Nokia recently announced that it is recalling 14 million phone chargers due to the possibility of the circuitry shocking the user. Imagine the challenges if the circuitry had to be removed from someone’s body before replacement. In many ways, implantable electronics can be compared with the electronics being used in a cell phone; both are finding ways to get maximum functionality into a minimal space.

Tom Zemites

May 7, 2010

19 Min Read
The Evolution of CRM Power Technology

These industries are driven by a number of market and technological pressures, both. Both the consumer and medical electronics markets continue to innovate and introduce new features and applications. What sets implantable electronics apart is the need for absolute reliability in addition to performance, size, and cost.
 

Over the last decade, double-digit revenue growth has been the norm in the medical implantable industry. The market for pacemaker-type products expanded in both application and geographic market. But in recent years, the growth rate has dropped below 10%. And in the past 10 years, the implant industry has become highly competitive and undergone consolidation. In addition, a healthcare environment of cost containment, managed care, large buying groups, government contracting, and hospital consolidation has added pressure to drive down costs. Significant investment in research and development is necessary to introduce new products. What can be done to drive the implant market back to double-digit growth?
 

Electronics have been used in implantable pacemakers since the invention and production of transistors in the late 1950s. The pacemaker grew in functionality and acceptance throughout the 60s and 70s. The implantable cardioverter-defibrillator (ICD) was first implanted in humans in 1981. Together, these two products compose the life-critical cardiac rhythm management (CRM) market segment. Throughout the history of CRM electronics, circuits and components have changed very slowly, with good reason. With lives on the line, patients and caregivers can’t be worried about the reliability of electronics inside the body. But the pressure for smaller size, increased functionality, and extended battery life requires improvements in the current packages.
 

Miniaturization is the key growth driver for implantable medical devices.1 To the patient, a small device is less intimidating than a large one. The incision is smaller, the procedure is less obtrusive, the body heals more quickly, and the implant is less noticeable. Moreover, with smaller electronics, more options can be fit into the package. Increased features in pacemakers and ICDs include RF transceivers for wireless communication, advances in sensors to optimally time pacing and defibrillation shocks, and backup systems in case the main system fails. Processing power and memory have also increased, and integrated circuits (ICs) are being stacked on top of each other. However, in most cases, the discrete components remain unchanged. With such innovation, discrete packaging has become a critical concern.
 

This article touches on the market pressures CRM manufacturers are facing and what is being done to address the market. Moreover, it explores innovative options in electronic packaging that enable discrete devices to improve in performance, size, and cost, while maintaining reliability.
 

Figure 1. The market for electronics in treating chronic diseases continues to expand.

Emerging Markets

The aging of the world’s population will play a key role in the need for implantable electronics. By 2050, more than 2 billion people will be over the age of 60 and more than 2 million will be over 100 years old according to the World Health Report. The average age of a pacemaker recipient is 70. These demographics continue to drive the implant market.
 

Worldwide expenditure for healthcare is on the rise. The United States spends nearly $7000 per person per year on healthcare. The Office of the Actuary estimates that U.S. healthcare spending is approximately 16% of the gross domestic product (GDP) and it is expected to continue its historical upward trend, reaching 19.5% of the GDP by 2017.
 

Remote and emerging markets are becoming increasingly affluent and will be one of the largest opportunities for implantable device growth. China healthcare expenditure increased from 3.7% of the GDP in 1995 to 5.6% in 2007. Currently, China spends $300 per person on healthcare, but as part of its stimulus package, it will spend $124 billion in healthcare upgrades in the next three years. Taiwan’s national healthcare expenditure increased to 6.3% of the GDP in 2005.2 India’s government proposed in 2008 to increase public expenditure on healthcare from 1% to 3% of the GDP. These countries do not produce their own advanced medical supplies and respect U.S. firms’ brand recognition, reliability, and technological superiority.
 

The 2009 medical electronics market is estimated at $2.54 billion.3 The medical diagnostic therapy market segment containing CRM electronics is estimated at $550 million and is growing at a 14.7% compoound annual growth rate. In comparison, due to pricing pressures and the lack of new applications, the CRM market growth is expected to remain below 10%.
 

