Operational and affordability requirements placed on current and future military systems are becoming more demanding based on the ever changing threats facing our soldiers and sailors in harm's way. Electronic systems and associated technologies are the enabler for new and improved capabilities that will ensure the Department of Defense is fielding systems that meet current operational requirements. Radio frequency (RF) systems and components are a key element of these electronic systems that enable more capable communications, radar, and sensor resources. Typically, defense applications require advanced capabilities in small form factors, which in-turn requires the design and manufacturing of complex RF circuit assemblies.

Complex RF circuit assemblies are typically mixed signal (digital and analog/RF) assemblies in combination with precision dielectric substrates with high performance characteristics. In order to achieve high performance characteristics and circuit isolation, all grounds, shielding, and controlled impedance traces and substrates must be well designed and fabricated for the final assembly. To withstand harsh military environments, hermetic protection must also be designed into the RF assembly. In order to meet current system budget constraints, affordable packaging and assembly methods must be integrated along with reliable RF interconnects.

Design for performance is critical for most defense RF systems. Traditional manufacturing processes, however, can wreak havoc on even the best of designs. Inconsistencies and anomalies in component placement, component and die attachment, and coating processes along with the manufacturing of large ground planes and shields can create variations in the final fabricated RF circuit assemblies that will have a direct impact on both RF performance and potentially on the reliability of the assembly. In addition to the three dimensional performance and behavior characteristic associated with RF components, there are manufacturing challenges associated with non-standard and hermetically-sealed components.

Common defects in the manufacturing of complex RF circuit assemblies can exist within the board fabrication process and in the subsequent assembly processes. Within the board fabrication process, inconsistencies in the fabrication of traces, internal vias (Figure 6-1), and large ground planes and shields can have a direct effect on both assembly reliability and RF performance. In the subsequent assembly processes, the soldering or bonding of nonstandard components onto circuit board substrates in association with large shields and ground planes can create significant processing challenges. Specifically, some common defects in RF assemblies are as follows:

• Non-optimal reflow profiles (due to extreme thermal gradients) for solder joints causing either excessive or insufficient solder reflow
• Improper placement and strain-relief of large non-standard components
• Incomplete cure of die and shield attach materials
• Marginal wire bond and flip-chip interconnect reliability
• Inconsistent hermetic sealing of solid state devices and assembly coatings

Reference

• Knadle, Kevin. "Reliability and Failure Mechanisms of Laminate Substrates in a Pb-free World." Proc. of PCB Carolina 2008. Web. 7 Oct. 2008. .

These defects will impact both the functional performance and the ultimate reliability of the RF assembly. In some complex RF assembly manufacturing processes, the RF test and tune process accounts for some of the highest cost associated with the manufacturing of the product. Designing manufacturable RF assemblies that will enable more consistent fabrication and assembly methods, in concert with tighter tolerances in interconnects and substrates, will have a direct impact on reducing the amount of time and effort expended in the RF test and tune processes. This will directly translate into a more affordable, higher performance, and reliable product.

The EMPF is offering a Design for Production of Complex RF Assemblies course that will provide both design related information in the form of design guidelines along with hands-on experience that will optimize the learning experience. This course is intended for both design and manufacturing engineers. The course will contain the following modules:

• Design Guidelines for Printed Circuit Board Manufacturers
• Design Guidelines for RF Circuit Board Assembly
• Advanced Packaging Methods for RF Components
• Design Guidelines for System Packaging and Reliability
• Introduction to Design for Testability

Contact the Registrar for current cost and scheduling at 610.362.1295 or via email at registrar@aciusa.org.