Electronic assemblies testing

Another important application area is the non-destructive defectoscopy of electronic components. Fig.2a shows an X-ray shadow image of a SMC LED. The 3-dimensional displacement of internal parts can only be visualized non-destructively in the tomographic reconstmction. Reconstructed cross sections through this LED are shown in Fig.2b. In the same way most electronic components in plastic and thin metal cases can be visualized. Even small electronic assemblies like hybrid ICs, magnetic heads, microphones, ABS-sensors can be tested by microtomograpical methods. 

In many regards, however, this study has raised more questions than it has answered. The lack of performance of all of the primed samples and the corresponding superior performance of the unprimed samples using types 1, 2, and 3 elastomers raises serious questions concerning the validity of adhesion testing as the primary criteria in the selection of silicones for corrosion control of electronic assemblies. The poor performance of the primers A, B, and C, is... 

Test performance. In electronic assemblies extensive accelerating tests are perforated (16) the most important are listed In Table III. 

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. 

A function of adhesives that is becoming increasingly important is to absorb and dissipate stresses produced in electronic assemblies due to changes in environmental conditions or accelerated testing. The need for stress-absorbing and dissipating adhesives has increased with advancements in microelectronics. Some advancements requiring low-stress adhesives are ... 

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. ... 

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. 

Whether these corrosive gas tests are reaUstic for materials other than those used for connectors or for operating electronic equipment is not clear. The test should be carried out, but the observation of no failures should not be taken to mean there will be no field failures in typical urban environments. Similarly, any failures that are observed should be carefully evaluated to ensure that the same mechanism would he operative in field situations. Connectors tire a somewhat unique part of an electronic assembly in that the active part is frequently a noble mettil and the sensitivity of the mated surfaces to failure may be lower thtin many other parts of electronic assemblies. Most failures in electronic assemblies attributable to the environment are due to ionic particle contamination in conjunction with atmospheric moisture. In 20 years of evaluating field failures in the United States, the author has never seen a failure that could be attributed to the effects of SOj, has seen a few caused by H2S or HCl, has heard of a few caused by NOx, and has seen several hundred that were caused by ionic contamination. Clearly, valid accelerated testing of electronic components, circuit boards, and assemblies must include ionic contamination. Emerging methods are discussed in the Fine Particle Testing section in this chapter. 

X-ray imaging of electronic assemblies can check for voids in the die-attach material to the substrate or to substrate-attach material to the package. There is no need to apply power to the DUT nor measure any electrical parameter. Devices may be potted or hermetically sealed for this test. 

Most standards that define acceptability have roots traceable to the various Department of Defense (DoD) military standards established in the 1960s and 1970s. The advent of complex electronics systems required a standardized approach to acceptability based on lessons learned through testing and product failure analysis that was accomplished by laboratory or research activities. The resulting definitions for acceptance became very restrictive, and although the requirements were initially intended only for harsh environment exposure or products that have a significant consequence for failure, they were used successfully to increase performance of all soldered electronic assemblies. Most commercial production did... 

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. 

IPC-TR-464 Accelerated Aging for Solderabihty Evaluations J-STD-001 Requirements for Soldered Electrical and Electronic Assemblies J-STD-002 Solderability Tests for Component Leads, Terminations, Lugs, Terminals and Wires... 

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. 

Active thermal cycling. As already mentioned, only the dwell time can be vari if an electronic assembly is to be tested in an accelerated test. If the performance of a single component is to be evaluated, one can enlarge the strain per cycle by mounting the component on a substrate with a larger ACTE to the component, but the temperature ramp has to be limited... 

Gloves that are used in electronic assembly are designed primarily to protect the product under manufacture, rather than protect the worker. Some standards relating to glove material cleanliness and to the testing of static electrical properties of materials are available and used in this area. 

Makeup of test devices and test systems Construction of harnesses for test devices Coating and potting of electronic assemblies LED-technology... 

The manufacturing processes for soldering electronic assemblies were developed over decades as electronics technology advanced. The substitution of lead-free solders for the standard tin-lead alloys has a considerable deleterious effect on manufacturing methods, the entire supply chain of materials and parts, and the environment. The main contributing factor is the higher temperature required to melt and use the lead-free solder alloys. Development and performance testing of... 

Solder pastes used for SMT assembly are subjected to a battery of tests which are listed in Table 2 to evaluate screenability, and the ability to maintain feature definition prior to reflow in a furnace [13], Several tests are specified by the IPC, (for example IPC-SF-818 and IPC-SP-819), while others have been adopted by the electronics assembly industry. 

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. 

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