Bare Die Used In Miniature Circuit Assemblies
The size of your thumb are roughly the dimensions to which electronic medical devices are shrinking. They are necessary to be easily implanted into people who are suffering from such life-altering disorders as chronic pain, and limb dysfunction. The new world of tomorrow is here!
Such devices require that circuitry be minimized, so that the device itself can be made smaller and more easily implantable or so that more features and capabilities can be added to an existing device. While market demands push devices to an even smaller size, manufacturers of these devices must keep in mind the need to make them reproducible and reliable.
As the use of neurostimulation devices gains momentum, consumers and medical professionals are demanding that implantable units be made smaller without sacrificing performance.
Classic reliable miniaturization techniques include the use of surface-mount devices (SMDs) on the top and bottom of a printed circuit board (PCB) instead of the use of through-hole components with packaged parts. However, these methods are no longer sufficient to meet the tiny size requirements. Instead, new techniques now take advantage of discrete components and the small size of semiconductor chips.
Miniature Circuit Assembly
Many advances in miniature circuit assembly technologies are the result of working with a bare die, or semiconductor chip. Although its circuitry is very tiny, the chip traditionally was put into a large package, negating the gains achieved through the circuit’s small size. This larger packaging was needed to accommodate the manufacturing methods in widespread use in electronic assembly fabrication facilities.
There are several ways to affix a bare die directly to a circuit board and thus eliminate the packaging. All methods require implementing production equipment such as a gold ball wire bonder or a flip-chip system that is capable of attaching bare die. It is essential to qualify any processes used for temperature and vibration variations. Employing these bare die–attachment techniques allows for up to 20 times the amount of circuitry to be placed in space traditionally taken up by large packaging.
3-D Chip-Scale Packaging
Yet another important gain in circuit assembly technology was achieved with the creation of 3-D chip-scale packaging, or 3D-CSP. With this process, considered the ultimate miniaturization technique, several COB or flip-chip circuits are assembled on flexible circuit boards. The boards are then folded or rolled to achieve the compact 3D-CSP package.
The smallest, most compact assemblies currently possible can be made using 3D-CSP, making it suitable for demanding size requirements such as those of implantable medical electronics. The technology is ideal for developing devices that need to incorporate wireless programming capabilities and a battery, as well as circuitry.
Two types of flip-chip bonding processes—solder bump and gold stud bump—are suitable for medical implantables (see Figure 1). For many types of medical implantable assemblies, the gold stud bump process is better suited. This process relies on the application of adhesive underfill to the substrate before the die is positioned, so the cleanliness of both surfaces can be ensured. The chip could be positioned and committed to final assembly with confidence that the underfill would be dispersed completely, free of contaminants and voids.
Flip-Chips In Action
Glued flip chip and stud bumping has been used in developing neurostimulation implants for maladies ranging from urinary incontinence and chronic pain to loss of muscle use. Such devices can be implanted using minimally invasive surgery. After implantation, a clinician can use a wireless programmer to set the stimulus parameter and timing patterns, which both the clinician and patient can use. A biocompatible battery is used to power neurostimulators. These can be recharged from outside the body by a recharging patch worn by the patient.
Some predict that the microminiaturization trend one day will produce nanobots, devices so minuscule they can be injected into the bloodstream to perform vital tasks, including unclogging blocked arteries and attacking tumors.
Minimizing circuitry means that devices themselves can be made smaller and be more easily implantable. More features and capabilities can be added. Traditional miniaturization techniques, such as the use of SMD, are no longer sufficient to meet the tiny size requirements. New techniques, including miniature circuit assembly technologies using Bare Die, 3D-CSP and flip chip, enable manufacturers to take advantage of discrete components and the small size of semiconductor chips.