Electronic assemblies reliability

Semiconducting devices, switches and miniaturised v.h.f. circuits are all particularly sensitive to the slightest reaction on critical surfaces, and in devices calling for the highest levels of reliability even the most inert of the phenolic, epoxide and silicone resins are not considered to be fully acceptablecorrosion of electronic assemblies may often be enhanced by migration of ions to sensitive areas under applied potentials, and by local heating effects associated with current flows. 

The following diagram (Fig. 3) illustrates the uses of ftiermal management materials. Special fillers are used to increase ftiermal conductivity values. As a consequence, the life and reliability of electronic assemblies are improved. 

First steps towards robust designs show the potential of the utilized FEA-based RSM/DOE approach to evaluate the thermo-mechanical reliability of various electronics assemblies in a more complex way giving at the same time a more solid basis for design optimizations. 

Mechanical attachment of components, devices, and other parts of an electronic assembly is the prime function of adhesives. Although adhesives are expected to bond a wide variety of materials for electronic applications, they do not need to be structural. They should, however, meet minimum tensile and shear strengths in order to withstand mechanical shock, thermal shock, thermal cycling, and vibration as specified for the intended application. For consumer and commercial products, these stresses may be minimal. For high reliability aerospace and medical systems, more severe tests as defined in MIL-STD-883 and other documents must be used. 

The reliability of an adhesive and its impact on the performance of an electronic assembly should be considered in the initial selection of the adhesive and the design of the system. The function that the adhesive must perform for a specific application, the environment it is expected to encounter, and its duration are all important. Various approaches may be used to predict and assure reliability. Key among these approaches is a basic understanding of possible failure modes and mechanisms. Most failure modes attributed to adhesives are now well understood and documented so that they can be avoided in the initial selection and qualification of the adhesive and in its processing. 

Although important for structural adhesive bonds, fracture mechanics is not as critical for non-structural low load-bearing adhesives as used in most electronic modules. For the most part, passing minimum specification requirements for peel and tensile strengths both at ambient conditions and accelerated test conditions are sufficient. However, computer-simulated modeling and reliability analysis have been used for evaluating electrically conductive adhesives as used in electronics assembly. ... 

The use of underfill adhesives has resulted in the development of the draft version of J-STD-030, Guideline for Selection and Application of Underfill Material for Flip Chip and Other Micropackages. The guideline covers critical material properties for underfill materials to assure compatibility in underfill applications for reliable electronic assemblies as well as selected process-related qualification tests such as thermal cycling. Table 6.9 summarizes selected materials requirements for underfill adhesives from the proposed JEDEC J-STD-030. ... 

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. 

BMW Group Standard GS 95003 Electrical/Electronic Assemblies in Motor Vehicles. lEC 600 50(191) (lEV) Dependability and quality of sevices. lEC 60605-4 Equipment reliability testing Part 4 Statistical procedures for exponential distribution Point estimates, confidence intervals, prediction intervals and tolerance intervals. 

Many standards cover printed circuit board assemblies. Most major electronics manufacturers have their own internally developed workmanship standards. Several industry and military standards also exist. However, the joint industry standard IPC-A-610D, Acceptability of Electronic Assemblies, is the standard most often referenced for criteria defining reliable solder connections. This standard was developed by the Institute for Interconnecting and Packaging Electronic Circuits (IPC) (www.ipc.org). 

Reliability of electronic assemblies is a complex subject.This chapter has touched on only one aspect of the problem understanding the primary failure mechanisms of printed circuit boards and the interconnects between these boards and the electronic components mounted on them. This approach provides the basis for analyzing the impact of design and materials choices and manufacturing processes on printed circuit assembly reliabihty. It also provides the foundation for developing accelerated testing schemes to determine reliability. It is hoped that the fundamental approach will enable the reader to apply this methodology to new problems not yet addressed in mainstream literature. 

