Materials in Electronic Manufacturing: Electronic Packaging
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Materials in Electronic Manufacturing: Electronic Packaging
Introduction of Surface Mount Technology
Robert C. Pfahl Jr. History of Electronic Packaging Electronic packaging involves using an appropriate combination of conductive and dielectric materials to electrically interconnect and mechanically support electronic components in a reliable and cost-effective manner. Since the invention of the integrated circuit in 1959 and mass wavesoldering in 1958/ the vast majority of electronic packaging has involved a planar substrate to which semiconductor devices in protective packages are attached by melting eutectic solder. The planar substrates or printed circuit boards (PCBs) were invented in 1940,2 but their widespread implementation was limited until the invention of mass soldering. PCBs use conventional epoxy-glass dielectric material with mass patterned conductive traces of copper, but alternative materials have been used for either enhanced electrical performance or lower product cost. The ever-increasing complexity of integrated circuits requires a similar increase in the number of leads or interconnections to access the circuit. This ever-increasing need for interconnections has been a driving force for advances in electronic packaging technology. The complexity of an integrated circuit is measured by the number of electronic functions or gates it contains. Rent's rule predicts that for random logic the number of leads, n, can be estimated from the number of gates, g, by: (1)
typically packaged in dual-in-line packages with up to 64 leads inserted in platedthrough-holes that pierced the printed circuit board (PCB) (Figure 1). Electrical and mechanical attachment was achieved by passing the assembly through a fountain or "wave" of molten tin-lead solder. Manufacturing defects were typically 4% or 40,000 ppm. This defect level, however, was largely determined by the time-study
During the late 1980s a significant change took place in electronic packaging, driven by the need for more leads, reduced size, faster electrical performance, and increased heat dissipation. Instead of using platedthrough-holes on a 2.5 mm pitch, the new surface-mount-technology, high-density components were typically square and had peripheral leads soldered to conductive pads on the surface of the PCB. Components now could be placed on both sides of the board and there were no large plated-through holes to restrict space for routing the wiring. From a design perspective, there was no change in the material system, but significant increase in the packaging density. The PCB now was a multilayered PCB with internal circuitry to increase interconnectivity (Figure 2). From a manufacturing perspective, there was a significant change in process and materials. Typically, the complex components had 60 to 200 leads with pitches down to 0.5 mm. Precise component placement equipment with sophisticated vision systems was developed. The components could no longer be attached by
Dudl-ln-Line Package (DIP)
Lead
Solder
Printed Circuit Board (PCB) P
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