Abstract

The rapid growth in the wireless communications markets together with the emerging need for high-bit-rate (≫ 100 Mb/s) multimedia/data services are pushing the usage of radio spectrum resources below 10 GHz to the uttermost limit. The lack of bandwidth has led to an extensive development of mobile/fixed BWA systems for the higher microwave and millimetre-wave regions up to the V-band frequencies (50–75 GHz). In order for these systems to have mass deployment and to meet cost-sensitive consumer markets’ requirements, their cost and size must be reduced from current levels. One of the most viable packaging approaches to satisfy these demands is the low-temperature co-fired ceramic (LTCC) based system-in-package (SiP) module technology combined with fully automated surface-mount assembly techniques. However, one of the main challenges of this approach has been previously associated with the broadband radio frequency (RF) and reliability performance constraints of the board-level solder joints in LTCC/PCB assemblies. In this thesis the primary focus is to tackle these limitations and significantly extend the feasibility of the LTCC module technology to various wireless/mixed-signal packaging applications.

The thesis is divided into three main parts. In the first part, design, modelling and implementation of vertical package transitions (BGA, flip-chip, substrate via) over a very wide frequency range are presented. In the second part, the emphasis is on the improvement of the thermal fatigue endurance of the board-level solder joints in the LTCC/PCB assemblies. In the last part, the results are merged to realize a high-performance LTCC module platform for use in a wide variety of SiP products in the telecommunication sector.

The flip-chip, substrate-via and BGA transition structures exhibited excellent signal transmission properties up to the V-band frequencies. The developed equivalent circuit models of the transitions matched well with the measurements. Cascading the transitions together allows the building of different combinations of vertical interconnection paths in SiP modules. To demonstrate this, a surface-mountable LTCC filter package for K-band radio link frequencies was implemented.

The developed composite BGA solder joint structure with plastic-core solder balls significantly enhanced the thermal fatigue life in LTCC/PCB assemblies in different thermal cycling conditions, indicating adequate board-level reliability for many practical LTCC-BGA packaging applications. Moreover, electromagnetic analysis showed that the use of the plastic-core solder ball has no detrimental impact on the RF performance of the solder joint.

Finally, based on the obtained results a reliable lead-free LTCC-BGA module platform was developed for broadband packaging applications. The BGA module platform with a size of 15 mm × 15 mm included 38 low-frequency and two wideband RF input/output connections up to the K-band frequencies. The module structure also allowed plenty of space to mount discrete SMD/bare-die components on the surface and/or to embed passive components in the 1.2 mm thick substrate. Preliminary thermal cycling results of the soldered LTCC/PCB assemblies demonstrated sufficient reliability for telecommunication applications. Therefore, the presented module platform can serve as a physical building block for various wireless/mixed-signal SiP products, and hence significantly reduce their development time and associated costs.