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PWB vendor to ensure that drilled and plated holes are sized appropriately for the press-fit process and that excess solder from the HASL process does not adversely alter the finished hole size or shape.
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Electrical Testing In the case of high-friction surface finishes, such as OSP-copper, forces applied during the press-fit process may be so great as to cause damage to the PTH barrel or adjacent inner- or outerlayer circuit traces. Check the board visually and by electrical test as part of a first article inspection.
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Check Tooling Frequently Check for excessive tool wear or damage. A damaged tool can ruin a connector or PWB and replicate the damage over many boards.
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Apply press-fit connectors after all SMT and wave-soldering operations. Do not try to solder press-pin connectors in place. Press-fit connector bodies may not be reflow-compatible. Soldering a press-fit connector will also make rework difficult or impossible.
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1. EU RoHS Commission Decision of 21 Oct 2005, Annex to Directive 2002/95/EC of the European Parliament and of the Council on the restriction of the use of certain hazardous substances in electrical and electronic equipment. 2. Parenti, D., and Mitchell, J., Validating Press-Fit Connector Installation, Circuits Assembly, April 2003, pp. 26 29. 3. Ocket, T., and Verhelst, E., Lead-Free Manufacturing: Effects on Press-Fit Connections, paper presented to the International Center for Electronic Commerce (ICEC) Proceedings, Zurich, September 2002. 4. Hilty, R. D., Effect of Lead Elimination on Press-Fit Interconnects, paper presented to the Institute of Electrical and Electronic Engineers (IEEE) HOLM Conference Lead Free Workshop, September 9, 2003.
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Gary M. Freedman
Hewlett-Packard Corporation, Business Critical Systems, Singapore
The land grid array (LGA) device is a solder-less package that relies upon pressure contact to interconnect it to the board mechanically and electrically. Interconnection is accomplished through a socket or interposer that is sandwiched between the LGA integrated circuit (IC) package and the printed wiring board (PWB). The socket has compliant, conductive contacts that match the LGA and PWB contact land patterns on each. A system of insulators, stiffeners, and fasteners keeps the LGA in intimate mechanical and electrical contact with the PWB. Care in assembly and cleanliness is of utmost importance for successful pad-to-land contact and subsequent assembly reliability. The reader should be aware that there are leadless soldered packages also called land grid arrays, but this chapter will deal with the pressure-interconnect variety of leadless package. Most often, an LGA-type interconnect is used for mounting a central processor unit (CPU) chip or other active device to a PWB. It is also used for some connectors. An advantage of this type of interconnect is that it makes it possible to exchange a chip without the need to unsolder and re-solder, facilitating rapid rework, engineering changes or field upgrades without soldering and without degradation in reliability. The LGA concept has been applied to connectors also. Although the technique has been in use for several years, it is still a niche technology but will likely increase in importance as input/output (I/O) counts continue to increase and package size limitations for soldering are met.
Since the LGA connection to the PWB is solderless, it is environmentally friendly, requiring no heating for solder, soldering, or rework. It is easy to move into compliance with the European Union s (EU) Restriction of Hazardous Substances (RoHS) requirements since solder considerations (at least external to the package) are absent.
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There are five key elements and several subelements that make the LGA system practical and reliable.These are pictured in Fig. 50.1, but be aware that there are many variations. Key elements will be described in a generic sense in this chapter.
FIGURE 50.1 Elements of an LGA stack.
PWB and LGA Issues Contact Lands. The PWB is much like any other board. It has lands to accommodate electrical interconnect of the LGA socket to the board. The lands correspond in size and shape to socket requirements. Most often, the LGA lands on the PWB are electrolytic nickel covered by electrolytic gold. Hardened gold is often used for added contact durability and reliability. Other surface finishes can also be used, but should be tested for reliable and long-lasting contact. Drilled Holes. LGA sockets have the requirement for two sets of drilled holes in the PWB. One set is used to accommodate fastening of the bolster plate through the PWB. The second set is used to orient and retain the socket on the PWB. The closely toleranced orientation holes correspond to molded locating pins on the LGA socket and they are meant to locate the socket contacts properly on the LGA lands of the PWB. The socket manufacturer will provide guidelines and drill pattern recommendations. It is good practice to make the locating pins different sizes and to drill the PWB accordingly to polarize the socket so it cannot be misoriented on the board. PWB Layout Considerations. There are some layout considerations such as avoidance of components in the vicinity of the LGA socket, bolster plate, and pressure plate. PWB flatness should be considered so that best contact can be made to the board once the LGA interconnect system is fully assembled.
50.3.2 Bolster Plate LGA technology relies on a pressure interconnect system, and the bolster plate is the rigid backside element that serves to anchor the LGA system to the PWB. It is inserted through predrilled holes in the circuit board and provides the means for fastening the clamping mechanism
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