement and the finish that supports the chosen attach process are therefore as critical as the routing, and the standards that govern them are described in PCB pad design standards.

Cleaning and Process Control

Residue that would be harmless in a consumer product is a reliability risk in a sealed implant, because it can corrode or migrate in the presence of the residual moisture. Cleaning is therefore specified in terms of measurable contamination rather than in terms of appearance, and the ionic cleanliness of the assembly is verified.

Process control follows the same logic. Every operation that could introduce contamination, from handling to the choice of a flux, is controlled and recorded, and the assembly is built in an environment whose cleanliness is monitored. The cleaning and the handling are part of the product specification, not an overhead on the manufacturing cost.

Reliability Testing and Qualification

Qualification is built around accelerated life testing. The assembly is subjected to temperature and humidity conditions that accelerate the mechanisms expected in service, cycled to expose mechanical weaknesses, and in some cases tested in a physiological solution. The number of samples and the duration are set by a statistical plan rather than by convenience.

The standard framework for the quality system is ISO 13485, which governs the design, the production and the traceability of medical devices and their components. A supplier working to that standard will be audited, will hold records for every lot, and will be able to show the process history of the parts it ships. The relationship between the solder alloy and the long term behaviour of the joints is a further consideration, and the trade offs are set out in lead free versus leaded solder.

Documentation and Traceability

Every unit is traceable. The laminate batch, the drilling and plating parameters, the inspection results and the test data are recorded against a serial number so that a device explained in the field can be traced back to its build and compared with the records of the whole lot. This is a regulatory requirement and also the only practical way to investigate a rare failure.

The documentation is therefore part of the design work. A change to a material or a process has to be assessed, verified and recorded rather than simply implemented, and the change control is what prevents a small substitution for cost or availability from becoming an unqualified change to a medical device.

FAQ

Is the board itself sealed? Sometimes, but the enclosure does the sealing. The board inside a hermetic can relies on the seals at the feedthroughs and the lid rather than on its own coating.

Can ordinary FR4 be used? Inside a hermetic enclosure it can be used for some designs, but flexible and thermally stable materials are preferred because of the assembly processes and the long service life.

What standard applies? ISO 13485 governs the quality system for medical devices. Together with the applicable class requirements it defines the traceability and process control expected of the supplier.