by Randy Frank
President, Randy Frank & Associates
All the advances and research occurring in micromachining would lead one to believe that breakthroughs in these areas will be sufficient to revolutionize sensing. Unfortunately, the problems associated with the basic sensor packaging are compounded when the sensor is combined with higher levels of electronics. These problems initiate at the lowest level of die and wire bonding and extend to encapsulation, sealing, and lead forming issues. Fundamental assembly differences frequently exist between sensor and microelectronics packaging and are among the problems that are being solved to achieve smarter sensors. These differences include die bonding for stress isolation instead of for heat dissipation and wire bonding procedures.
Packaging is essential to establishing the reliability of the sensor. Therefore the reliability requirements must be taken into account in the design of the package, especially for custom packages in specific applications. Testing of the sensor and circuitry combination also requires combining test capability from both technologies. This chapter will address sensor packaging, especially technology from the semiconductor industry that is being applied to add the smarts and solve reliability and testing issues for smarter sensors.
Semiconductor packaging applied to sensors
Many sensor packages in the late twentieth century resembled semiconductor packages of the 1980s or even the 1970s. The semiconductor industry and sensor manufacturers have made significant progress in high-density plastic encapsulated packages. The almost universal use of surface-mount technology (SMT) is among the more important changes. To achieve increased functionality without increased silicon complexity, available silicon technologies are being combined at the package level in packages based on semiconductor, not module manufacturer, assembly techniques. These multichip modules (MCMs) are used for several applications, including automotive.
As the use of surface-mount technology increases, a decline has occurred in the use of the previously popular package. Other through-hole packages like single in-line plastic (SIP) and pin-grid array (PGA) will also decrease. Today, ball-grid array (BGA) and land-grid array (LGA) are predominately used for MEMS sensors. SMT approaches, such wafer-level packages (WLPs), wafer-level chip-scale packages (WLCSPs), stacked die, and 3-D packaging are the focus of present packaging development. For highly integrated components, packaging techniques must take into account more complex, system-level requirements as well as assembly requirements. The ability of the sensor industry to adapt to the newer semiconductor packaging approaches to sensors will determine the acceptance of smart sensor technology and the future growth of the industry.
The type of sensor, amount of circuitry, and application can change the cost significantly. The cost of an accelerometer in Figure 1 (below) is based on a separate control chip and g-cell in an SMT package. The die cost in that case is 30% for the g-cell and 24% for the ASIC. However, the greatest challenge with the accelerometer for automotive air bag applications is inexpensive testing. Yole Developpement estimates that packaging, assembly, test, and calibration steps account for nearly 35% to 60% of a total MEMS packaged module’s cost. For more complex packaging such as pressure sensor where media isolation is required, the cost of packaging, assembly, and testing can climb to 95% of the total cost.
Sensor packages have basic requirements that are similar to semiconductor devices. The variety of harsh sensor applications makes packaging more difficult than packaging for a semiconductor device. However, the basic package operations occur in similar order.
Figure 1. Cost of the elements of an accelerometer in a surface-mount package.
A completed sensor wafer has a final processing step that prepares it for packaging. This could include thinning the wafer and attachment of a backside metal such as a gold-silicon eutectic. Sensors tested at the wafer level that do not meet minimum specifications are identified as rejected units by an ink dot. Sensors are then separated into individual die from the wafer by sawing or scribe-andbreak techniques. Good sensor dice are placed in carriers that allow automatic pick-and-place machines to transfer dice from the carrier to the final package where a die bond attaches the sensor firmly to the package. Wire bonds connect the electrical contacts on the die surface to the leads of the package that allow the sensor to interface to external components. The package is then sealed if it is metal or ceramic, or encapsulated if it is molded plastic. Lead plating and trim (or singulation) occur next and are followed by marking and final test operations.
The actual design of the package must take into account sensitive areas of the semiconductor device and the sensor's specific function. The following list identifies characteristics that affect sensor packaging. The sensitivity of the semiconductor to light must be minimized in packaging for an accelerometer but optimized for a photodiode. Similarly, the package’s sensitivity to stress must be taken into account during its design to prevent this stress from affecting the offset and sensitivity in a stress-sensitive pressure sensor. For smart sensors, the key item in the list below, which shows the characteristics that affect sensor packaging, is the fact that integration level affects the sensor's package.