Diversification

The medical community is working to create new customers for CRM devices. Most patients who receive a defibrillator have late-stage heart disease. By conducting clinical trials on patients with early-stage heart disease, they hope to reveal whether implanting defibrillators will yield health benefits.4 If treating early-stage heart disease patients with these devices can provide better health and longer lives while keeping people out of the hospital, insurance companies are more likely to approve the procedure.
 

Figure 2. A block diagram of an implantable cardiovertor defibrillator (ICD) shows the pulse generators, which contain all of the electrical circuits for the device. To see a larger version, click here.

The top three producers of pacemaker and ICD systems—Medtronic, Boston Scientific, and St. Jude Medical—generated $26 billion in combined revenue in 2008. Nearly 35% or $10 billion of their revenue was generated from the CRM market. Compare this with 12 years ago when nearly 60% of revenue was created by CRM products.
 

Much of the new growth of these companies comes from efforts to expand the therapies they treat (see Figure 1).5 By modifying the electronics found in pacemaker products, new applications have been developed to treat the neurological system of the body. Neurostimulators do not cure underlying causes but instead mask or block symptoms. For example, devices block chronic back pain, leg pain, and migraines. Others modify behavior associated with depression, anxiety, obsessive-compulsive disorders, and bulimia.
 

CRM Applications

Pacemakers replace the electrical pulses generated by the normal healthy heart sinoatrial (SA) node. Arrhythmias occur when the heartbeat is too fast, too slow, or irregular. The pacemaker unit delivers an electrical pulse with the proper intensity to the proper location to correct arrhythmias.6 During periods when the SA node produces its own electrical signal, the pacemaker does nothing but monitor. Some pacemakers are also rate-adaptive, meaning that they can monitor the activity level and change heart rate accordingly. A pacemaker may have one or two leads. A pacemaker with one lead is called a single-chamber pacemaker. Where the lead sits depends on where the signal problem in the heart is located. A pacemaker with two leads is a dual-chamber pacemaker, one lead in the right atrium, and the other in the right ventricle. Which type of pacemaker is needed depends on the kind of rhythm disturbance and the overall heart function.
 

ICDs have all the functions of a pacemaker but also send a high-voltage shock to the heart when the muscles lose their natural rhythm and start to fibrillate. Advanced electronics apply a large dc electric current to the heart that stops all erratic electrical activity and provides the SA node an opportunity to take control of the heart rhythm.
 

A block diagram of a typical ICD is shown in Figure 2. CRM housing typically contains a battery, a pulse generator, and a connector block. The pulse generator, shown in the block diagram, contains all the electrical circuits of a CRM device. Power management in a CRM device is critical. The goal is to have the battery last 5–10 years before replacement.
 

Sense and Control Components. The sense and control portion contains a microprocessor for computing, memory for storage, a pulse generator to supply the shocks, and a sense amplifier to monitor when shocks are needed. These components are combined into one or more ICs for size and cost savings. Most ICs operate at very low voltage to conserve energy; typically less than 3.3 V. These low-voltage circuits are sensitive to electrostatic discharge (ESD) and must be electrically isolated and protected.
 

Sensing technology is incorporated into the electrode or implantable sensors. Electrical impulses are transmitted to the heart via a lead, which is attached to the pulse generator via the connector block.
 

High-Voltage Charging. In the charging stage, power is taken from a lithium-based battery and is boosted from approximately 4 V to ultimately more than 700 V. The high voltage is used to defibrillate the heart. When a fibrillation episode is sensed, power is drawn from the battery to charge up one or more storage capacitors. This power is then released and directed via the switches to the heart leads. High-voltage rectifiers are used to steer the voltage in this stage.
 

Switching Electronics. Switches are used to route the high-voltage pulse from the charging stage to the heart leads. Various high-voltage devices are used in the switching stage including insulated gate bipolar transistors (IGBT), silicon controlled rectifiers (SCR), metal oxide semiconductor field effect transistors (MOSFET), rectifier diodes, and remote gate thyristors (RGT). Selecting which device to use requires the designer to choose between the complexity of the drive circuit, device performance, and device overall circuit board footprint.
 

