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Engineering Change Methodology for LGA Compression Connector Disclosure Number: IPCOM000015691D
Original Publication Date: 2002-Aug-21
Included in the Prior Art Database: 2003-Jun-21

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As higher density connectors are implemented on systems, there is a growing demand for connector schemes that permit significant I/O densities while at the same time provide for excellent signal integrity interconnection to the next level of packaging. With the advent of Land Grid Array (LGA) compression connectors as defined by Cinch, Thomas and Betts, etc., the density and signal integrity problems are addressed. A new problem for providing a suitable method of Engineering Change (EC) for rewiring or repairing system boards is required. In the early stages of hardware build, there are times that engineering changes are needed for the system boards. These modifications are due for a number of reasons including architecture changes, added function, or early user availability from imperfect hardware. The LGA connector requires an electrically insulated backing structure to counterbalance the forces used to compress the chip carrier (i.e., Multi-chip Module (MCM) or Single-chip Module (SCM) to the system board. In the common application of this arrangement, an insulator sheet of FR4 and metal plate are used, which by nature of their design, entirely covers the access to the connector area on the board and hence the I/O’s of the chip carrier. In most applications, this is permissible but when access is needed for Engineering Changes it is necessary to provide space for the repair wires (both DC and high frequency coaxial cable) while maintaining excellent electrical measurement characteristics provided by the LGA connector. Two basic approaches have been employed previously to facilitate the addition of the EC wire. The first is the custom machining of the insulator to facilitate routing of the wire between the Printed Circuit Board (PCB) and the insulator. This routing is unique per repair and provides the necessary clearance for the EC wire to prevent mechanical loading of the wire by the LGA actuation forces. This customized repair technique is both prone to error as well imposes significant cost. Second, is the introduction of a flex-circuit based interposer specifically designed to provide access to the signal pads. This design is temporary and therefore requires removal of the circuit before final product assembly. To address the problem, a pre-molded LGA has been conceived to a facilitate the routing of EC wires without the necessity for customized machining of the insulator. Figure 1 below provides an exploded view of a typical LGA actuated system assembly. This assembly includes the PCB w/stiffener, the insulator, the LGA connector, the MCM and its cooling apparatus. Figure 2 provides plane, isometric and x-section views of the insulator concept. Figure 3 depicts a X-section view of the new repair technique. As shown, the pre-molded insulator provides the necessary relief areas to facilitate the routing of the EC wire without custom machining. This technique also offers the advantage of maximizing the number of wire routing paths. 1