Electronics solder joints

It is beyond the scope of this chapter to provide a detailed explanation of SPC. Rather, there will be a qualitative discussion of process control that addresses those factors affecting circuit board assembly, beginning with defect types in electronic solder joints. Then the discussion will turn to process control as it pertains to the basic assembly steps (e.g., dispensing, pick-and-place, etc.). The various material sets (e.g., circuit board, solder paste, flux, etc.) will be incorporated in the discussion. Also, the use of Pb-free soldering technology will be addressed where applicable. 

Zribi, A. Cotts, E.J. Diffusion controlled growth of ternary intermetallic compounds in Pb-based and Pb-free electronic solder joints. Proceedings SMTA International Conference, 2001 810 pp. Zribi, A. Clark, A. Zavalij, L. Borgesen, P. Cotts, E.J. The growth of intermetallic compounds at... 

In contrast to soldering, conductive adhesives are used for special electronic applications. Conductive adhesives simultaneously establish mechanical and electrical joints between PCBs and components by means of a particle-filled resin matrix. Whereas the polymer matrix is responsible for the mechanical interconnection, the fiUing particles (silver, palladium, or gold particles) provide the electrical contact between PCB and component. Therefore, in contrast to solder joints, conductive adhesive joints have a heterogeneous structure. 

There are other clues as to how well a sensor will withstand shock and vibration. Excessive shock and vibration cause sensors to fail by breaking electronic components or solder joints inside the sensor. Sensors with many internal components and/or open spaces inside the enclosure are more susceptible to failures than less complex sensors that have any open space inside the enclosure filled with epoxy or some other substance. Sensors that are epoxy-encapsulated are injected or filled with epoxy in one of the final manufacturing steps. Eliminating open space within the sensor enclosure makes the sensor more rugged. Often this step also seals the sensor against the environment. Another process similar to epoxy encapsulation is potting. A sensor that is potted or has potted electronics has been filled with a substance that hardens and protects the internal components. 

Brighteners. Sometimes Ag is added to solder to improve wetting, making the solder joint smooth and shiny. When not used as a scavenger, the addition of Ag should be avoided as it is not usually needed for most electronic joining applications with Sn-Pb. 

The IPC-A-610 standard describes the acceptability criteria for quality electronic assemblies. It does not define process requirements, although the methods used must produce a completed solder joint conforming to the acceptability requirements as defined. Consistent with other IPC standards, the IPC-A-610 document details the acceptance criteria for each of three classes. 

Components and their packages influence many of the field failures of electronic assemblies. Packages are primarily designed and selected for their ability to protect the electronic components inside for example, ceramic packages may be selected over plastic ones for their greater hermeticity.This section will discuss the ways in which package selection can influence solder joint and cleanliness-related failures. 

A. Emerick, J. Ellerson, J. McCreary, R. Noreika, C. Woychik, and P. Viswanadham, Enhancement of TSOP Solder Joint Reliability Using Encapsulation, Proceedings of the 43d Electronic Components and Technology Conference, lEEE-CHMT, 1993, pp. 187-192. 

Deepak Goyal.Tim Lane, Patrick Kinzie, Chris Panichas,Kam Meng Chong, Oscar Villalobos, Failure Mechanisms of Brittle Solder Joint Fracture in the Presence of Electroless Nickel Immersion Gold (ENIG) Interface, Electronic Components and Technology Conference, 2004, pp. 732-139. 

Kozo Harada, Shinji Baba, Qiang Wu, Hironori Matsushima, Toshihiro Matsunaga, Yasumi Uegai, Michitaka Kimura, Analysis of Solder Joint Fracture under Mechanical Bending Test, Electronic Components and Technology Conference, 2903 pp. 1731-1737. 

Zequn Mei, Mudasir Ahmad, Mason Hu,Gnyaneshwar Ramakrishna, Kirkendall Voids at Cu/Solder Interface and Their Effects on Solder Joint Reliability, Electronic Components and Technology Conference, 2005, pp. 415-420. 

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