Electronic circuits reworking

Polymer adhesives have found their place in numerous electronics applications. Major uses include eommercial/consumer products computers and military, space, automotive, medical, and wireless communications. Some adhesives may be used aeross several applications while others have been formulated to meet applieation-specific requirements. For example, reworkability is not a consideration for high-production, low-cost consumer products such as cell phones or calculators, but is important for high-value, high-density printed-wiring boards (PWBs) used in military and spaee electronics. Further, thermal stability at high temperatures is required for near-engine electronics in automobiles, aircraft, and for deep-well sensors, but not for office computers. The major applications for polymer adhesives are to attach and electrically insulate or to electrically connect components, devices, connectors, cables, and heat sinks to printed-circuit boards or to thin- or thick-film hybrid microcircuits. In addition, over the last several decades, new uses for adhesives have emerged for optoelectronic (OE) assemblies, microelectromechanical systems (MEMS), and flat-panel displays. 

Pb-Free Solders. The equipment types described in the preceding sections are often combined to form an electronics assembly line. The term line implies that the equipment is physically located end to end, thereby allowing for the movement of circuit boards between the different functions or stations. Although this scenario is, in fact, typically the case, it need not always be so. Under some circumstances, it may be advantageous to locate some stations a distance away others on the shop floor, thus requiring operators to move product between the two sites on wheeled racks. This is usually the case with inspection and repair/rework functions as well as for some cleaning processes due to health or safety concerns. The setup and hence the flow of a product assembly process are referred to as the (assembly) line architecture. 

The second issue is that of mixing Pb-free and Sn-Pb solder. This situation would be less likely to occur with newly fabricated product, when repair and rework procedures are performed immediately after assembly inspection. The same solder would be used for both processes. Rather, the concern arises particularly with regard to the rework of legacy products that are returned from service. Older, field-return electronics were most likely assembled with a Sn-Pb solder. In those cases where Pb-free rework procedures are in full use, there is the likelihood of intermixing Sn-Pb and Pb-free solders in the interconnections. In terms of the rework process itself, the poorer solderability of Pb-free solders will be improved by Sn-Pb residues remaining on the circuit board pads. (The I/O finish of the replacement component wUl likely be Pb-free, unless the component was in storage as a life-of-program-buy [LoPB] for that system. Then, the Sn-Pb finish on the I/O will also improve Pb-free solderability.)... 

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