By Brian Vicich and Sai Uppuluri
 Figure 1. A common application for the LGA connectors is to use a socket, LGA chip and frame with lid to terminate the chip to the PCB. |
The key advantage of using nonlinear analysis in designing LGA connectors is the possibility of evaluating design parameters without performing physical tests.
Personal computer (PC)-based nonlinear finite element analysis was used in the design of land grid array (LGA) connectors. The beryllium copper compression contact used in the connectors needed to deflect at three times its thickness under a very small amount of normal force while avoiding a permanent set. In the past, this type of problem could only be addressed with expensive and difficult-to-use host- or workstation-based nonlinear analysis tools. In recent years, similar types of tools have been developed for PCs that provide equal capabilities at lower costs and are easier to use.
For these LGA connectors, the model was created using design analysis software to calculate the spring rate (amount of force needed to compress the spring of the connector) and other design parameters. This made it possible for the device manufacturer to develop the product in far less time than would have been required if using the conventional build-and-test approach.
The connectors terminate LGA chips to a printed circuit board (PCB). The system is solderless, although the socket can also be terminated to the PCB via solder balls. This system can be used to test several different grid arrays without having to solder the chip to the PCB; it can be used to change or upgrade LGA chips; and it allows sockets to be used on first-run PCBs and then removed without changing the pad layout on the PCB. The connectors are used in mezzanine and riser card applications and in test, emulation and debug applications.
Compression Contact System
The connectors utilize a Z-beam contact — a beryllium copper compression contact. The Z-beam contact is pressfit into a plastic grid array, which is the interface between the PCB and LGA chip. A common application uses the socket, LGA chip and a frame with lid to terminate the chip to the PCB (see Figure 1). A frame, made of a material such as anodized aluminum, is placed on the PCB; it holds the compression connector set and chip in alignment. The LGA socket is placed in the frame, and the LGA chip (or ball grid array [BGA]-style chip) is placed on the LGA socket. A screw-down lid compresses the leads on the chip to the top of the socket, thus compressing the leads on the bottom of the socket with the pads on the PCB. The lid can be incorporated into a heat sink.
Designing the Z-beam contact system was the most challenging part of developing the connectors. An important characteristic of LGA chips is that they concentrate a large number of connections, roughly around 600, in a very small area, perhaps 1 sq in. This makes it necessary, of course, for the contact system to be very small, in this case 0.003" thick and 1.5 mm high. At the same time, the amount of force required to deflect the contact system needs to be maintained at very low levels because of the high concentration of pins. A conventional contact system requires 100 g of normal force, which in a 600-pin connector amounts to between 132 lb per sq in of pressure, more than a typical PCB is capable of withstanding.
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In order to meet the requirements of this application, the engineers had to reduce normal force to something on the order of 40 g at 0.010" deflection. This reduces the pressure generated by the contact system to about 52 lb per sq in, which can be accommodated by the PCB.
Another important requirement of the contact system is that it not take a permanent set (the spring does not spring back) when the pin is inserted. If a permanent set occurs, the socket can only be used once. These connectors are targeted at test and measurement systems into which LGAs chips need to be continually inserted and removed.
Use of Nonlinear Analysis
In order to address these concerns, engineers used PC-based nonlinear finite element analysis in the process of designing the connectors. The key advantage of nonlinear analysis is that it makes it possible to evaluate key design parameters, such as the spring rate of the connector, without the necessity of performing physical tests.
The utility of nonlinear analysis has been increased by the advent of PC-based software that provides capabilities that previously were limited to host- or workstation-based software. The latest PC-based nonlinear analysis tools include many analysis capabilities that were previously limited to high-end software yet maintain the simpler user interface and high level of integration with computer-aided design (CAD) software that is characteristic of PC-based analysis tools.
The nonlinear analysis tool used in this design, the NSTAR Module of COSMOS/M from Structural Research & Analysis Corp. (Los Angeles), provides the ability to analyze nonlinear static and dynamic problems with geometric and material nonlinearities. These include large deflection, large strain plasticity, hyperelasticity, creep, thermoplasticity and viscoelasticity. The software also analyzes nonlinear contact problems involving surface interactions of 3-D models with or without friction.
Integration with CAD System
Engineers created a solid model of the proposed contact design in the CAD system. Using the NSTAR module, preprocessing was performed within the CAD system. The contact part was inserted into a CAD assembly and a block, which simulates the flat metal piece that pushes down upon the contact, was created. A few modifications were made to the model to remove material that was redundant to the analysis. This reduced the size of the finite element model and lowered computational time without sacrificing accuracy.
The removed material did not affect the deformation of the contact. The block was positioned just slightly above the contact in order to minimize computational times while avoiding the creation of a welded joint between the two components.
The assembly was meshed with tetrahedral elements. Because the contact underwent heavy bending deformation during the compression, four elements were used through the thickness to obtain an accurate solution. The flat portion of the contact, where it is attached to the motherboard body, was restrained. The prescribed displacement of 0.01" was not applied on the block because this displacement needs to follow a loading curve, which can be defined only in the Geostar interface (the graphical user interface [GUI] for activating controls for advanced finite element analyses, such as nonlinear analysis and dynamic analysis).
Performing the Analysis
The assembly consisting of the contact, block, material properties, restraints and mesh was imported into the Geostar interface. Options for large displacement and large strain plasticity were chosen for the contact. A loading curve, which acts as a load multiplier, was applied so that the block was displaced downward by 0.01" and then returned to its original position. The surface-to-surface gap elements were automatically generated. The analysis was then run and took approximately 5 hours to complete.
The results were validated using two approaches. First, the displacement results, both at total compression of the contact and at the end of the retraction, were found to compare well with experiments. Next, the nodal stress at compression and the element stress at compression were found to correlate well. This verified the accuracy of the mesh. The nodal stress and the element stress at the end of analysis were also found to match. Table 1 shows the results at total compression and at permanent set.
The ability to rapidly assess the spring rate and make sure the spring did not take a permanent set was crucial in the development of the LGA connectors. Physical testing would have been expensive and difficult because of the small size of the connectors and need to build a mold to construct each prototype iteration. Instead, engineers were able to evaluate the performance of alternative designs in a minimal amount of time simply by changing the solid model within their CAD system and repeating less than 10 commands to perform the analysis. In fact, the software allowed the engineers to analyze all four of their designs in one day. The result was that the design process was successfully completed in time to meet the product launch window.
BRIAN VICICH is New Product Engineering Manager, Samtec Inc., P.O. Box 1147, 520 Park East Blvd., New Albany, IN 47151-1147; (812) 944-6733; Fax: (812) 948-5047; Web site: www.samtec.com. SAI UPPULURI is Consultant Engineer, Structural Research & Analysis Corp., 12121 Wilshire Blvd., 7th Floor, Los Angeles, CA 90025; (310) 207-2800; Fax: (310) 207-2774; E-mail: info@srac.com; Web site: www.srac.com.
SPEC SHEET
End Applications:
Mezzanine and riser card applications; test, emulation and debug applications
Related Products:
LGA connectors, LGA chips, PCBs
Main Point:
The ability to rapidly assess the spring rate and make sure the spring did not take a permanent set was crucial in the development of these LGA connectors. Physical testing would have been difficult to do because of the small size and the need to build a mold to construct each prototype iteration. Instead, engineers were able to evaluate the performance of designs by changing the solid model within their CAD system and repeating less than 10 commands to perform the analysis.
