Journey Through Embedded Active Components in PCB Substrates with Pacemakers
With 200,000 pacemakers implanted in the United States per year, the surgical process to correct heart abnormalities has become routine. When preparing for the process, cardiologists choose from three different incision types to determine the best implant method. Each type of incision impacts patient comfort and the amount of risk involved with the surgery.
The incision provides access to a vein and allocates space for the pacemaker. A cardiologist embeds the pacemaker by enclosing the device within a pocket formed from human tissue. A surgeon can choose to form a pocket within the tissue layer just under the skin by using one or two fingers to gently spread the fleshy tissues apart after an incision.
Another method involves placing a pacemaker below the pectoral muscle and begins with a shallow incision in the major muscle. The technique finishes with blunt dissection to create the pocket. In both cases, wound closure and the healing process allow the tissue to encapsulate the pacemaker.
The concept of embedding microcontrollers, MOSFETs, voltage regulators, integrated circuits and other active components within the substrate of a PCB mirrors the process of implanting a pacemaker within a human being. With integrated module board technologies, an SMT component implants in a cavity on the surface of a conventional rigid substrate.
Technological advancements have made cavity sizes more precise and allowed PCB designs to incorporate different cavity shapes corresponding to component dimensions. Using lasers to remove dielectric material offers positional accuracy and precise cavity depths. Small, precise milling and routing tools also provide the control needed to produce cavities that have a tight tolerance for the component.
Mechanical, chemical, and electrical compatibility between the component, the substrate, and buildup materials must exist for proper circuit operation. After aligning and placing the component, your next steps involve filling the cavity with molding polymers that include isotropic solder. The mix of polymers and solder ensures compatibility. Laminating the core substrate with resin-coated copper allows for microvia fabrication.
Using strong PCB design software will help track your via fabrications.
Embedded wafer-level packaging (EWLP), embedded chip buildup (ECBU), and Chip-in-Polymer (CIP) processes completely embed the active component within a multilayer PCB during manufacturing. Rather than drilling cavities into the dielectric material, the second embedding technique places thin wafer packages directly into the buildup dielectric layers.
The thin package die-bonds to the substrate followed by the PCB manufacturer applies liquid epoxy or resin-coated film as a dielectric to mold the component into the substrate. While EWLP requires fan-in and begins at the wafer level, the ECBU method mounts active components face down to a fully-cured polyamide film mounted to a frame for dimensional stability and coated with polymeric adhesive. Then, the manufacturer builds the interconnect structure.
The CIP method, on the other hand, places thin components directly on top of the core substrate, bonds the chips with an adhesive, and embeds the devices into the polymer buildup layers of the PCB. Laser drilling establishes the vias to component contact pads and facilitates the mounting of passive devices directly over the embedded active component.
Life is Full of Tests
Cardiologists cannot assume that a pacemaker works. After ventricular and atrial lead placement occurs in a pacemaker implant, the cardiology team conducts pacing checks. Part of a pacing check involves verifying the “demarcation current” or the electric current from the central part of the body to the injured heart. A large current indicates that good contact between the lead tip electrode and myocardium has occurred.
Then, the pacing check tests for the proper millivolt sensing signal, the correct impedance, the appropriate pacing threshold, and the stability of the lead connections. Each of these tests ensures that the pacemaker senses the intrinsic rhythm of the heart, paces the ventricle correctly, and provides the energy needed to electrically capture the myocardial tissue.
Embedded active components require the same thorough approach to testing. Although embedding pays dividends by reducing the size of components and PCBs, the process can introduce defects. Smaller, thinner solder joints can crack. An inadequate amount of solder paste or incorrect soldering temperatures can also produce weak bonds and intermittent connections.
Decreasing the size of the PCB may increase the possibility of short circuits between traces. Mechanical stress on the PCB can crack the substrate while increased surface tension during soldering can cause tombstoning.
Given these possibilities, your test routine should check for open traces, short circuits between traces, and micro shorts. Because the embedding process often involves heat and vacuum pressure, you should also check for deformed traces or non-conductive vias. You may also want to use functional low-voltage tests for the active components. Newer versions of flying probe testers provide four probes on each side and can perform comprehensive functional tests on embedded active components.
Ensuring adequate testing routines when working with your circuit design can save you hassle in the long run.
There’s Another Side to All This
Late-1950s versions of pacemakers required an additional cart to hold the large vacuum tube-powered machines. With external leads attached to their chests, patients often complained about receiving constant electrical shocks. Today, miniaturized pacemakers have allowed heart patients to lead normal lives and introduced new procedures and design rules.
The introduction of embedded active components into PCB design introduces flexibility that changes fabrication processes, design rules and the approach taken by EDA providers. Managing this flexibility requires design tools that synthesize the electrical requirements, material requirements, and physical dimensions of a component for accurate placement and alignment. The design tools must also provide the capability to manage and configure layer properties.
Stackup and material changes occur earlier in the PCB design during the placement and interconnect phases. PCB designers benefit from this approach by gaining control over component size and placement. However, the different dimensions of active components and the use of wirebonding requires design tools that give the flexibility to move wirebond pads and to generate wirebonds from the silicon die to the PCB.
With the use of embedded active components, you also gain the capability to minimize electrical path lengths for high-frequency circuits. Minimizing path length by positioning passive components directly underneath the active component pin reduces parasitic inductance, capacitance, and noise. Additionally, you can integrate EMI shields directly around embedded components to reduce noise.
Altium Designer® assists you with your PCB design by managing how embedded components impact the layer stack through calculations and design rule checking. Stack management occurs through the creation of a stack for each unique combination of placed and cut layers needed by the embedded components included in the stack.
Embedding a component within the layers of the board automatically creates a Managed Stack. From there, Altium Designer checks for embedded components, tests the suitability of available managed stacks, and creates a new Managed Stack if necessary.
To learn more about using Altium Designer to manage embedded active components, talk to an expert at Altium.