Buried via

FIGURE 5.4 Cross-section multilayer board with buried via holes. Buried vias are built into each of the double-sided boards that make up the final multilayer structure. 

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. 

For blind via apphcations, resin-coated copper foil can be used to form the external circuit layer and dielectrics between layers 1 to 2 and n to n-1, using laser or plasma processes to form the vias. Buried vias could be formed in sequential processes. Two basic types of resin-coated copper foil are available. The first type uses one layer of partially cured resin (see Fig. 9.9). 

Drill layers drill (through drill layer) and dr3-4 (buried via connecting sig3 and sig4)... 

Drill data locations, spans (for blind and buried vias), tolerances, and plating thickness... 

Structure This factor determines the number of signal layers and the combination of through and buried vias that permit interconnection between layers and the complex blind, stacked, and variable depth vias available in high-density interconnection (HDI) technologies. 

This generation of printed boards is characterized by very small blind, buried, and through vias made by techniques other than mechanical drilling. To turn blind vias into buried vias, these process techniques are repeated and the layers are built up, hence the name build-up ox sequential build-up circuits (SBU). 

Figure 22.7 shows the compatibihty of laser via, photovia, and plasma via methods with four basic surface dielectric structures on which microvia holes are to be formed. Although laser via methods can cope with all four dielectric structures, photovia and plasma via methods are applicable to only one structure, respectively, as shown in the figure. This is one reason why laser via is more widely used today. Another wiring layer is built over the existing microvia holes, which become buried via holes (BVHs). 

Circled processes 3,4, and 5, as well as squared 3 through 10 in Fig. 23.1, utilize a laser drill via generation technique. This process was first used in the late 1970s to drill small holes in G-10 laminate for buried vias in mainframe computer boards by IBM for its 3081 system and by Burroughs. (Attempts to find pictures of these products proved unfruitful.) The Hewlett... 

CLLAVIS. The CLLAVIS build-up technology is marketed by CMK of Japan. This laser-drilled microvia technology is the most common of HDI processes. The cross-sectional view in Fig. 23.13 shows the filled buried vias in the multilayer core, as well as the optional filled microvias that can be stacked. This structure is also available with the simpler, unfilled staggered microvias. 

Structure. The structure of DYCOstrate PERL is shown in Fig. 23.28. The core material can be standard FR-4 multilayer innerlayers or a through-hole-plated two-sided or multilayer board. Multiple layers can be built up to increase density with the resulting buried and bhnd vias. Current production ranges from 4 layers to 12 layers with various buried board and buried via constructions. As with DYCOstrate, many materials are plasma-etchable and the resin butter-coated copper foils come in many thicknesses and resin types, such as BT, cyanate ester, and polyphenelene-ether (PPE). 

New structures or advancements on existing structures are then needed to make these interconnections. These structures—such as blind and buried vias, multi-lamination (sub-laminations), and build-up technologies—directly impact multilayer processing. Deeper bhnd vias and buried vias have caused a major increase in the demand for reliable via filling materials and methods. Relatively new materials, equipment, and processes have and are being developed to address this need. A new section in this chapter, Filled Via Processing and Sequential Lamination, addresses filled vias internal to the PCB. 

Type 3 This is a mnltilayer board without blind or buried vias (see Fig. 27.4)... 

FIGURE 27.7 Type II (six-layer HDI MLPWB) high-density multilayer board with vias as well as buried vias in the core and through-vias connecting the outerlayers. 

Type III >2[C]>0 may have buried vias in the core and may have through vias connecting the outeriayers (see Fig. 27.8). 

FIGURE 27.15 Type 4 (eight-layer B/V ML-PWB), an eight-layer board with two buried via layers. 

FIGURE 27.16 Type 4 (eight-layer blind and buried via ML-PWB). The design contains buried vias connecting L5 and L6. Blind vias connect layers LI and L2. 

A second application for buried vias is to ensure complete side-to-side electrical isolation. This is particnlarly important in wireless designs where the RF circuits must be shielded from other circuits. Through vias allow RF electric fields to escape from a shielded region. A blind via eliminates this problem and allows RF fnnctions to be combined with logic and control functions on the same board. 

The High-Density Stack-Up. The buried via stack-up design came about for a... 

The requirement to fill vias is driven most often by ronting density. When high-density-area array components are ntihzed, the quantity of vias per sqnare inch greatly increases in the local area under the device. Buried vias or blind vias are freqnently the solntions to throngh-via starvation. 

An innerlayer detail is essentially a thin double-sided printed circuit. The standard innerlayer process contains no plated holes because it is produced by usiug a priut-and-etch process. Bliud, buried via layers aud laminated cores contain holes that must be plated in either a pattern-plate or a panel-plate process. Figure 27.31a shows a typical flowchart for four innerlayer process options. Processes 1 and 2 support standard innerlayers, and processes 3 and 4 can be used for buried via iuuerlayers or core subassembhes. All four processes start with a bare copper-clad laminate and end with a patterned double-sided circuit.The patterned circuit must be iuspected and treated to enhance adhesion prior to further ML-PWB lamination. All four of these sequences work equally well with any of today s materials systems. 

Photoresist Photoresist is supphed in both dry-film and liquid forms. Dry films are a popular choice because of the simphcity of apphcation. For buried via innerlayers, dry films are preferred because Uquids are difficult to use with through holes. The major weakness of dry-film resists is sensitivity to surface flaws and a tendency to lift on poorly prepared surfaces. 

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