White Papers

3D Transitions and Connections

Abstract:

Vertical transitions are now very common in many RF and microwave products. This was certainly not the case a few decades ago for at least two reasons. First, the simulation tools required to design vertical transitions were not commonly avail- able. Until the early 1990s, three-dimensional (3D) electromagnetic analysis using numerical methods was, in general, restricted to university settings and some research labs. However, by the mid 1990s, 3D electromagnetic simulation and the required computing power were widely available. Second, the fabrication methods and materials were not available or not known to be useful for RF and microwave products. For instance, low temperature co-fired ceramic (LTCC) became widely used after the early/mid 1990s. LTCC is important for vertical transitions since it allows for both blind and buried vias which enables optimized design and layout. In the early 1990s, designers began to experiment with other materials such as high temperature co-fired ceramic (HTCC) in aluminum nitride and alumina to deter- mine their usefulness for RF and microwave applications. Because of widely available 3D electromagnetic simulators and fabrication methods, vertical transitions are used in a variety of RF, microwave, and millimeter-wave products.

Traditionally, RF and microwave signals on transmission lines were routed on planar boards and the signals did not transition out of the x–y plane except for very small z-axis travel for interconnect to integrated circuits. A vertical transition is formed when the signal is transitioned from a transmission line in one plane to a transmission line in another plane. A very common vertical transition is from microstrip to buried stripline which is illustrated in Fig. 3.1. Note how the top transmission line is connected to the buried transmission line using conductive vias in the substrate material.

In this chapter, we will consider four types of 3D transitions. Each transition type has its own challenges, but each follows the design principles of keeping the transition as compact as possible and using transmission lines as the transition element. The transitions considered here are:

1. Vertical Transitions Between Planar Transmission Lines: We will consider the microstrip to stripline transition in co-fired ceramic substrates.

2. Vertical Transitions Using Stacked Die and Through Silicon Vias: In this approach, integrated circuits are stacked on top of each other and interconnects are made between them.

3. 3D Transitions Using Connectors: Transition connectors transfer the RF signal from one layer to another layer in the packaging. The fuzz button connector and elastomeric connectors were mentioned in the previous chapter. In this chapter, the SMP connector is described.

4. Vertical Transition Using Balls or Bumps: These vertical interconnects are sued with flip chip ICs.