In the automobile industry, AEC (Q) 100 is used for complex components. It is a standard for the qualification of electric components. Simple components as resistors or capacitors are not covered in this standard. Since these simple components would often push aU statistic boimdaries through their variety of elements, such statistic observations are often insufficient for safety engineering. The risk for such simple components is that harmful components can be delivered to the production undetected. This is why the eligibility and whether the components are actually sufficiently dimensioned for their case of application are tested in the context of the qualification of the entire electric assembly group. The value for failiue rates is taken from the reliability handbooks. However, for the correct qualification including the proof of lifetime efficiency of the entire electronic assembly group it is assumed that the simple components is within the constant phase of failure rates of the bathtub curve. 

The worldwide drive to remove lead from electronic assemblies adds challenges to an already complex and difficult technology. Well beyond simple structures, solder interconnects provide at least as many challenges as the components they connect. Constant new demands of size, speed, power, and new materials continually direct the developer onto the frontiers of science to provide reliable interconnects. 

NPRD-95 - The Non-electronic Parts Reliability Data (NPRD-95) databook is a widely used databook published by the Reliability Analysis Center that provides a compendium of historical field failure rate data on a wide array of mechanical assemblies. 

The handbook includes a series of empirical failure rate models developed using historical piece part failure data for a wide array of component types. There are models for virtually all electrical/ electronic parts and a number of electromechanical parts as well. All models predict reliability in terms of failures per million operating hours and assume an exponential distribution (constant failure rate), which allows the addition of failure rates to determine higher assembly reliability. The handbook contains two prediction approaches, the parts stress technique and the parts count technique, and covers 14 separate operational environments, such as ground fixed, airborne inhabited, etc. 

Visual inspection, refined over the years to identify defects and faults that have been correlated to mechanical property behavior and reliability in the field, is a very important aspect of the electronic assembly process. Accordingly, a set of visual inspection criteria has been identified and generally practiced across the industry for eutectic Sn-Pb solder joints. However, the mechanical properties, visual appearance, and solder joint geometry of lead-free solders, in combination with lead-free and lead-containing terminations, are markedly different from those of PbASn, as listed in Table 28. Therefore, new visual inspection guidelines must be developed [19]. 

Many metallic elements are contained in electronic assemblies either as terminations or coatings for component devices or as the electrical circuit, terminations, or coatings on PWBs. The elimination of lead in electronic products requires substitution by other metals that can provide the performance and reliable properties characteristic of traditionally used lead-bearing alloys. There are a number of metal resources that can be selected as substitutes for lead in solders and as coatings for lead-free electronic assemblies. These metals include tin (Sn), silver (Ag), copper (Cu), bismuth (Bi), antimony (Sb), gold (Au), indium (In), nickel (Ni), palladium (Pd), platinum. 

Historically, the electronics assembly industry was developed around the ability of tin-lead solder alloy to make highly reliable connections between components and the printed wiring board. The component terminations are often made of difficult-to-solder metals, such as nickel-iron or beryllium-copper, but they can be coated with more easily soldered metals, such as silver, tin, tin-lead, nickel, palladium, or gold. The copper circuitry is usually covered with a solder mask except for the termination pads that are coated with tin-lead solder, either by electrolytic plating or by hot dipping. 

The widely recognized use of Pb-Sn solders in electronics assembly is in the Level 2 interconnections. Level 2 interconnections are solder joints that attach device packages to the printed circuit board. The solderability and, ultimately, the reliability of Level 2 interconnections are determined by the materials used for the package input/output (I/O) and those used for the circuit board. 

Mawer, A. Koschmieder, T. Hodges, D. Can we assemble reliable Pb-free assemblies Packaging Assembly Symposium New Millennium Electronics-Manufacturing Challenges Binghamton, NY, Nov. 2000. 

Reliability Aspects of Lead-Free Solders in Electronic Assemblies... 

Dunford et al. investigated the metallurgical and reliability aspects of Pb-free mixed technology electronic assemblies utilizing area array packages, leadless ceramic chip passives, and small outline integrated circuits with conventional Sn-Pb terminations on a thin PWB with Au/Ni surface finish utilizing Sn-Ag-Cu-Sb solder as the interconnection alloy. The Sn-Au and Sn-Ag... 

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