Power switches have common characteristics. First, they are large; these switches can be rated as high as 1600 V and 50 A. Some have described them as silicon rocks. An ICD delivers an incredible energy pulse for a very short period of time, typically only milliseconds. There is little time for heat dissipation so the silicon must absorb the energy. Second, both sides of the die are electrically active and need connection, which presents assembly challenges. This is different from ICs, which have one active side and need electrical connection only on the top part. Third, a high voltage pulse has a will of its own and can arc to unwanted places. Spacing between components, wire bonds, and a protective coating becomes an important consideration.
 

Protection from Electrostatic and Transient Voltages. Transient voltage suppression (TVS) diodes are used to protect sensitive electronic devices. The diodes shunt to ground stray electrical pulses picked up inductively by the heart leads or the case. These pulses can come from straying too close to strong magnetic fields from sources like medical equipment, arc welding equipment, car engines, or external defibrillation devices.
 

Energy pulses generated inside the case are a concern. When the ICD releases its high-voltage pulse, the sensitive IC electronics must be protected. The control stage is protected by power switches that block any stray energy. These are called blocking switches and typically MOSFETs are used to control them.
 

Figure 3. As electronics advanced, chip-on-board assembly paved the way for chip-on-chip and led to 3-D packaging. All three configurations are used throughout the industry.

Evolving Electronic Packaging

In September 2009, at a medical electronic symposium, Paul Gerrish of Medtronic Microelectronics Center commented that CRM device manufacturers know how to package ICs densely.7 Gerrish’s big concern was what else could be shrunk in the next-generation packaging products. Work is being done on shrinking discretes, transformers, capacitors, and batteries. The author wonders whether power discrete manufacturers are working on the right solution.
 

Improvement in substrate assembly has allowed medical device manufacturers to continually shrink CRM devices.8 Chip-on-board assembly, chip-on-chip, and now advanced 2-D and 3-D packaging are in use throughout the industry (see Figure 3). It is estimated that these techniques reduce the overall circuit space by 60–80%. Die stacking decreases interconnects, improves testing, and allows the mixing of wafer process technologies in a small area. The trade-off for the footprint reduction is cost. Expensive materials and cumulative yield issues tend to drive up costs.
 

As discussed previously, power discrete devices such as IGBTs, SCRs, MOSFETs, and rectifiers provide unique layout and packaging challenges to circuit designers. These include making electrical connections on both sides, controlling high-voltage arcing, and creating a small footprint. Moreover, remember that a large die size is necessary to handle the required power (silicon rocks). Discrete packaging needs to evolve into a chip-scale package that can be incorporated into stackable designs and at the same time be manufactured easily to help lower costs. There are various options for such packaging that OEMs can use, each of which exhibits benefits and limitations.
 

Chip and Wire. Chip and wire is the traditional packaging method for implant applications. It requires purchasing a thoroughly tested and inspected chip from a vendor, making a connection to one side by attaching it with conductive epoxy or solder to the circuit board, and connecting the other side into the circuit by using thin wire. A protective coating is placed over the die and wire bond to help prevent high-voltage arcing through air and to secure the wire bond firmly in place. Chip and wire is one of the most cost-effective ways to attach power devices. The major drawbacks are the extra space for wire bonding and the reliability of the wire bonds. Other common challenges with chip-and-wire packaging include handling, marking, pick and place, breakage, and conformal coating.
 

Chip and Clip. Chip and clip is used in power package technologies. The clip is a ribbon of copper connecting the top side of the die to the circuit board. The copper ribbon expands the contact region allowing for larger current-carrying capability and improved heat dissipation. Performance improvements when using a clip are negligible for CRM devices due to the short duration of the pulse. Arcing remains an issue because the clip lies flat on the die, limiting the space between the terminals.
 

Figure 4. A power silicone on insulator (PSOI) enables designers to eliminate back-end manufacturing steps.

