Automatic die bonder - Courtesy of Beckemus Technologies
Die Bonding is the process of attaching the semiconductor die either to its package or to some substrate.
Wafer dicing is the process by which die are separated from a wafer of semiconductor following the processing of the wafer. The dicing process can be accomplished by scribing and breaking, by mechanical sawing (normally with a machine called a dicing saw) or by laser cutting. All methods are typically automated to ensure precision and accuracy
Additional Packaging Issues for Microwave and Millimeter-wave Frequencies:
• Design of the metal pattern and dielectric thickness to maintain required line impedance.
• Short interconnect lengths to minimize reflections.
• Careful material selection to minimize effect on electromagnetic fields in integrated circuits
• Coupling between traces and package resonance
• Active devices often have high power dissipation
“Normal” Packaging Issues:
• Choose compatible materials for reliability
• Die attach method and interconnect method
• Metal system, CTE matching
• Sealing and die encapsulation
thermal power density
Another major concern when packaging at high frequencies is the thermal power density that is often associated with high frequency components. This is especially an issue for RF power amplifiers, which can have power densities of hundreds or thousands of watts per square centimeter.
Often these requirements work against each other. For instance, the requirement to design for distributed effects and the need to provide thermal paths for high power devices are usually in conflict. The designer of high power packaging often spends a large amount of effort balancing these, often conflicting, requirements.
Material selection is an important part of the packaging process at high frequencies. One of the reasons is material selection will affect the line impedance and insertion loss of transmission lines. In addition to the material itself, the thickness of dielectric layers and metals layers will affect the design of the transmission line. Also, the materials must be selected to minimize interaction with integrated circuits as in the case of a glob top or under-fill application.
Coupling and Radiation
Another layer of complexity is coupling and radiation. Adjacent metal traces, traces next to integrated circuits and traces between layers can couple energy. This can be a useful effect and circuits such as directional couplers and baluns can be created using coupling. However, coupling is often the hidden enemy of the designer of microwave and millimeter-wave packaging. Coupling and radiation effects can be difficult to model and often only reveal themselves during electrical test. Even during electrical testing, it can be a challenge to precisely determine the location and cure for a coupling or radiation problem. Such effects can lead to resonances or amplifier oscillation.
The requirement to treat metal traces as transmission lines and interconnecting vias as signal transitions adds another layer of complexity. For instance, features as minute as a bend in a signal trace will degrade performance if not carefully designed. A via carrying a signal from one layer to the next can create a signal transition which can limit electrical bandwidth. Often times, extreme effort is required to accurately model and predict the performance of interconnects and transitions. The use of three dimensional numerical simulation tools such as the finite element or finite difference method is common.
Packaging at microwave and millimeter-wave frequencies has the same challenges as packaging at lower frequencies except that there are several additional complexities. An example is distributed effects. This is an issue at high frequencies because circuit features and components can have dimensions that are an appreciable fraction of a wavelength. This causes circuit elements to have electrical characteristics that change as frequency increases. For instance, a wire bond is a simple connection point at low frequencies. However, at microwave frequencies a wire bond performs more like an inductor and at millimeter-wave frequencies it can perform more like a resonator or antenna. Distributed effect concerns dominate the design procedure for packaging at high frequencies.
Fundamentals of Packaging at Microwave and Millimeter-Wave Frequencies
The thirst for higher data rates and greater bandwidth has resulted in increased interest in millimeter-wave systems as a means for local and wider area information transport. At the same time there is a real need for lower cost and more compact systems. These requirements have led to the development of a highly integrated millimeter-wave System In Package (SIP), which operates beyond 40 GHz. This solution uses low cost ceramic packaging as well as optimized interconnects and transitions to allow for wide-band electrical performance.
When referring to fundamental engineering limits to very high speed electronics, packaging and interconnect constraints figure significantly. Ever increasing data rates are transforming digital technologies into what are essentially RF systems.
The once arcane tools of the RF discipline are becoming increasingly applicable to electronic systems in general, motivating many new considerations. RF systems, despite their small device count, have traditionally been voracious, inefficient consumers of power, creating significant challenges for packaging engineers to deal with heat dissipation. Most digital devices have been much more frugal, but speed and high levels of integration turn these devices into significant heat sources as well.