Micro-Miniature Medicine

Reducing the size of electronics increases implantable possibilities

—edited by Stephanie Gooch

Few markets have witnessed as significant an impact from the miniaturization of electronics as products for the medical profession. With consumer devices, the reduction in form factor enhances the portability of “fun” objects like CD players and cameras. The military depends on size reduction for increasing the tools of the foot soldier. In medicine, however, a smaller package permits new avenues for treating a patient directly at the source of the complaint. In time, micro-miniaturization may even produce the nanobots of today’s fantasies, devices injected into the bloodstream to clear arterial blockages and assault tumors.

For now, however, advances in packaging technology have brought powerful implanted medical devices to a scale where they can be installed comfortably and safely within a patient. Tiny pacemakers and intra-cochlear devices (packages that offer the possibility of restoring hearing to certain patients) have been evolving for a few decades. The use of implantable stimulators for the treatment of chronic pain, Parkinson’s disease, tremor, etc. also offers great opportunity for miniaturization.

One company that concentrates on the design and execution of these devices, contractor Hi-tronics Designs, Inc., of Budd Lake, New Jersey, has been involved in the development of such products as pacemakers, neurological stimulators and arrhythmia monitors, for some of the most noted names in medical treatment products. One of their strategies has been a long-time partnership with Valtronic USA, Inc, a multinational electronic manufacturer that specializes in miniaturization for industrial and medical applications. “We’ve been working with Valtronic since the mid 1990’s,” remarks Gregg Turi, Vice President of Engineering at Hi-tronics. “Although we do custom IC designs in-house, we had not done Chip-on-Board or Flip Chip technology in-house. Initially, Valtronic was our SMT vendor, stuffing boards prior to our purchasing our own SMT line. As our relationship evolved, more of Valtronics packaging and miniaturization capabilities were introduced. They’ve worked with us on packaging one of our arrhythmia monitoring devices. More recently, we worked with their designers on size reduction issues for another implantable device.”

Implantable stimulators

Neurological stimulators have been in existence for many decades, even being part of the annals of “quack medicine” during the early days of generated electricity. In its present state of refinement, mild applications of electricity can be used as treatment for pain and illnesses such as epilepsy.

More recently, researchers found that a weak current, generated by pulsed electromagnetic fields or capacitive coupling, could be passed through a target site to encourage the restoration of bone. While the first test device used external generators and implanted electrodes, it was quickly found that an invasive device could be constructed using DC current. The product is gaining acceptance in many areas, including use as an adjunct to spinal fusion surgery.

HDI has been involved in development and manufacturing of implantable products for both of these markets. And when they wanted to further diminish the size of one neurostimulator package, they teamed with Valtronic, who responded with their Chip-on-Board strategy for significant size reduction in a portion of the circuitry.

Methods of miniaturization

Valtronic has successfully demonstrated packaging techniques that take components to smaller dimensions. Surface Mount Technology (SMT) is the one of the oldest miniaturization methods, having been introduced around 1980. SMT consists of mounting leadless components and packaged ICs directly to pads on a circuit board, either through wave soldering, or more recently, with specialized adhesives. The technique lends itself to automation, as “pick and place” machines can be used to position spooled devices on the PCB prior to attachment. ICs used in SMT are packaged, but the usual through-hole leads are replaced with contact pads arranged around the periphery of the chip or device.

SMT is often used alone for the assembly of products not requiring a high degree of miniaturization, such as computer motherboards, peripheral cards, and other types of consumer electronics. When included in more aggressive miniaturization strategies, SMT is used chiefly for the attachment of resistors, capacitors, and other discrete components. SMT components themselves can be miniaturized, as with the recent introduction of “0201” (20 × 10 mil) components destined to replace a significant portion of the current “0402” (40 × 20 mil) variety.

Chip-on-Board and Chip-on-Chip

The next level of miniaturization is similar in concept to SMT, in that ICs and discretes are mounted directly to the PCB. However, significant space savings are realized by using unpackaged ICs. With the Chip-on-Board (COB) process, bare dice are mounted to the PCB with contact pads facing outward, and fine wire interconnects are bonded from the die to corresponding attachment points on the PCB.

The Chip-on-Chip (COC) method can be thought of as “stacked COB.” COC bonds a smaller chip to the top of the first chip, also with contact pads facing outward. These pads are also connected to the PCB with bonded wires. For both COB and COC, the chip is sealed to the board and encapsulated to protect the assembly. The COB and COC techniques can provide size reductions of approximately 50% as compared to conventional assembly techniques. However, a limitation of COB and COC is that the bonded wire interconnects around the chip(s) consume PCB area that prevents closer placement of chips. In addition, COC cannot use same-size or overlapping die because these chip placements interfere with bond wires between the PCB and the lower IC die.

The Flip Chip process escalates the level of miniaturization by eliminating the wire leads between chips and the PCB. Flip Chip is older than SMT, having been introduced by IBM as the “C4” process in the 1960s. However, the adoption and further development of Flip Chip has grown dramatically since the mid 1990s. Furthermore, Flip Chip forms the basis of more aggressive miniaturization methods, such as three-dimensional chip scale packaging described later.

Generically, Flip Chip refers to several related, but significantly differing, methods for attaching unpackaged integrated circuit dice, face down, to circuit board substrates. Common flip chip bonding methods include solder, adhesives laced with conductive fillers, and non-conductive adhesives used in conjunction with metal bumps previously applied to IC contact pads.

The ultimate miniaturization technique is three-dimensional chip-scale packaging, or “3D-CSP.” The definition of standard (flat) chip scale packaging is somewhat loose, as there are currently 50 or more varieties of CSP. However, two common elements of most CSP techniques are the use of a solder ball grid array (BGA) attachment technique, and a finished package size only slightly larger (about 1.2) the raw chip size. As with Flip Chip, solder connections are located between the chip and PCB, resulting in virtually zero lead length, low power consumption, and excellent circuit speed.

Three-dimensional chip scale packaging (3D-CSP) is a development of Valtronic. 3D-CSP achieves the smallest, most compact assemblies currently possible. With 3D-CSP, Flip Chip based circuits are first assembled on flexible CBs along with SMT devices. Components can be mounted on both sides of the PCB. Afterward, the circuit board is folded or rolled to achieve a three-dimensional package. For successful 3D-CSP, boards must be designed with great consideration of bend radii, chip spacing, trace routing, power distribution, and other parameters. The result is a 75% to 80% size reduction, as compared to traditional assemblies.

What’s made the partnership between Hi-tronics and Valtronic so cost effective, says Turi, is that “Valtronic delivers fully assembled and tested modules,” not just design proposals. Valtronic is continuing development of packaging technology for next generation designs. One concept is called “Minext,” in which critical PC board real estate is gained by mounting a second chip, contact pads up, on top of a Flip Chip. The second chip is then attached to the board’s contact pads using conventional bonded wire leads. Additional space savings have not yet been calculated.