Conventional Epoxy Molded Packaging. A plastic (epoxy) surface mount, ball grid array (BGA), and quad flat pack no leads (QFN) solve some of the problems of chip and wire, but generate others. Internal wire bonds bring the front and back of the die to the same surface. Epoxy encases the die, which helps prevent high-voltage arcing. Moreover, the epoxy covering protects the die during shipping and can withstand standard pick-and-place equipment. Reliability can be ensured with the proper inspection, burn-in, temperature cycle, and testing. There is a shared economy of scale with the commercial world, although medical OEMs have to be wary of product life cycle differences. (Commercial cell phone models last only a few months and are quickly replaced with new designs.) The major drawback is again size. Adding a lead frame, wire bonds, and an outer epoxy package increases the overall footprint of the device, especially for power devices.
 

Sidewinder Epoxy Molded Package. Sidewinder is an unconventional plastic surface-mount package that turns the die on its side and connects it directly to the lead frame. The greatest benefit it offers over a standard plastic package is an x-y footprint with reductions up to 60%. The trade-off is that the height of the package may grow up to 50% because the die is essentially flipped on edge. Moreover, because it is a nonstandard packaging technique, it is more costly than standard plastic.


Flip Chip. Flip chip describes a chip-scale package if all the contacts of a die are on the same side. The engineer applies solder and simply flips the die over and reflows the solder making attachment to the board. Since CRMs are a power device in which the backside is active, such a connection poses a problem. This technology is used in the CRM industry today, but so far it has primarily been introduced in low-voltage products (less than 30 V).
 

One method is to solder the die on a metal carrier to bring the backside contact to the front. The copper carrier is bent into an inverted L shape, which brings the backside contact to the same plane as the die. Solder bumps are placed onto the die and the carrier to allow for flip chip attach. The method complicates the x, y, and z planarity because the die can move or tilt when soldering to the carrier. For high-voltage applications, arcing from the die to the metal carrier is a concern.
 

To eliminate the planarity problems created by the carrier, another method to bring the backside contact to the front is to create a path through silicon. The die size is expanded to include nonactive silicon adjacent to the active silicon region. A channel is created through the nonactive silicon by creating a sinker or a heavy dope region through the epitaxial (epi) layer. This provides the path to bring the backside of the wafer to a front-pad location. By applying backside metal, current can flow from the active region, through the channel, to the front side. The die size is larger, but not as much as when a carrier is used. This method works for low voltage and a couple of amps, but it isn’t feasible for a 1000-V, 50-A pulse.
 

There are no flip chip high-voltage packages on the market, so something new must be developed to use this technology in CRM. The major difficulty is creating a conduction path for high current flow. One solution may be the use of metal plated through-hole vias.9 Using plated through-hole vias is a proven manufacturing process.10 A specialized metallization process prevents the conducting metal from migrating and contaminating other process steps. The top of the die is conducting current, so stacking die must be done carefully. A second option for high-voltage flip chip is an epi mode diode. This method uses standard processing steps. Standard die thickness of 12–20 mils would be incorporated. The issue is that the epi would be thicker and larger than standard processing, thereby adding to the cost. Moreover, the common problems with handling, marking, pick and place, breakage, and conformal coating still apply.
 

Power die stacking is applied today. It requires starting with two or more known good die and soldering the die together by the use of a solder preform that fits in between the die.11 The final die stack is soldered on the bottom and wire bonded on the top, like a typical chip and wire, but only requires half the board space. It allows mixing of wafer process technologies in a small area. The process dramatically reduces the x-y footprint over other approaches, but at a higher price. The stacked-die manufacturing yields are a function of the yields of the die being packaged. Naturally, cumulative yield losses drive costs higher.
 

Process Integration of Power Discretes. This process would solve many of the packaging challenges faced by medical OEMs, e.g., combining the high-voltage rectifier with an IGBT or SCR. Such integration could result in fewer components, simplified testing, higher reliability, and improved cost structure. So why isn’t it practical? Basic wafer process steps are needed to form active regions on a particular device. The process steps of forming a rectifier and a SCR are different enough to prevent combining the two parts into one component.
 

