Blind via

Eliminating or minimizing the acid has a second important beneficial effect. Since the sulfuric acid carries most of the current within the bulk electrolyte, its removal shifts the transport number of the copper ion from about zero to 0.5, thus effectively doubling the copper transport rate (Eq. 9, below). This chemically induced transport enhancement is particularly important for providing adequate copper transport within the blind vias. 

Sr, Si 0.03 Anode blinding via the formation of SrS04 and Si02 deposits... 

Murao, K. Fabrication of printed circuit boards provided with blind via-holes plugged with copper by electrodeposition. Jpn. Kokai Tokkyo Koho JP 2006(X)9079, 2006 Chem. Abstr. 2006,144, 116371. 

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). 

Surface mount/mixed 1-sided blind vias, gridless CAD Up to 25%... 

Information This information may consist of a single or mnltiple file(s), and defines the location and tool nnmber used for each hole in the PCB.The files reqnired shonld define all plated, nnplated (which can be combined with plated if fnlly defined), bnried via, and blind via layers. 

Information This information describes the size, plating statns, layer-from, and layer-to (in the case of buried and blind vias) drill-data format and filenames. This information is referenced against the drill drawing. 

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)... 

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). 

Blind via push/shove during manual routing... 

Copper- Clad Dielectric Materials. Due to relative ease of implementation, copper-clad dielectrics are used on a iarger scale than unclad dielectrics. Copper-clad dielectrics provide a method that requires the ieast number of changes in manufacturing flow because they typically use the same dielectric and reinforcements found in standard PWBs. Copper-clad-based materials have a longer history in making blind vias than any other method. This makes many designers, OEMs, and PWB fabricators more comfortable with copper-clad-based materials. 

FIGURE 23.9 Structure for the HDI multilayer substrate with lasered blind vias and direct connection to the ICs. 

Structure. Figure 23.21 shows the structure and cross-sections for an ALIVH product. The PCB consists of laser-produced blind vias. The core material is an epoxy-aramid laminate. The man-made aramid filaments are ideal to be cut with a CO2 or UV laser. If the DuPont Kevlar filaments are added, then the resulting material wiU have a very low coefficient of thermal expansion (CTE).This is useful for mounting ceramic packages and for direct attachment of flip-chip integrated circuits. The structure can be as simple as a two-sided PCB or as complex as a many-layered PCB. The vias consist of a copper-epoxy paste that connects the top and bottom copper foil. If used as a prepreg layer without copper, the vias connect the various ALIVH layer pairs into a multilayer structure. This is not a sequential build-up process, but rather a parallel build-up process. 

FIGURE 23.27 The DYCOstrate multilayer structure and cross-sections of through and blind vias. 

Structure. Figure 23.27 shows the structure of DYCOstrate.The core material can be polyimide film with plasma-etched holes, drilled epoxy fiberglass, or other plasma-etchable materials such as liquid crystal polymers. Multiple layers can be built up to increase density with the resulting buried and blind vias, but two-sided DYCOstrate structures are also used in products because of the very high density that 0.075 mm through holes allow. 

For blind vias, reduce the copper-foil thickness to eliminate the copper overhang. 

Figure 23.27 shows microsections of a plated through hole and a blind via in polyimide film.The plasma-etch procedure is basically an isotropic process, as indicated by the undercut seen on the through hole considering the actual dimensions, however, this undercut is too small to cause any plating problems. 

FIGURE 27.6 Type I (six-layer HDIMLPWB) high-density multilayer hoard with blind vias from top and bottom layers and through vias connecting the outerlayers. 

HGURE 27.8 Type III (eight-layer HDI MLPWB) high-density multilayer board with blind vias as well as buried vias in the core and through vias connecting the outerlayers. 

The Blind Via Stack-Up. Another via option is the blind via, which connects the surface layer to one or more internal layers but does not go through the board to the opposite outside layer. Figure 27.16 shows an example of an eight-layer ML-PWB with both blind and... 

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 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. 

It is not compatible with the plating processes used to make buried and blind vias. 

All of these limitations are avoided when the blind vias are drilled through a two-sided board used for the outerlayer and next innerlayer in a stack-up. These holes are drilled prior to lamination just as a thin two-sided plated through board. In this case, the process sequence for a bUnd via layer is identical to the sequence for a buried via layer. Such layers can be stack-driUed. 

There is no aspect ratio limitation and the need for controlled-depth drilling is completely eUminated.To be able to driU bUnd vias prior to lamination, a clad outer stack-up is required. One side of the outer component layer is patterned prior to lamination, while the other is patterned after lamination.This means that when the blind via is metallized, the unpattemed side is blanket-metallized. This is true for either pattern plate or panel plate. This outside layer is metalUzed again when the holes in the finished ML-PWB are metallized. The result can be very thick plating on the exterior of a bUnd via board. To minimize this problem, the fabricator should plate the blind via iimerlayer with the minimum possible current density on the blanket-metaUized side. 

The system fits readily into existing in-house equipment and in-house know-how. Figure 29.12 shows an exaiiqjle of a blind via that is 5.0 mil in diameter and 3.0 mil deep, plated with this type of bath at 20 ASF for 90 min. 

FIGURE 29.12 Example of a blind via that is 5.0 mil in diameter and 3.0 mil deep, plated with chemically mediated bath. 

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