Medical device engineers at the front of the product development process will likely soon — if not already — be encountering revolutionary new and different product design, assembly, and manufacturing considerations not dealt with before. At the same time, these newer, more advanced assembly and manufacturing techniques and capabilities allow medical OEMs to go beyond prototyping and into pilot, medium, and high production runs. Consequently, medical OEMs can maintain manufacturing in the Untied States and keep their intellectual property (IP) safe and secure.
Advanced and small medical electronics — wearable, portable, implantable, ingestible, and insertable — devices are state of the art and at the forefront of a new round of technology trends involving printed circuit board (PCB) assembly and manufacturing. Many of these medical devices are extremely small, often the size of a thumbnail and sometimes, even smaller.
Examples of these miniaturized devices include implantable defibrillators, cardiac pacemakers, programmable infusion pumps, continuous glucose monitoring (CGM) devices, wellness and emergency response safety monitoring systems, and even smaller devices like the so-called ingestible smart pill as shown in Figure 1.
But It’s Not Traditional PCB SMT Assembly
A wide array of conventional medical electronics equipment and products are largely based on traditional PCB surface mount technology (SMT) assembly and manufacturing practices. However, with the onset of miniaturized medical devices, the industry is rapidly moving toward PCB microelectronics assembly and manufacturing to help medical OEMs produce these newer products (see Figure 2).
Medical miniaturization is coming to the forefront because doctors, hospitals, and other medical care facilities require more functionality, increased portability, and more robustness to upgrade outpatient practices, surgical procedures, and mobile monitoring equipment. Moreover, these highly advanced devices operating directly with the human body are driving the need for greater medical miniaturization.
A PCB microelectronics assembly line fitted with a number of efficient operations must be able to deal with this emerging device miniaturization efficiently and reliably. Among them are such operations as precise epoxy dispensing, highly accurate component placement during die attach process followed by accurate wire bonding, excellent inspection systems, and final testing tools.
Precise epoxy dispensing is important for effective die attach and encapsulations. Some component placement systems require tolerances of within ±5 µm accuracy. Inspection systems must use inprocess verification methodologies. This allows a product to have self-verification and correction mechanisms built into it for the full manufacturing cycle, thus enabling optimal product creation. Also, final testing tools such as bond shear testing and pull testing are critical so that final product testing can be automated, allowing time to ship the product to be reduced.
New Technology Terminology
When it comes to PCB microelectronics assembly, medical OEM product development engineers are becoming familiar with such terms as multi-tier wire bonding, chip on board (CoB), die attach, and flip chip ball-grid-array (FCBGA), among others. These technologies are all geared toward consuming less PCB real estate but can also significantly increase the total circuitry’s functionality.
However, design advantages and disadvantages, as well as trade-offs, are closely associated with these technologies, depending on specific medical device application. In these instances, it’s prudent for medical OEMs to work closely with their PCB microelectronics assembly management to determine appropriate assembly methodologies.
Wire bonding goes back to the transistor age and has been used to connect a silicon die to its package, most recently to such packages as BGAs. However, now that dramatically increased functionality is in demand for medical miniaturization, multi-tier wire bonding is entering the process to provide such extra functionality.
Today’s complex microchips are designed to provide that needed functionality. Combining a single die with multi-tier wire bonding captures that additional functionality. Simultaneously, this combination significantly reduces chip count, thus decreasing printed circuit board real estate.
There can be two, three, and four levels of wire bonding, in some cases called stacked wire bonding (see Figure 3). Also, multi-tier wire bonding offers OEMs a solution when the number of inputs/outputs (I/Os) are far beyond the traditional ones that are used in the single wire-bonding application.
Meanwhile, CoB provides an extra measure of space on the PCB because device packaging is not involved, and the chip or die is directly attached to the board. Since a package is much larger than the die, itself, the form factor is significantly decreased. A prime feature of CoBs is their flexibility, which allows interconnection changes to be made.
For higher thermal medical device applications, CoBs provide high levels of heat dissipation through the use of various epoxies. Moreover, fanning out of CoB is easy and not complicated for medical device PCB designers. Fanning out means assigning optimal paths from the die bond pad to the substrate pad.
When CoB fanout is optimally designed, traces coming out of the die can be optimized, using a straight and short path toward the substrate pad. Plus, those interconnects can be more optimal compared to wire bonding. Again, this is one advantage CoB has over wire bonding for medical devices and other applications.
Also, for high-speed, high-frequency medical device applications, wire bonding isn’t as effective due to the capacitance and inductance it creates. CoB operates more optimally in higher-frequency, high-speed ranges because it has less capacitance or inductance built in while traveling along the forward and return current path.
Die attach closely resembles CoB. But its application is more extensive since this process, as the name implies, attaches a die to a package, substrate, rigid or flex circuit, another package, or even a die attached to another die. There are three different die attach methods: epoxy, eutectic, and solder.
In the epoxy method, a fine and extremely precise dispenser very accurately dispenses the epoxy. Eutectic die attach uses an aluminum or gold metal layer. Solder attach is a common type of die bonding due to better thermal conductivity of the solder material itself. After die attach is completed, wire bonding is performed to create the joint that connects the die to the substrate.
The combination of a flip chip and BGA packaging is a third major technology applied during microelectronics assembly (see Figure 4). This combination reduces the conventional BGA package by one-third to one-fourth the original size. However, all necessary connections are efficiently accommodated in that space. As a result, less real estate is used in a miniature flex or rigid-flex circuit board.
In this instance, the medical OEM customer and PCB microelectronics assembly management must be on the same page. Both must have an understanding of new advances in the redistribution of layers (RDL). In the case of a flip chip, in particular, RDL conveys signals from a die out to pins that solder it to the circuit board and through the BGA, effectively reducing the amount of real estate involved.
In summary, an EMS company that provides PCB microelectronics assembly should be knowledgeable about newly emerging medical products like wearables, implantables, insertables, and ingestible electronic devices, as well as all the nuances associated with them. Because this is an emerging area, considerable research still must be performed to ensure that these devices produce desirable results without creating harmful effects in the human body.