The increased functionality of today’s medical devices is astounding. Optical devices, for example, analyze chemicals, toxins, and biologic specimens. Semiconductor devices sense, analyze, and communicate. Microelectromechanical system (MEMS) devices utilize inertial methods to detect motion, direct light, and move components over short distances. Radiofrequency (RF) devices communicate wirelessly to other devices directly and remotely over the Internet. Handheld acoustic devices scan the body and build a virtual 3D model that shows conditions in the body. The innovation currently happening in the medical device industry is staggering, limited only by imagination and finding technical methods to implement the vision.
To achieve this advanced functionality, a broad variety of disparate components to sense and communicate must be integrated and combined with electronics that process and analyze data to provide all of the necessary functional capabilities. Engineering and manufacturing these complex devices requires heterogeneous integration; the process of combining multiple, different parts that together have the functionality required that gather process and communicate the important information.
The Design Stage: Understanding Factors That Affect Heterogeneous Packaging and Assembly
As with most products, optimizing performance while lowering the cost starts with, and is dominated by, good design of the medical device. Heterogeneous integration (HI) increases the functionality of the device, though it makes the design and manufacturing process more complex. Considering HI during design is essential to ensure functionality, user friendliness, manufacturability, scalability, and reliability — all of which affect cost. The design stage is the time to make decisions that impact down-the-line production and use of the device in the field. Consider the following factors when designing packaging and assembly:
Software. Recognize that the use of design software, particularly functional simulation, may not always be usable in designing devices that heterogeneously integrate components with electronics. Some software may not function at the level of sophistication needed when designing HI devices. The design team may need to experiment with a combination of HI-enabled software combined with traditional and custom software.
Materials. Know that good data is essential on material choices and their properties. Without this, a blind spot may emerge. This commonly happens, for example, during the design of optical devices that require extreme mechanical stability to avoid unwanted optical effects over the life of the device. Optical attenuation, for example, happens during a thermal, stress-induced motion and from yellowing, crazing, etc. RF properties of many materials (the dielectric constant and loss tangent), as another example, vary with frequency, water content, and in other ways that may be poorly documented in the early stages, before they happen. Medical and biotech devices often directly come into contact with fluids or organic materials that may either be contaminated by materials in the device or may cause the device to deteriorate in some manner. One extreme example is an implantable device where the materials that come into contact with tissue must cause no harm. Designing to avoid detrimental phenomena is important but can be difficult, even when the material properties are well known.
Components and materials. Sourcing heterogeneous components is complex due to the wide variety of parts, the number of suppliers, the length of the supply chain, the cost of margin stack from multiple vendors, and the need for traceability and compliance to a variety of quality requirements. Developing and managing the supply chain should begin with design. But that is only the beginning because good design is of no value if the parts, materials, and equipment required are not available in a timely manner with acceptable cost and quality, over the long term.
Cost. One of the most frequent and demanding asks from OEMs is to reduce the size of heterogeneous devices. While the primary reason for reducing the size is usually to make the device easier to use, size reduction also minimizes the amount of material required to build the device. However, cost increases by the number of parts, the complexity of the supply chain, the number of assembly processes, and the increased precision required by the small size of the device. Making intentional, thoughtful decisions at the design stage helps manage expectations — and costs — over the production cycle.
Heterogeneous Integration in Medical Device Assembly
While mechanical assembly methods utilizing clips, screws, snaps, and other parts is used for multifunctional devices, electronic methods (soldering, thermosonic metal-to-metal bonding, thermally and UV-cured epoxies, acrylates, and other organics) are often advantageous. These methods require a minimum of space, are compatible with most of the new heterogeneous components, are low-cost, well-developed, widely understood, and readily available (see Table 1).
The specific and unique characteristics of the parts that HI devices utilize often require improvements and modifications to assembly processes such as:
Reducing the temperature that components are exposed to during assembly, below the typical electronic range of 240 °C, sometimes as low as 40 °C. This severely limits the number of electronic processes and materials and sometimes requires the development of new joining methods, materials, and equipment.
Locating parts, such as optical fibers to lasers, with submicron accuracy and maintaining that location throughout the operating life of the device.
Sealing interfaces to prevent liquids from leaking because the volume for analysis is small, or the liquid is potentially harmful to the device, user, or environment.
Modifying equipment, fixtures, and processes to build the product economically when the number of units to be built is small compared to the typical run quantity for electronic devices.
Providing information systems to track, monitor, gather, store, and report information that highly functional devices require and produce.
Building an assembly operation capable of running the increased number of processes often encountered when the number and variety of added components, along with their unique assembly process requirements, are added to conventional electronic methods.
Unusual electrical requirements such as extremely high currents (hundreds of amps), low currents (picoamps), very low voltages, or extreme electronic and noise minimization.
HI Manufacturing Process Steps
HI usually requires a greater number of assembly steps. At Promex, for example, it is common to need as many as 50 (or more) steps for heterogeneous assembly of a single device.
Figure 1 illustrates the overall heterogeneous assembly process. Details of that process are shown in Figure 2, Figure 3, and Table 2. Assembly often starts with the conventional surface mount technology) SMT process (Figure 2), which results in an SMT subassembly. Figure 3 illustrates how that SMT subassembly is often used as a platform on which die, which are often custom, are attached and connected. To build the final device, the subassembly is then processed further, with the addition of specialized parts using the appropriate processes. Table 2 provides examples of unique assembly issues and solutions for a variety of special components.
In addition to the highly specialized processes in Table 2, more common packaging methods can also be used for heterogeneous devices, including:
Die stacking with and without spacers.
Flip chip.
2.5D integration utilizing silicon interposer.
3D integration using through-silicon vias.
System in package (SIP) utilizing multiple interconnected die and parts.
While the assembly steps and packaging methods shown in Figures 2 and 3 are a good starting point, HI often requires combining these conventional processes with specialized processes specific to the device. For example, while most electronic devices are built on printed circuit boards, many HI devices are physically small and thus must utilize an equally small circuit board or other types of substrates — such as thick-film ceramics, thin-film ceramics, silicon, glass, or flex circuits. Some of these are panelized, while others are provided as individual parts. Because processing multiple devices in parallel is a good way to reduce cost, substrates that are not panelized can often be placed in fixtures that not only enable parallel processing, but also minimize handling or enable mechanizing of handling, which can mitigate damage during processing.
In the future, it is likely that a greater number of devices will be designed, developed, and manufactured using the heterogeneous integration philosophy due to the functionality that results from combining the data gathering, analysis, storage, and communication capability of electronics with components that interact with the user and their environments to gather information and communicate information.
Although hurdles must be overcome to cost-effectively build robust medical devices with heterogeneous integration, experienced microelectronics development and assembly specialists have invested time and resources in breaking down the hurdles to unlock the benefits of the technique.
If a device combines semiconductor devices with specialized components that interact with the environment to gather information or incorporates activators (lights, displays, RF, sound source, etc.) to communicate results, HI may benefit the project.
This article was written by Richard Otte, President and CEO of Promex Industries, a provider of microelectronic assembly for the medical and biotechnology markets. Otte holds BSEE and MSEE degrees from MIT and an MBA from Harvard University. For more information, e-mail
