Substrates with Elements and Channels Embedded

Space requirements of active components can be reduced to a minimum by using chip size packages (CSPs) or flip chips. Further miniaturization, however, requires 3D integration of components. The obvious benefits, to name only a few, are shortened wiring, increased reliability by reduction of solder joints, and better electrical performance by reduction of parasitics, especially for high-frequency applications on the order of several GHz. Em-bedded passive and active chips or components in substrate has been described to have advantages over classic SMT and LTCC because it offers smaller form factor with a high level of passive integration, thus achieving a lower cost system in package, especially in high-frequency applications. SIP products realize all the system functions on an ultraminia-turized, multifunctional and high-performance microelectronics package by integrating both active and passive components into a single package substrate. Since the early 1990s several

56 Chapter 3 Substrate Technology embedded passive and active chip technologies have been reported, and a few have been com-mercialized by universities, research institutes, and companies. For example,the Packaging Research Center (PRC) at the Georgia Institute of Technology demonstrated various sub-strates with passive and active embedded the for SOP.^[27]Imbera Electronics has developed several integrated module board (IMB) technologies for organic, low-cost PCB substrates with discrete elements embedded. Since 2003, Imbera has focused on third-generation tech-nology providing a low-cost, flexible platform for multiple component types, including Si, GaAs, and discrete C, R, in the range of 2 to 350 I/Os^[28].

Embedded active technology, in which thinned active chips are directly buried into a core or into high-density interconnect layers, is gaining more interest for ultraminiaturization, increased functionality, and better performance of SIP. The current technology provides organic substrates with high-density build-up layers and microvias, assembled on both sides with surface mount passive and active components.

It is very important to improve the tolerance of embedded elements in order to meet the requirement from high-frequency electronics for stringent tolerance of passive components embedded in a substrate. T. Kim and his colleagues from Samsung Electro-Mechanics Co. Ltd. proposed a new design and process method providing an acceptable level of improvement in each embedded passive component’s tolerance by studying mainly the factors coming from the metal layer’s line variations. It is reported that resin-coated copper foil type material is used as the embedded capacitor material, while the dielectric material of the embedded capacitor is composite material with ceramic powder and epoxy binder.^[29]

The LTCC technology provides a convenient medium for fabricating three-dimensional (3D) structures, such as cavities and channels, and for embedding electrical elements of capacitors, resistors, and 3D interconnections. These structures, capable of being easily embedded or integrated into the substrate for ICs, can become integral components in liquid-cooling systems for packaged ICs, and biomedical analysis (or reaction) systems, and can also be good candidates for the implementation of mechanical sensors and actuators, such as accelerometers and grippers. These functional modules can then largely enhance the performance of those substrates for cutting-edge packages.

For example, chemical sensors, physical sensors and micropumps by Ilmenau University of Technology^[30] and thick film accelerometers by Dresden University of Technology were fabricated by using LTCC technology,^[31] while the internal microchannels have been used for fluidic micromixers, liquid separation, optical detection, and biochemical reaction in LTCC-based microfluidic systems by Wroclaw University of Technology and Sandia National Laboratories.^[32−35]Thelemann et al. found that the integrated microchannel cooling system in LTCC substrate can decrease additional temperature more than 80%, while the silver thermal vias only reduce additional temperature by about 30%.^[36] With increasing heat flux, the cooling ability of a microchannel becomes much better than the metal thermal via.

The design of the LTCC microchannels should consider both the fabrication process and the mechanical reliability of the substrate. The traditional and effective cooling microchan-nels are straight, serpentine, and fractal-shaped microchanmicrochan-nels. In the study of Beijing University’s LTCC group,^[37,38] six types of microchannel networks are supplied, including straight, serpentine, spiral, and three other fractal-shaped microchannel networks (curved, I-shaped, and parallel), as shown in Figure 3.9. The dimension of the microchannels was decided according to the theories of laminar flow and heat conductivity and the limitation of the fabrication process. The microchannel network was fabricated in the fourth and fifth layers of substrates with a cross section of 200 μm × 200 μm. A thick-film resistor of 2.0 cm × 2.0 cm as the heating source was placed on the center of the substrate surface. The experiment was controlled by the mass flow rate at the inlet and heat flux of the surface heat source.

3.6 Advanced Substrates 57

Inlet Outlet

4 cm

4 cm

2 cm Heating surface

(1) (2)

(3) (4)

(5) (6)

Outlet Inlet

Microchannel

Figure 3.9 Scheme of LTCC substrate with microchannel

An X-ray was used for the observation of the internal microchannels. Figure 3.10 shows that the machining process is good enough to fabricate the microchannels accurately, even for the fractal-shaped microchannel networks. Damages such as sink, block, and distortion were found neither in the profile of the microchannel nor in the dislocation between layers.

Figure 3.10 X-Ray observation of internal microchannel and optical observation of microchannel profile.

The finite volume method (FVM) was used to calculate the temperature, fluid pressure, and flow velocity fields. A mixture of mesh was selected and refined on the sidewalls of the channels, including tetrahedral, hexahedron, pyramid-shaped, and wedge-shaped meshes.

The simulation results shown in Figure 3.11 agree well with the experimental results.

The experiment results showed that the LTCC straight cooling microchannels reduce 73.4% of the temperature addition at a heat flux of 1 W/cm², i.e., from 79 K to 21 K, as shown in Figure 3.12. Under the same fluid mass at the inlet, the serpentine and spiral microchannels have the best ability to dissipate heat and the largest fluid pressure drop, which requires a more powerful pump. The heat dissipation ability of fractal-shaped mi-crochannel channels is worst, which becomes the best when the pressure drop increases to the same level with the serpentine and spiral microchannels, while the temperature field of fractal-shaped microchannels is more homogeneous than other microchannels. The heat dissipation ability would also be greatly enhanced with the appearance of turbulent flow.

However, more investigation should be made into the complex mechanics of fluid flow and heat dissipation of fractal-shaped microchannels before commercial application.

With the development of high-power microsystems and heat fluxes above 10 W/cm², 3D microchannels become more important and realizable for cooling applications in the

micro-58 Chapter 3 Substrate Technology electronics industry, with the advantages of small hydraulic radius, high heat conductivity, homogeneous thermal field, easy liquid control and detection, and various types of networks for different microelectronic chips.

Figure 3.11 Temperature distribution at the heating surface of LTCC substrate with (a) straight, (b) spiral, (c) I-shaped microchannel networks and liquid temperature distribution in (c)

80

Figure 3.12 Additional temperature under cooling and no cooling condition for the straight microchannel networks