Microvia

Wurth Group Allied Companies TWINflex Microvia FLATcomp... 

Nanoparticles of silver and carbon are also being studied as replacements for copper or gold plating or for copper filling of microvias in interconnect substrates. ... 

Mark III Microvia, directional etching 0-10,000 to 0-40 KW/ 40 kHz 8-32 panel loading (18 x 24-in) or custom 1 %-in shelf spacing... 

Kresge, J. S. Lauffer, J. M. Russell, D. J. Laser ablatable epoxy resin with dyes for adhesion to copper in formation of microvias for integrated circuits. US Patent 6361923, 2002 Chem. Abstr. 2002,136, 271651. 

For example, the development of high-mesh-coimt screens and microvias in multilayer ceramics has recently allowed for much higher densities of interconnects than was traditionally the case [52-54]. These same technologies also enable the tight tolerance fine lines that are required by high-frequency designers [53-55]. Finally, there has been tremendous activity in the last few years regarding the use of microfluidic and MEMS-based devices based on multilayer ceramic materials [57,58]. 

A high-density interconnect board with sequential bnild-np microvias, buried and blind... 

While the materials already discussed are used in blind and buried via applications using conventional processes, additional materials can be used to increase density using more spe-ciahzed process techniques. The specialized processes used to form microvias include laser ablation, plasma etching, and photoimaging, with laser formation by far the most common. 

The third process technique used in these applications involves photounaging a permanent dielectric material in order to form the microvias. These photoimageable dielectrics resemble plating resists but must be able to be catalyzed for subsequent plating operations that will form the external circuit image, and must adhere sufficiently to the rest of the multilayer circuit to provide long-term reliability. 

IPC 4104, Specification for High-Density Interconnect (HDI) and Microvia Materials, 05/99 This document contains specification sheets for materials used in HDI and microvia applications. 

Microvias respond to current the same as a through-hole via. The cross-sectional area is the parameter to match to a current level and temperature rise. 

Printed wiring boards (PWBs) with microvia hole structures are called different names, such as HDI, SBU (sequential build-up), and BUM (build-up multilayer). However, HDI covers a broader range of high-density wiring boards such as extremely high-layer-count multilayer boards (MLBs) without microvia holes. MLBs with microvia holes are not necessarily built sequentially, nor do they necessarily have build-up structures.These definitions are not appropriate for the discussions in this chapter, and therefore we shall address MLBs with microvia holes simply as microvia hole boards (all microvia hole boards are essentially multilayer boards). 

Some trade and academic organizations define the microvia hole to be a hole of a certain diameter or less. For example, IPC defines a microvia hole as a hole with a diameter equal to or less than 150 pm. However, when a surface blind via (SBV) hole is formed between layer 1 (LI)... 

The purpose of this chapter is to examine a variety of microvia hole formation technologies, structures, and materials. 

However, a greater emphasis will be placed on the laser-drilling process laser via hereafter) since it is the most popular process today and it seems its popularity will grow in the future. It must be understood that via hole formation is just one element of fabricating HDI wiring boards. Fabrication of HDI wiring boards with microvia holes involves many new processes not common to conventional board fabrication. Therefore, additional emphasis will be placed upon these new fabrication processes that are common to other microvia technologies. 

The interactions among printed circuit boards, components, and assemblies are best seen by the packaging technology map (see Chap. 2 Fig. 2.2). Components are characterized by the average I/Os per part, assembhes by the components per sq. cm and I/Os per sq. cm, and the printed circuit by its wiring density in cm per sq. cm Fig. 22.1 shows the approximate crossover between the traditional printed circuit board and the next generation with microvias. ... 

FIGURE 22.2 Relative price and density comparison between conventional through-hole (TH) boards and HDI microvia boards with different structures. The relative cost index (RCI) is actual production pricing of boards normalized to the eight-layer price. The density (DEN) is a measure of the maximum average pins (leads) per sq. in. (for both sides of the PCB).The diagonal lines are equivalent density boards. These are for FR-4 printed circuits. 

IPC-2315 Design Guide for High Density Interconnecting Structures and Microvias ... 

IPC-2226 This specification educates users in microvia formation, selection of wiring density, selection of design rules, interconnecting structures, and material characterization. It is intended to provide standards for nse in the design of printed circuit boards utilizing microvia technologies. 

TABLE 22. 1 Ten Process Methods Utilized to Produce Microvias... 

FIGURE 223 Representative microvia profiles and the processes that produce them. 

The core, defined as [C], can be identified as an A, B, or C type core. Thns, [CA] is a core with internal vias only redistribntion makes contact with the snrface. [CB] is core with internal and external (throngh microvia strnctnres). High-density interconnecting structures make contact with the innerlayers of the core. [CC] is passive core, in which there are no interconnections. 

FIGURE 22.4 IPC-2226 microvia structures from Type I to Type VI. 

Typical HDI design rules are given in Fig. 22.5. This figure bridges the two design categories from the specification IPC-2226, Design Standards for HDI and Microvias. This is a Type III HDI structure. 

HDI and microvias place a new bnrden on printed circuit design. The various IPC types impose significant changes in the multilayer stack-up from a conventional board. But additionally, microvias can be implemented in many different ways and with different design rules. This section presents just a few of the design issues. 

For interconnecting IC substrates with hundreds of I/O terminals, a single layer of microvia holes may not be sufficient to satisfy interconnection requirements. Double, triple, or even quadruple microvia hole layers may be necessary whether the board is a motherboard or a chip package substrate. When microvia holes are made only between adjacent layers (see Fig. 22.6), all three microvia processes— photovia, plasma via, and laser via—can be used. However, when a design requires microvia holes that must connect beyond adjacent layers—such as LI and L3 (skipvia)—laser via processing is the only practical option. 

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