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FIGURE 50.2 Stainless steel bolster plate (l) and insulator (r). (Courtesy of Hewlett-Packard.)
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that holds the LGA stack together. It is typically fashioned from a stiff metal such as stainless steel or thick aluminum (see Fig. 50.2). The bolster plate may be orthogonal or sculpted to accommodate components in the vicinity of the bolster plate or to permit access to test points for in-circuit testing (ICT) or diagnostics. The design of the bolster plate must be such that it is rigid upon fastening and must flatten out any local bow or warpage in the vicinity of the LGA contact field on the PWB to ensure that all electrical contacts are made and mechanical stability is imparted to the LGA stack. Usually an insulator is applied over the surface of the bolster plate and positioned against the underside of the PWB to prevent electrical shorting of the metal bolster to any electrical features on the secondary side of the PWB (through-hole vias, test pads, and so on). It is best not to include any components on the secondary side of the board beneath the bolster plate. If components must be included, the bolster plate will have to be relieved for clearance so that these components will not be crushed during LGA assembly. 50.3.3 Active Component (LGA IC) The LGA is an IC package usually made from a rigid substrate such as ceramic. It looks like a ceramic ball grid array (CBGA) without solder interconnects (balls) on its underside (see Fig. 50.3). In all other respects, it is like any other ceramic package. It typically has gold pads on its underside that serve as the contact points for the LGA socket. The contacts are usually electrolytic nickel overplated with electrolytic gold. Sometimes a hardening agent such as cobalt is co-plated with the gold to enhance contact durability for improved long-term reliability.
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FIGURE 50.3 LGA package topside (a) and underside (b). The package is similar to a CBGA without any solder interconnects. (Courtesy of Hewlett-Packard.)
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Socket Many types of LGA sockets (also referred to as interposers) are available. There are interposers with traditional dimpled leaf-spring contacts, Fuzz Button contacts, and even metal-filled polymer contacts. The contacts are held on a molded insulating substrate. The contacts and socket frame are customized to fit the LGA package and corresponding PWB footprint (see Fig. 50.4).
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FIGURE 50.4 LGA sockets: (a) a socket with metal-filled polymer contacts; (b) a socket with Fuzz Button contacts; (c) closeup of a single wire-wound Fuzz Button.
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Each contact is interconnected through the molded substrate in some fashion, and the socket can be considered double-sided for interconnecting both the LGA IC and the PWB (see Fig. 50.5).
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FIGURE 50.5 Interposer sockets electrically connecting the PWB with an LGA IC: (l) spring contact; (r) Fuzz Button contact.
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Additional subelements such as fasteners and springs may be required to enable or enhance fastening, shock and vibration resistance, and thermal management. Although Fuzz Button technology is among the most reliable of LGA interconnect schemes, some defects are common to this socket style. Pulled wires (see Fig. 50.6) as well as leaning and missing contacts have been seen, although the interconnect reliability is quite high if the socket is in good condition prior to assembly and if the LGA stack is assembled correctly. If too much pressure is applied to metal-filled polymer contacts, they can deform and flow over time, lessening contact pressure, and result in open or intermittent contact failures.
FIGURE 50.6 Fuzz Button LGA socket with a stray strand of wire, which can cause electrical shorting.
Pressure Plate The pressure plate is fastened atop the LGA package and couples to the bolster plate. It forces the LGA package in contact with the socket and the socket to the lands on the PWB for completion of the electrical circuit. Besides being responsible for interconnecting the socket to package and board, coupled with the bolster plate, it imparts the necessary mechanical rigidity to the LGA stack for shock and vibration resistance and reliable operation. Of course, shock and vibration resistance will be dependent on bolster plate and pressure plate designs, including any springs, fasteners, and such. Force on the stack has to be such that it damages neither the LGA IC nor the PWB. Guidance is needed from the LGA package manufacturer and from the LGA socket manufacturer as to how much force can be applied to the stack without damaging the board, socket, or IC package.
Thermal Management Frequently, thermal management solutions are built into the LGA stack. The heatsink, if constructed properly, can be used as the pressure plate. A thermal interface material such as graphite sheet or other thin thermal transfer material can be used between the heatsink and the IC package to enhance heat dissipation.
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