• Wafer thickness and wafer stack (e.g., single, silicon-silicon, silicon-glass);
• Environmental sensitivity/requirement for physical interface;
• Physical vulnerability/stress sensitivity;
• Heat generation;
• Heat sensitivity;
• Light sensitivity;
• Magnetic sensitivity;
• Integration level.
A passivation layer that is deposited near the end of the wafer fabrication process protects the active area of semiconductor devices. Figure 2 (below) lists the common terms used to describe this layer. Silicon dioxide and silicon nitride doped with boron, phosphorus, or both are two materials used for the passivation process. For the semiconductor sensor, the mechanical properties of these layers must also be taken into account. For example, in silicon pressure sensors, implanted or diffused elements are protected by a passivation layer, but the diaphragm area is masked to avoid the dissimilar material interface and dampening effect that would be caused by the glass layer.
Figure 2. List of the common terms that describe the passivation layer that is deposited near the end of the wafer fabrication process.
For semiconductor components, the package provides protection from the environment that can include moisture and gaseous or liquid chemicals. These packages are further protected in applications like automotive underhood-mounted modules by additional epoxy and silica potting compounds or conformal coatings (e.g., acrylics, polyurethane, silicone, and ultraviolet curing compounds) that cover the printed circuit to which the component is mounted. However, semiconductor sensors frequently have to interface to this environment. Pressure sensors, for example, that respond to static and dynamic pressures must have protection techniques that allow the transmission of the pressure signal to the force collecting diaphragm with minimal damping and distortion of the signal. Distortion can be caused by compressible fluids. Additional protective materials for the die surface and the wire bonding connections, such as a compliant, thin conformal parylene coating or hydrostatic methyl-silicone gel, are frequently used as the means to transmit pressure to the top surface of the sensor. Parylene deposition is a vacuum process where the reactive vapor is passed over room temperature sensor and coats it with the polymer. The equipment used to perform this process is quite sophisticated, especially relative to gel coatings. The kind of media to which parylene and gel-protected sensors can be interfaced is limited by the properties of the protective material.
Increased pin count
One of the more difficult problems that must be solved when additional electronic circuitry is integrated with or interfaced to the sensor is the requirement for additional pinouts. Integrated circuits including MCUs have industry-accepted standard packages that allow a large number of pinouts. To increase the density in rapidly increasing surface-mounted applications, pin pitches at 50 μm and below are being pursued. Tape-automated bonding (TAB) is one of the technologies used to address fine pitch requirements, especially for the MEMS portion. Sensors, on the other hand, are usually limited to eight or fewer pins. Furthermore, the packaging varies considerably from manufacturer to manufacturer, with no standard form factor. The requirements for a mechanical interface for pressure, force, flow, or liquid level cause additional packaging problems.
Additional circuitry, whether it is simple signal conditioning or more sophisticated approaches that include ASIC and microcontroller capabilities, has an impact on sensor packaging. A "sensor only" versus integrated sensor plus control circuit is one level of differentiating packaging requirements. However, increased functionality through additional circuitry, either on the same chip or from a separate chip included in the ultimate sensor package or module, affects the pin count, normally increasing the number of package pins. An exception occurs for simple amplification and temperature compensation circuits for pressure sensors, where the number of pinouts is actually reduced from four to three for piezoresistive sensors.
Randy Frank is the principal of Randy Frank & Associates, Ltd, a Scottsdale, Arizona based consultancy that focuses on sensors, power, and automotive electronics. He is an IEEE Fellow and an SAE Fellow.
Mr. Frank has been the chairman of the Sensor Standards Committee of the Society of Automotive Engineers (SAE). He is also a former member of the IEEE standards Association, IEEE Sensor Terminology Taskforce, and the Association for the Advancement of Medical Instrumentation (AAMI) Blood Pressure Transducer Committee. He has taught Advanced Instrumentation and Control at the University of Michigan that included the reliability aspects of sensors. He has three patents issued and has published over 300 technical papers and articles, and several book chapters on semiconductor products and applications.
Reprinted from Understanding Smart Sensors, Third Edition with permission, © 2013 Artech House, Inc.