barcode scanner asp.net mvc Copyright 2008 by The McGraw-Hill Companies. Click here for terms of use. in Software

Printer Denso QR Bar Code in Software Copyright 2008 by The McGraw-Hill Companies. Click here for terms of use.

Copyright 2008 by The McGraw-Hill Companies. Click here for terms of use.
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impact upon the manufacturing process. Traditional tin-lead solder melts at 183 C, whereas the most common lead-free alternatives have a much higher melting temperature, including tin-copper (227 C), tin-silver (221 C), and tin-silver-copper (217 C). This affects the substrate materials, the flux formulations, the cleaning materials and processes, among other things. This chapter will review the assembly, soldering, and cleaning processes, briefly touch on choice of surface finishes for both printed boards and components, discuss the soldering flux in terms of its role in soldering, review the variety of flux chemistries available along with their advantages and disadvantages, and comment on the challenges for flux formulation and cleaning due to lead-free soldering temperatures.
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The basic elements of an electronic assembly are the PWB, the components and the interconnecting metallization. The majority of PWBs are made of epoxy-glass substrate with copper or solder-coated copper metallization. In many cases, a solder mask covers most of the circuit traces, leaving exposed only those patterns where soldering will occur during assembly. Boards designed for through-hole components usually contain copper-plated through holes that may be solder-coated; those designed for surface-mount components will have metallized pads for interconnecting the surface-mount parts to form the assembly.
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Assembly Process Issues The assembly process for electronics includes wave soldering, reflow soldering, and vapor phase soldering as well as selective soldering. Components range from through-hole and surface-mount BGA, m-BGA, CSP, and stacked packages to chip-on-board (COB). The high density and reduced size of today s electronics has led to system-in-a-package (SIP), where the integrated circuit (IC) package contains several active devices, and system-on-achip (SOC), where all the functions of an electronic system are integrated on a single chip. These package types increase the complexity of the electronic assembly process, requiring adhesive attachment for bottom-side discretes, underfill for CSP, wirebonding and encapsulation for COB, and solder paste, paste flux, and liquid flux for various applications. When manufacturing complex printed wiring assemblies (PWAs), the thermal process chosen must take into account the thermal mass of the assembly, the component density, the solder flux and solder paste characteristics, and the maximum temperature limitation of the components. Most of the components assembled to the board in the soldering process have a maximum temperature of 240 C for traditional Sn/Pb soldering. Some electronic components, such as electrolytic capacitors and plastic-encapsulated components, are not rated to experience the high temperatures required to process with lead-free solders. The resulting heatinduced degradation can result in early field failures. Also, the higher temperatures required for lead-free solders are not compatible with many optoelectronic components. The increased heat can cause a variety of conditions with these components, among them electrical variances, changes in silver-epoxy die attach properties, delamination between plastic and leadframe parts, deformation of plastic encapsulants and plastic lenses, damage to lens coatings, and changes in the light transmission properties.
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43.2.2 Assembly Temperature Profiles IPC/JEDEC2 has developed a recommended reflow profile for nonhermetic packaged semiconductor components. These recommendations are based on the temperature taken at the top side of the packaged device, and also based on package volume, excluding external leads,
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or solder balls in the case of BGAs, and nonintegrated heat sinks. Table 43.1 lists the recommended range for Sn/Pb and Pb-free assemblies.
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TABLE 43.1 Reflow Profiles Recommended for Sn/Pb and Pb-free Assemblies Based on Temperatures Taken on the Package Body Profile feature Average ramp-up rate Preheat Temperature min. Temperature max. Time Time maintained above melting Temperature Time within 5 C of peak temperature (Tp) Ramp-down rate Time 25 C to Tp Sn/Pb assemblies 3 C/sec. max. 100 C 150 C 60 120 sec. 60 150 sec. 10 30 sec. 6 C/sec. max. 6 min. max. Pb-Free assemblies 3 C/sec. max. 150 C 200 C 60 180 sec. 60 150 sec. 20 40 sec. 6 C/sec. max. 8 min. max.
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The maximum recommended temperature for the package depends on the package thickness and volume. Table 43.2 lists the recommended reflow temperatures for Sn/Pb processing, whereas Table 43.3 lists the recommendations for Pb-free processing.
TABLE 43.2 Recommended Maximum Package Reflow Temperatures for Sn/Pb Process Package thickness <2.5 mm 2.5 mm Volume <350 mm3 240 +0/ 5 C 225 +0/ 5 C Volume 350 mm3 225 +0/ 5 C 225 +0/ 5 C
TABLE 43.3 Recommended Maximum Package Reflow Temperatures for Pb-Free Process Package thickness <1.6 mm 1.6 2.5 mm 2.5 mm Volume <350 mm3 260 +0 C 260 +0 C 250 +0 C Volume 350 2,000 mm3 260 +0 C 250 +0 C 245 +0 C Volume >2,000 mm3 260 +0 C 245 +0 C 245 +0 C
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