Surface Mount Substrate Material

The surface forces apparatus (SEA) can measure the interaction forces between two surfaces through a liquid [10,11]. The SEA consists of two curved, molecularly smooth mica surfaces made from sheets with a thickness of a few micrometers. These sheets are glued to quartz cylindrical lenses ( 10-mm radius of curvature) and mounted with then-axes perpendicular to each other. The distance is measured by a Fabry-Perot optical technique using multiple beam interference fringes. The distance resolution is 1-2 A and the force sensitivity is about 10 nN. With the SEA many fundamental interactions between surfaces in aqueous solutions and nonaqueous liquids have been identified and quantified. These include the van der Waals and electrostatic double-layer forces, oscillatory forces, repulsive hydration forces, attractive hydrophobic forces, steric interactions involving polymeric systems, and capillary and adhesion forces. Although cleaved mica is the most commonly used substrate material in the SEA, it can also be coated with thin films of materials with different chemical and physical properties [12]. 

Some adhesive materials and processes are used across many apphcations. For example, adhesives are used to attach bare die, components, and substrates in assembling commercial, consumer and aerospace electronic products. Adhesives are also widely used for surface mounting components onto interconnect substrates that serve numerous functions for both low-end consumer products and for high rehability applications. Underfill adhesives are used to provide stress relief and ruggedize the solder interconnects for almost all flip-chip and area-array devices, regardless of their function as integrated circuits. 

Conductive adhesives are one of the feasible alternatives to lead for electronics assembly. Isotropically conductive adhesives are suitable for standard pitch (50- to 100-mil) surface-mounted components and numerous commercial materials are available (see commercial suppher Ksting, Section VI.E). Anisotropically conductive adhesives are more suited to flex to rigid connections, fine pitch components (15- to 20-mil pitch), and flip-chip assembly (4- to 12-mil pitch) [22]. Adhesives are not ready to replace solder throughout the electronics industry, however, due to questions that remain concerning the reliability of electrical interconnections. Their implementation is currently limited to low-cost applications using polyester substrates and specialty appHcations where solder cannot be used. Additionally, the lack of equipment for large-volume assembly with anisotropically conductive adhesives, which require the simultaneous appUcation of heat and pressure, impedes the acceptance of these promising materials. 

Microfluidic Boards Microfluidic board interconnection technologies seek to mimic the electronic printed circuit board for ease of use, reliability, and versatility. In microfluidic board technologies, the substrate contains the passive microfluidic channels and reservoirs, with active components (e.g., sensors, valves) mounted on top (e.g.. Ref. [2]) (see Fig. 1) similarly to electronic components mounted on a printed circuit board (PCB) or multichip modules (MCM) in surface-mount technologies (SMT). Electronic coimections between components are provided on top of the board, while fluidic interconnection is usually provided in the board material itself. Microfluidic circuit boards can be made from a variety of materials, including silicon, glass, and polymers, e.g., polydimethylsiloxane (PDMS), epoxy, or polymethyl methacrylate (PMMA). Microfluidic boards can be as simple... 

Most metal-core PWB consumer applications are made of aluminum substrate, which comes as a copper-clad material. PWBs made of such material do not have through-holes, and components are usually surface-mount types. These circuits are frequently formed into three-dimensional shapes. 

ML-PWBs often have surface-mount devices on both sides of the board, and they receive three or more solder operations during the assembly of connectors and devices. In addition, because of the value of a completed assembly, the board must be able to withstand additional soldering operations needed for occasional removal and replacement of defective devices. Boards made with difunctional GF epoxy can suffer from lifted lands, cracked PTH barrels, or substrate blisters during these multiple soldering operations. The solution is to use materials with low moisture absorption and high thermal degradation temperature. 

Materials with Enhanced Mechanical or Conduction Properties. In cases where the device package (leadless area array with ball grids) is incompatible with the CTE of a standard material, a low-expansion snbstrate mnst be nsed.This can be achieved in several ways. In the past, leadless surface-mount technology (SMT) focused on replacing the woven glass in standard FR-4 with woven quartz or aramid fibers. Although this reduced the expansion of the substrate, both materials were expensive and difficult to process. 

IPC-3406 Guidelines for Electrically Conductive Surface Mount Adhesive IPC-3407 General Requirements for Isotropically Conductive Adhesives lPC-3408 General Requirements for Anisotropically Conductive Adhesive Films lPC-4101 Specification for Base Materials for Rigid and Multilayer Boards lPC-4103 Specification for Plastic Substrates, Clad or Unclad, for High Speed/High Frequency Interconnection... 

The trend towards further miniaturisation and surface mounting of components in electronic construction is likely to lead to increased use and development of high-purity silicones as coatings to provide protection against the environment. Flexible silicone adhesives will also play an important role in bonding components to substrates where differing thermal expansion coefficients preclude the use of rigid materials. 

One of the most critical requirements for electronic surface-mount components is the CTE. In this respect, LCPs are unique in that their thermal expansion properties can be tailored by the judicious incorporation of reinforcements (such as short glass fibres and chopped carbon fibres) and fillers (such as talc, silica and calcium carbonate). Figure 7.9 illustrates the thermal expansion of LCPs and how they compare with other engineering materials. The CTE can be adjusted within the wide limits to practical requirements by varying the processing conditions, and can approach that of FR4 epoxy/glass fibre - one of the most widely used substrates for printed circuit boards. Other application-related examples are discussed in section 7.5. 

Clearly, it is important that the CTE of materials used in the production of components for surface mounting should match the CTE of substrates as closely as possible. The through-hole assembly process currently allows a level of imprecision and CTE mismatch since any differential expansion can be accommodated by the compliance of the component leads. Much greater precision is required in surface mounting, where any difference in CTE between mating parts can result in failure due to poorly controlled part-to-part dimensions, stressing of components, device or board damage, or poor solder joints. 

Although the resolution of atomic force microscopy (AFM) is basically inferior to that of STM, the technique has the advantage that insulating materials can also be used as substrates. In AFM the forces acting between the tip and the sample surface are detected. The probe tip mounted on a flexible cantilever scans over the sample. AFM can be operated in contact mode, exploiting repulsive forces, as well as in non-contact mode, exploiting attractive forces. In the contact mode the probe tip is in direct contact with the sample surface (Fig. 7.8). Either the tip is passed over the sample surface at constant height (CHM,... 

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