Printed-circuit assemblies

MIE-I-46058C(6) Insulating Compound, Electrical for Coating Printed Circuit-Assemblies), U.S. Dept, of Defense, Washington, D.C., Nov. 8,1982. 

In one of the leading UK pharmaceutical companies, a printed circuit board assembly in a plant controller was replaced after a failure with an ostensibly identical unit. To everyone s complete surprise, this simple action, regarded as relatively routine, led to a major operational incident. The cause was found to be an EPROM chip on the replacement board. This chip not only carried an upgraded version of the software on the original but also possessed different operational parameters. Despite the fact that the replacement printed circuit assembly had been successfully tested (in accordance with quality procedures), it was the lack of effective configuration management that allowed the change in the software to elude detection, which led directly to the failure. 

Kastner M. Adhesive application and curing. Printed Circuit Assembly. Dec. 1988. 

Fuel injector compronents Graduated capillary spacing test assembly Heat exchangers Printed circuit assemblies Wax... 

Contaminations from printed circuit assemblies are removed to achieve long-term... 

Kastner, M., Adhesive Application and Curing, Printed Circuit Assembly Pec. 1988)... 

Objective or mission of the item under analysis (e.g., printed circuit assembling... 

Freda, Michael, and Furlong, Jason, Application of Reliability/Survival Statistics to Analyze Interconnect Stress Test Data to Make Life Predictions on Complex, Lead-Free Printed Circuit Assemblies, EPC 2004, October 2004. 

FIGURE 14.2 This is a block diagram of a digital device (in this case a disk drive) that has been segmented to its assembly levels. The dashed lines represent initial partitioning of the entire product to printed circuit assemblies and also shows the expected interface (connector) requirements. 

Printed Circuit Assembly. In a similar situation, assembly tooling requires the checks and analysis of supplied BOM, machine files, and artwork for the assembly process. These actions are specifically designed to locate potential manufacturability problems during... 

The most suitable photosensitive dielectric was the photo-imageable solder masks, due to their proven compatibility with printed circuit assembly processes, ability to withstand exposure to service environments, and possession of the necessary via-imaging characteristics. SLC technology was originally developed and implemented with a liquid PID solder mask applied by curtain coating. In 1995, it was also qualified with a dry-film PID with virtually identical electrical properties. The dry-film version did not require the extensive surface grinding processes that the liquid PID process required. 

M., Bixenman, Miller, E.,and Rued, F, Lessons Learned and Best Practices Developed for Cleaning Pb-Free Flux Residues from Printed Circuit Assemblies and Advanced Packages, paper presented for the IPC/JEDEC International Conference on Lead-Free Electronic Components and Assembly, 2006, Singapore. 

Now, due to the EU s RoHS directive and similar legislation elsewhere in the world, Sn-Pb is being phased out from mainstream electronics manufacturing. Only a small fraction of printed circuit assemblies (PCAs) will remain as Pb d solder assemblies as permitted by RoHS exclusions and exempted military applications. As time progresses, it will become increasingly difficult to source Pb-bearing parts for use with RoHS Pb-exempt circuitry. 

FIGURE 46.2 A BGA solder joint rife with voids from flux-derived gases and improper soldering profile. These voids can detract from the reliability of the solder joint. The IPC-A-610 workmanship standard for printed circuit assemblies (PCAs) should be consulted on void acceptability. (Courtesy of Hewlett-Packard.)... 

Boosted largely by cost, floor space, and environmental concerns, no-clean fluxes now dominate printed circuit assembly. They are generally composed of rosins (e.g., natural acidic plant residues such as pine pitch) and/or resins. Often, but not exclusively, the term resin is used to denote a manmade rosin analog. 

Hwang, X S., Controlled-Atmosphere Soldering Principle and Practice, Printed Circuit Assembly, July, 1990, pp. 30-38. 

Morris, J. R., and Bandyopadhyay, N., No-Qean Solder Paste Reflow Process, Printed Circuits Assembly, February 1990, pp. 26-31. 

FIGURE 47.1 Cross-sectional view of reflow oven with top and bottom heater assemblies, cooling module, vent stack, insulated tunnel, and printed circuit assemblies (PCAs) atop motorized conveyor. 

Soldering flux is applied prior to component placement and soldering. Flux choice should be tailored to the hot-bar process.The flux can be either liquid or paste. TTie flux chosen should be tested to resist charring and the development of polymerized decomposition products, or lacquers, which adhere to both circuit board and hot-bar. Residue build-up on the bar can adversely inhibit hot-bar performance by diminishing thermal transfer. The residue can also become thick and uneven enough to prevent the hot-bar from squarely contacting component leads. Baked-on aqueous-clean flux residues can make flux cleaning difficult. With no-clean flux, overheated residues may detract from the visual appearance of the printed circuit assembly. 

This chapter covers why manufacturers inspect printed circuit assemblies, how they have implemented and enhanced visual inspection, what automated inspection systems they are using, and how they have implemented these antomated systems. The scope of this chapter includes only inspection of printed circuit assemblies during the assembly process, as typically shown in Fig. 53.1. Thus, it includes inspection of solder paste after the paste printing process step, components after the component placement process step, and solder joints after the solder reflow process step. Not included, however, is incoming inspection of components and the bare printed circuit board (PCB). The focus of this chapter is on prodnction nse of inspection, not the collection of measurements dnring process development in a research and development (R D) environment. 

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