William P. Trumble

Chemical conversion coatings on electronic equipment

Publisher Summary This chapter focuses on chemical conversion coatings, which are used throughout the electronics industry to protect the base metal of enclosures, housings, and other assemblies from the oxidative effects of the environment. Increasing environmental awareness throughout the electronic industry has identified chromate conversion coating as a process that creates excessive toxic wastes in all phases of the product cycle, and causes problems with the recycling of metal parts at the end of their life. In Europe, the political pressure that is exerted by these environmental concerns extends to the point where returned equipment treated with chromate is not allowed to be disposed in landfill, but must be stored in a dangerous goods storage site. However, alternatives of chromate coating are now available, which have much less environmental impact, and also have electrical properties that are better suited to the demands of high-speed digital equipment. An alternative conversion coating includes anodizing, phosphatizing, and Group IV metal oxides. The chapter takes a quick look at the topic of conversion coatings in general before looking into chromate coatings and their alternatives.

No-lead solder assembly

Publisher Summary This chapter discusses the use of eutectic tin–copper alloy as a viable alternative to eutectic tin-lead solder for some applications. Because of the higher melting temperature of the tin–copper alloy, there are limitations to the types of components that can be used. However, it appears that standard SMT assembly equipment can be used to process the lead-free alloy. The only unique requirement is a nitrogen-inerted atmosphere for soldering. Component lead finishes that are very thick, and react with tin to form intermetallics, should be avoided for use with the eutectic tin–copper solder alloy. The recommended finishes for use with this alloy are tin or tin-copper. The tin–copper solder might be used in applications that are required to withstand 4,000 thermal cycles, but extensive product testing is recommended to ensure reliability. The eutectic tin–copper alloy probably is not appropriate for applications that require the solder joints to withstand 6,000 thermal cycles.

Selection of plastics using cost, structural, and environmental factors

Publisher Summary Plastics, when used within their design envelope, are very environmentally friendly and cost efficient. When used within the boundaries of their physical design properties, plastics are more effective than glass, ceramics, wood, or metal. They are a good electrical insulator, lightweight, and some of them can be molded into very large complex shapes that would require many parts and assembly operations for other materials. The primary classes of plastics are thermoset and thermoplastic polymers. Some typical thermoset plastics are epoxies, phenolics, polyesters, polyimides, and rigid urethanes. These materials are formed by combining two or more components and applying sufficient heat to cure the material to a rigid mass. Most thermosets are rigid but not very strong, so most are reinforced with substances such as glass fibers, glass matrices, carbon black, other fibers, or other fillers.. Filled thermosets have low coefficient of thermal expansion, and have the ability to maintain a significant proportion of their mechanical and electrical properties over the thermal operating ranges of most electronic systems. Thermoplastics are characterized by a loss of mechanical properties, and then softening with the application of heat.