Surface-mount assembly

Klein Wassink, R. J., and Verguld, M. M. F. (1995), Manufacturing Techniques for Surface Mounted Assemblies, Electrochemical Publications, Port Erin, Isle of Man. 

For example, solvent or chemical resistance of adhesives used in surface-mount assembly may be tested according to Method 4.5.6 of ANSI/ lPC-SM-817, General Requirements for Dielectric Surface-Mounting Adhesives Although this method specihes specific solvents including isopropanol 1,1,1-trichlorotrifluoroethane an azeotrope of 6% methyl alcohol and 94% trichlorotrifluoroethane and a 10% alkaline... 

In this section we discuss three applications of electrically conductive adhesives die attach adhesives, anisotropically conductive adhesives for liquid crystal display (LCD) assembly, and conductive adhesives for surface-mounted assembly of packaged components on printed wiring boards (PWBs). These applications were selected based on overall... 

C. Surface-Mounted Assembly of Printed Wiring Boards... 

Figure 11.24 shows one of the completed PWBs after surface mount assembly. Table 11.8 shows the fabrication steps for making the LCP film multilayer board. The board has been coated with a conventional solder mask with good... 

In the surface mount assembly process, type 11 and type III boards will always require adhesive to mount the SMDs for passage through the solder wave. This is apparent when one envisions components on the bottom side of the substrate with no through hole leads to hold them in place. Adhesives will stay in place after the soldering process and throughout the fife of the substrate and the product, since there is no convenient means for adhesive removal once the solder process is complete. Additionally, adhesives can be used to enhance both thermal and electrical conduction between device features and board features. This means use of an adhesive must consider a number of both physical and chemical characteristics ... 

Rowland, R. 1993. Applied Surface Mount Assembly. Van Nostrand Reinhold, New York. 

When Fig. 2.2 is used to describe surface-mount assemblies, the vertical (y-axis) dimension (shown as a vertical oval) indicates how complex it is to assemble the board by number of components per square inch or square centimeter for the surface area of the PWB. This vertical oval can vary from 1 to over 100 parts per square inch. As the parts become smaller and closer together, this number naturally goes up. A second assembly measme is average leads (I/Os) per square inch or square centimeter. This is the x-axis value multiplied by the y-axis value. (For a further description of these issues, and equations for quantifying them, see Chap. 18.)... 

Perimeter array I/O packages at 0.050-in pitch can be readily surface-mount assembled. Both 0.4-mm pitch perimeter I/O QFPs and 0.3-mm pitch perimeter I/O QFPs are now in manufacture. Reviewing Fig. 3.8, which shows QFP size versus lead pitch and pin count, it is apparent that the usefulness of QFPs is limited to around 400 FOs. 

Coefficient of thermal expansion Tee.—Surface-mount assembly process subjects the printed wiring assembly to more numerons temperature shocks than typical through-hole processes. At the same time, the increase in lead density has caused the designer to nse more and more layers, making the board more susceptible to problems concerned with the base material s coefficient of thermal expansion Tce- This can be a particular problem with regard to the z-axis expansion of the material, as this induces stresses in the copper-plated hole, and becomes a rehabihty concern. Figure 13.12 shows typical z-axis expansion for a variety of printed circuit base laminate materials. 

An advantage of through-hole technology is reduced cost in some appUcations. That cost benefit may be realized by lower labor costs in some parts of the world that support hand assembly, which is relatively easy with the larger components and products, and lower board densities. Even when fully automated—either by wave soldering, selective soldering, or paste-in-hole/reflow—the capital equipment and manufacturing costs can stiU be lower than those required for surface-mount assembly. 

DCA refers to the assembly of an unpackaged active semiconductor device directly onto the circuit board. The interconnections are made either by wirebonding or by solder bumps on the front side of the die. The latter application is referred to as FC attachment and can be implemented into a surface-mount assembly process as long as existing equipment can accommodate the placement accuracy required for the very small solder bumps and pads. Otherwise, a dedicated cell may be required. 

Specifically suited to surface-mount assembly of packages with lead frames, hot-bar soldering has been in use for several years. The technique rehes on a resistance-heated element to push component leads into contact with solder and bonding pads, simultaneously reflowing the solder. Compression of the leads onto the circuit board lands is continued as the heat is ramped down. Upon cooling, the solder solidifies and the heating element is withdrawn from the newly formed solder joints. The heated element is commonly referred to as the hot-bar, although the term thermode is also in widespread use. 