Power Silicon on Insulator (PSOI). PSOI is a sealed chip-scale package that takes a different approach to bringing the electrical connections to the same side (see Figure 4). PSOI develops the active regions on the same side using standard processing steps but joins the regions with a top metallization. The top side is then sealed and protected by attaching a top side insulator. The die can then be sawn in any form (e.g., single, duals, and quads). The concept is to eliminate any back-end manufacturing steps. After sawing in wafer form, simply test and ship the product in containers (waffle or gel packs) for automatic pick and place.
 

Top, bottom, and side insulators isolate the junction from environmental contaminates and moisture sensitivity. The process eliminates wire bonds and protective coating, reduces overall chip size, and can be manufactured with through-hole vias for stacking. It provides desirable thermal characteristics (i.e., thermal resistance path of <40ËšC/W) and provides small size while maintaining surge performance. This process provides die-to-die electrical isolation and reduces parasitics. Overall yields have to be on par with standard wafer yields to match costs. Depending on the technology, overall circuit footprint can be reduced 20–55%.
 

Conclusion

New CRM products are packed with new features and benefits, but maintaining a clear competitive advantage grows more difficult in today’s market. There has been a drive to take CRM devices and diversify into new markets. Coupled with the aging world population and the increase in medical spending in developing countries, CRM remains a strong market for implantable medical devices.
 

Miniaturization, performance, and quality remain as leading technological challenges for today’s design engineers. Reducing the size of power devices cannot be accomplished using the next-generation lithography node. It requires advanced 3-D circuit packaging and stacking of flip chips on flexible substrates. Stacking power devices is a proven—but high-cost—method due to the accumulation of electrical and mechanical yields. A new chip-scale package that can handle high voltages and bring contacts to the same surface is needed. It must have reliability, manufacturability to generate good yields, space efficiency, even with the required high-voltage spacing, and cost-effectiveness.
 

Several options exist for creating planar contacts on a power device. What looks most promising to meet these criteria are metal plated through-hole vias, epi mode diode, and PSOI packaging technologies.
 

References


1.R Srinivasan, “Implantable Devices: Challenges and Opportunities,” Medical Device Technology 13, no. 9 (September 2009).
2.“Technological Change and the Growth of Health Care Spending,” Congressional Budget Office (Washington, DC: January 2008): 13–15.
3.“Implantable Medical Devices,” American Institute in Taiwan (Taipei, Taiwan: 2005).
4.P Benesh, “Medical Device Maker Looks Forward to Expansion of Heart Market,” Investor Business Daily, May 6, 2009. Available from Internet: www.investors.com/NewsAndAnalysis/Article.aspx?id=476091&Ntt=.
5.P C Tortorici, “Keynote Address: Implantable Medical Devices; Past Successes, Current Status, Future Possibilities and Challenges,” MEPTEC and SMTA Medical Electronic Symposium (Tempe, AZ), September 2009.
6.Case History “The Rhythm of Life,” The Economist Technology Quarterly (London: March 2009).
7.P Gerrish, “Keynote Address: Implantable Medical Electronics: A Leading Application for Integrated 3D Systems,” MEPTEC and SMTA Medical Electronic Symposium (Tempe, AZ), September 2009.
8.K Takahashi and M Sekiguchi, “Through Silicon Via and 3-D Wafer/Chip Stacking Technology,” Symposium on VLSI Circuits Digest of Technical Papers (Honolulu, HI, 2006).
9.J Van Olmen, et al., “3D Stacked IC Demonstration using a Through Silicon Via First Approach,” The Interuniversity Microelectronics Center, August 2009.
10.S Ramaswami et. al., “Through-Silicon Via Technologies: Challenges and Solutions,” Panel Discussion, Semiconductor Today, May 2009.
11.J Ohneck, “The Shrinking World of Implantable Medical Electronics,” Medical Electronics Manufacturing, Fall 2007.
 

Tom Zemites is strategic marketing manager for Microsemi in the Scottsdale, AZ, office.

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