Microvia material

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. 

This section provides an overview of the dielectric and apphed conductive materials used in microvia and via filhng. Some of these materials can be used in both IC chip carrier and PWB HDI apphcations. The discussions are focused on the HDI PWB arena and on materials for which information is readily available. In section 22.5.2, cross-references are made to the relevant material specifications of the IPC/JPCA-4104 specification for HDI and microvia materials. A brief material roadmap discussion is included to illustrate material property trends. 

To facilitate the many choices of laminates and their associated properties, industry standards groups such as the IPC have defined minimum performance specifications and have issued several specifications to inform the selection process. Some of the most commonly used material specifications are those that deal with laminate,prepreg, and copper foil. IPC lOl, Specification for Base Materials for Rigid and Multilayer Printed Boards, and IPC-4652, Metal Foil for Printed Wiring Applications, are the primary specifications for clad laminates, prepregs, and foils. Another specification, IPC- 4104, Specification for High Density Interconnect (HDI) and Microvia Materials, deals with many of the new materials for HDI, such as epoxy-coated microfoils, as discussed in this chapter. 

IPC-4104 Specification for High Density Interconnect (HDI) and Microvia Materials... 

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]. 

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. 

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

IPC-4104 Standard for Qualification and Conformance of Materials for High Density Interconnection 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. 

Nonphotoimageable, Nonreinforced Dielectric Materials. This group can be laser-drilled, plasma-etched, and/or mechanically treated to form microvias. As stated earlier, many of the photoimageable dielectrics are laser-drillable. 

Aramid-reinforced laminate and prepreg allow fast microvia hole formation and at the same time maintain the performance characteristic of a smooth surface for fine-line conductor imaging. The ablation speed of non-woven (aramid) laminates and prepregs is close to that achieved when using nonreinforced materials such as resin-coated foil, dry film, or liquid dielectrics. Since aramid laminates are very stable, they allow the fabrication of doublesided, very thin, etched innerlayers, which are then pressed to a multilayer package in a single... 

FIGURE 22.14 Photos of conventional and skip-microvias laser-drilled in Thermount materials. (Courtesy of DuPont)... 

Prior to dielectric material coating by any of the methods described previously, the copper surface of the base core must be treated by an adhesion promotion process to ensure good adhesion of dielectric material to the copper surface. Today, very few manufacturers use oxide treatment for this purpose. The most popular adhesion promotion treatment is a special etching process offered by many supphers of chemicals. This step is common to aU microvia processes. 

However, there are many variations. For the purpose of drilling microvia holes, there are four laser systems UV/Yag laser, CO2 laser, Yag/C02, and CO2/CO2 combinations.Then there are three dielectric materials RCC, resin only (dry-film or liquid resin), and reinforced prepreg. Therefore, the number of ways to make microvia holes by laser systems is driven by the permutation of four laser systems and these three dielectric materials. 

Gonzalez, Ceferino G., Materials for Sequential Build-Up (SBU) of HDl-Microvia Organic Substrates, ,The Board Authority, June 1999, pp 56-58... 

PID technology is based on photoimageable polymeric systems to form blind microvias in dielectric material between layers of circuitry. The use of PIDs allows all microvias on a panel to be formed simultaneously, with no incremental-per-via cost. Its use is particularly advantageous on apphcations with high densities of vias (e.g., more than 50,000 on an 18 in. x 24 in. panel). 

Glass Transition and Modulus of Materials Used in High Density interconnection (HDI) and microvias—DMA method ... 

Glass transition temperature and thermal expansion of materials used in high density interconnection (HDI) and microvias—TMA method Decomposition temperature (Td) of laminate material using TGA Glass transition temperature and cure factor by DSC ... 

Since TMA measures Tg as a function of the expansion of the material, and will also provide the z-axis CTE, it is the method fabricators use most commonly for Tg determination to infer a materials thermal stabUity during assembly. For dielectric materials used for the formation of HDI layers (thinner layers, = 0.006 in., adjacent to or forming the outerlayer, and having microvias to provide interconnection with the balance of the construction), IPC-4104 states that the preferred method for determining Tg and CTE above and below Tg is Test Method 2.4.24.5. 

IPC-2221 Generic Standard on Printed Board Design. There are many ML-PWB strnctnres. This section discnsses the methods and materials for the basic and several of the advanced strnctnres. The IPC has two comprehensive standards for the design of rigid printed circnits, IPC-2222 and IPC-2226. The classification system within these standards for the ML-PWB by stmctnre is shown. The distinction between the two is the focus on microvias in the later standard. 

Electrodeposited copper (see Fig. 29.2) has become an effective method of filUng microvias, replacing the alternative via filling techniques, especially as the conductor patterns become finer and the pitch density of the component packaging increases. Resin or condnctive pastes have been used for via filUng, but the reliabihty problems associated with these materials include subsequent pohshing, entrapped air within the blind micro via, and void formation during the reflow process. 

HDI applications require finer traces and microvia holes in severe manufacturing and application conditions. For manufacturing convenience, flexible circnit prodncers prefer thinner conductors and substrates. However, physical performances of thinner materials may not be optimal. Thinner materials impact both the performance and manufacturing yield of the final flexible circuit. 

These hquid polyimide resins could be the major dielectric materials of special high-density flexible circuits that demand extremely high density, down to 5 micron pitches with 10 micron via holes. The cost of these materials is higher than that of polyimide film. However, they have broader capability for meeting nonstandard requirements such as ultrathin substrates with microvia holes. The properties are very dependent on the manufacturer. 

The UV YAG laser is another choice for generating microvia holes smaller than 50 /mi in diameter on flexible materials. It has a higher productivity rate than the excimer laser. It can drill through both copper foil and flexible substrates. An issue with the UV YAG laser is that it takes a long time to generate large holes. 

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