Hot-bar soldering is best suited for low-thermal-mass, single-sided surface-mount assemblies. Each component type requires its own hot-bar bonding head assembly. These can be expensive, with the price dependent on the complexity, materials of construction, precision required, and overall size of the bonding head. 

Hutchins, C. L., Soldering Surface Mount Assemblies, Electronic Packaging and Production, Supplement, August 1992, pp. 47-53. 

FIGURE 53.3 Typical process defect spectrum for surface-mount assemblies. Process defects marked E" can be detected by electrical tests, those marked M can be detected by mechanical or structural inspection or testing, and those marked by E and M can be detected by either method. 

Clech, J-E, Noctor, D. M., Manock, J. C, Lynott, G. W., and Bader, F. E., Surface Mount Assembly Failure Statistics and Failure FTeeTime, Electronic Components and Technology Conference, 1994, pp. 487 97. 

Today, almost 50% of the semiconductors encapsulated in plastic packages are made for surface-mount assembly that subjects the devices to a considerable thermal shock during the soldering process. Within a few seconds, the internal package temperature rises to 215-260°C and the moisture absorbed by the plastic encapsulant and the organic adhesive evaporates explosively. This sometimes results in package cracks that start at the interface between the chip and the die pad or in delamination within the die attachment layer. To investigate the relationship between the chemical structure of epoxies and this so-called popcorn elfect , a series of polyfunctional resins has been evaluated [5]. They include new experimental epoxy novolacs whose chemical formulae have been previously displayed [4]. 

The surface-mount assembly of fine pitch integrated circuits is a potential market for anisotropic adhesive films [103,104]. The best conductive adhesives exhibit a volume resistivity of 2 to 5 X 10 " cm with a contact resistance of 10 mfl, although AI Technology claims volume resistivities of 5 X 10 -5 X 10 n cm, similar to that of metallic solder joints. This allows a current carrying capabdity of 50-60 A mm compared to 20-30 A mm for typical... 

Lead-free solder has a rougher surface finish and generates a different-shaped fillet. It also is more prone to voids and tombstoning. These and other deviations can require adjustments to commonly used inspection techniques, such as automated optical inspection (AOI). While the results of a National Physical Laboratory (NPL) study confirm that AOI systems can be used to inspect lead-free surface mount assemblies, many defects created by lead-free processes are not visible. The added loss of visual and electrical access due to the growing complexity of PCBAs compounds the problem. 

In addition to the step series, SMT also includes its lead-free series of columns published in 2005. As the world prepares for lead-free manufacturing and the enactment of the Restrictions on Hazardous Materials directive, massive changes are happening for the surface mount assembly sector. The inclusion of these articles represents a year s worth of counsel, technology and research at a critical time for the industry. 

Best practices and innovative ideas for surface mount assemblers... 

In April 1999, Marconi Communications initiated production of a telephone manufactured with lead-free solder, hailed as the first lead-free line in Europe. The telephone utilized a Sn-Ag-Cu alloy that was introduced to Marconi s existing surface mount assembly line. The only modihcation required was a slight alteration to the convection reflow profile [21]. 

FIG. 24 Lead-free surface mount assembly of a discrete component. 

Harrison, M.R. Vincent, J. IDEALS improved design fife and environmentally aware manufacturing of electronics assemblies by lead-free soldering. Proc. IMAPS Europe 99, Harrogate, GB, June 1999. Stam, F.A. Davitt, E. Barrett, J. Reliabihty testing of SnAgCu solder surface mount assembly. Proc. IMAPS 32nd International Symposium on Microelectronics, October 1999 259-263. 

Bath, J. Crombez, E. Surface mount assembly evaluations with lead-free solder pastes. Proc. of SMTA Symposium on Lead Free Interconnect Technology Boston, MA, June 2000 88-97. 

Over the past decades, development and use of electrically conductive adhesives have expanded in the microelectronics industry. The major applications are die attachment, liquid crystal display and surface-mounted assembly of packaged components on printed wiring board. Recently, isotropic conductive adhesives (ICA s) have been widely used in electronics applications to replace lead-based soldering due to toxicity of the lead in Sn-Pb solders [1]. 

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