High-reliability applications

In the period since 1962, the use of polymer packaged devices has increased from purely consumer applications to industrial and other high reliability applications. The pressures exerted by commercial and industrial users on semiconductor suppliers have significantly improved the reliability of the polymer encapsulated semiconductors (2). Today, it is estimated that nearly 60% of all semiconductor devices are encapsulated in polymers (3). 

However, up to now, most flexible circuit boards have been based on either polyester or polyimide. While polyester (PET) is cheaper and offers lower thermal resistance (in most cases reflow soldering with standard alloys is not possible), polyimide (PI) is favored where assemblies have to be wave or reflow soldered (with standard alloys). On the other side, the relative costs for polyimide are 10 times higher than for polyester. Therefore, a wide gap between these two dominant materials has existed for a long time, prohibiting broad use of flexible circuits for extremely cost-sensitive, high-reliability applications like automotive electronics. Current developments in the field of flexible-base materials as well as the development of alternative solder alloys seem to offer a potential solution for this dilemma. 

Shukla RK, Mencinger NP. A critical review of VLSI die attachment in high reliability applications. Solid State Technol. Jul. 1985 67-74. 

Together with substrates, ceramic packages compete with polymers but are superior in terms of thermal conductivity and hermeticity and are used in high-reliability applications. ... 

Shukla, R. K.,andMencinger,N. P.,ACriticalReviewofVLSIDieAttachment in High Reliability Applications, Solid State Technology, pp. 67—74 (Jul. 1985)... 

Uses. Polyimide coatings are used in electrical applications as insulating varnishes and magnet wire enamels in high-temperature, high-reliability applications. They are also used as alternatives to fluorocarbon coatings on cookware, as shown in Fig. 10.5. 

The MAXI 924 Protector from Maxim Integrated offers factory-programmed monitoring thresholds so that incorrect settings are prevented, which is useful in high-reliability applications. Additional features include positive power-path current interrupt MOSFET control so that commimications with the MAX1924 remain intact even after a fault condition (Figure 16.7). 

For the development of safety-critical software, the choice of programming language makes a significant difference in meeting the requirements of exacting safety standards and, ultimately, high-reliability applications. 

Silver tends to suffer from electromigration and therefore has not been widely adopted in most high-reliability applications even though it is far less costly an gold and, xmlike copper, can be fired in air. To alleviate this problem, several material suppliers have developed mixed metal systems that allow the use of silver inside the cofired ceramic and gold on the external surfaces. By effectively encapsulating the silver in the ceramic the impact of electromigration is reduced. 

Plastic components were accepted at the outset by the consumer industry virtually without reservation because of their principal attraction of low cost. However, the computer, telecommunications and military sectors have long had strong reliability reservations and it is only in very recent years that economic pressures, which fortuitously coincided with significant improvements in quality and reliability, have led to the acceptance of plastic components by these industries. The only areas where plastic encapsulation is unlikely to be used are for the high reliability applications, such as military and aerospace and for undersea repeaters, where the need for guaranteed reliability takes precedence over cost. 

A common metallization scheme for connector contacts used on printed circuit boards is a gold electroplate over copper or copper alloy substrate, usually with an intervening thin diffusion barrier layer (1.2-2.5 pm) of electroplated nickel or nickel-phosphorus alloy. Ordinarily this metallization scheme performs well and is frequently used in high reliability applications. A less reliable and infrequently used metallization has silver as the diffusion barrier. A potential problem with both metallizations is caused by the inevitable pores that exist even in the best quality gold elec-... 

This paper addresses some of the problems involved in the design of software for distributed processors, particularly where there are implications for safety. Modern software engineering techniques and languages are used to consider possible approaches to the design of such systems, and to discuss methods of providing fault tolerant structures for high reliability applications. 

The final general characteristic to be discussed from Table 8.1 is cost. In many small volume, high-reliability applications, cost is not an issue and Au-based materials may be selected. However, high-volume consumer and commercial applications will generally utilize Ag and Ag-Pd based materials as well as certain base metal conductors. One aspect of cost not factored into Table 8.1 is the associated processing costs of base metals. Refractory base metals such as W and Mo not only require specific controlled atmospheres, but also high temperatures and, therefore, significantly more power consumption. Base metals such as... 

Complicating the situation further is the different stress dependency of the creep rates for Sn-Ag-Cu and Sn-Pb eutectic alloys (Ref 11), leading to different comparisons between the two alloys at low and high stress levels (Ref 12-13). Recent studies (Refs 11 and 14) also suggest that both primary creep and tertiary creep are important for the Sn-Ag-Cu alloy. Such considerations are particularly important for high reliability applications (such as aerospace, automotive, military, etc) and long life products (such as telecommunication infrastructure equipment). 

IDI Composites International announced that LARRY LANDIS will head the development team for a new line of engineered structural composites (ESC) targeted to high-reliability applications in the military, transportation and industrial markets. 

Dry film photoresists have largely been used in high performance and high reliability applications. The largest markets for PWBs fabricated using RISTON have traditionally been computers, telecommunications, aerospace and military. 

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