Area-array devices/packages

ICs, chip carriers, and packages may have their I/O connections formed throughout the surface area (area arrays) as opposed to peripheral I/Os as with wire-bond pads on die or axial leads on packages. Area arrays include flip-chip, BGA, and pin-grid arrays. Area-array devices provide very high I/O counts per surface area, which are necessary for increased functionality. 

Area-array devices have one thing in common their solder joints are hidden by the package body and it is impossible to repair an isolated open solder joint beneath the package. Nitrogen provides added insurance that best solder wetting will result if soldering conditions and materials are process-appropriate. 

Laser is a recent innovation in rework and repair. The fundamentals of laser for initial soldering or for repair have already been discussed in Chap. 47. In the commercial incarnation of this technique, a laser beam is quickly scanned around component leads or, package surface, in the case of area-array devices such as EGAs and chip-scale packages (CSPs). It is most effective for plastic packaged components rather than the thermally massive CBGAs, CCGAs, and so on. In this technique, the component body is heated. Otherwise, there is little difference between this and alternative rework or repair techniques. 

S.K. Sitaraman, R. Raghunathan, and C.E. Hanna, Development of Virtual Reliability Methodology for Area-Array Devices used in Implantable and Automotive Applications, IEEE Transactions on Components and Packaging Technologies, Vol 23 (No. 3), Sept 2000, p 452 61... 

Due to the ever increasing I/O counts for 1C devices, packaging trends have been changing to surface mountable area array second-level interconnection, namely BGA and CSP. The driving force for these types of second-level mounting is... 

Materials with Enhanced Mechanical or Conduction Properties. In cases where the device package (leadless area array with ball grids) is incompatible with the CTE of a standard material, a low-expansion snbstrate mnst be nsed.This can be achieved in several ways. In the past, leadless surface-mount technology (SMT) focused on replacing the woven glass in standard FR-4 with woven quartz or aramid fibers. Although this reduced the expansion of the substrate, both materials were expensive and difficult to process. 

Part range 0201 (English) passive chip devices to 10 mm area-array packages... 

Hot air or hot gas (nitrogen) has been used to reflow, simultaneously, all solder joints of fine-pitch (high I/O count), peripheral leaded packages (e.g., QFPs) as well as those of area-array packages that include EGA component CSPs, and FC components. An example of a hot-gas nozzle apphed to a EGA package is shown in Fig. 40.31. Control of the hot-air flow maximizes heat dehvery to the solder joints and minimizes the temperatnre rise in the component body. The shroud also limits the exposure of neighboring devices and their solder joints to elevated temperatures. 

Other factors also affect PCB inspection. Increases in component density are making it difficult to use ICT. Average density of wireless devices has reached a point where ICT cannot be used effectively. Higher speeds also reduce electrical access because electrical test points affect signal integrity above 500 MHz. The move to area-array packaging is driving visual loss of access. The proportion of joints that cannot be inspected with AOI solutions is expected to rise to about 50% by 2007. 

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