PC-nonlinear FEA makes the connection
Engineers use PC-based nonlinear analysis to help design land grid array connectors
By John Lewis, Northeast Technical Editor -- Design News, September 17, 2000
Beryllium copper compression contacts, used in new connectors, have to deflect at three times their 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 tools have migrated to personal computers where they provide equal capabilities at a far lower cost and are also much easier to use.
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Samtec uses SRAC's NSTAR module of COSMOS/M to design land grid array connectors to demanding specifications. |
For example, Samtec (New Albany, IN) uses the NSTAR module of COSMOS/M, from Los Angeles-based Structural Research & Analysis Corp. to design land grid array (LGA) connectors to demanding specifications. "In this application, the model was created using a design-analysis software suite to calculate the spring rate of the connector and other design parameters," says Brian Vicich, Samtec's new product engineering manager. "This made it possible for the device manufacturer to develop the product in far less time than would have been required using the conventional build-and-test approach."
Samtec's LGAF and LGAM Series are connectors that terminate LGADs (Land Grid Array Devices) to a printed circuit board (PCB). The products are available on two centerlines—0.050 inch (LGAF Series), and 1 mm (LGAM Series). It is a solderless system, although the socket can also be terminated to the PCB via solderballs.
This system can be used to test several different grid arrays without having to solder the device to the pc board. It can be used to easily change or upgrade LGA devices, and it allows sockets to be used on first-run boards and then removed without changing the pc-board's pad layout. Used in mezzanine and risercard applications, such devices are targeted at test, emulation, and debug applications.
New contact system. Both connectors use Samtec's Z-Beam contact system, a beryllium copper compression contact. The Z-Beam contact is press-fit into a plastic grid array, which acts as the interface between the pc board and LGA. A common application uses the socket, LGA device, and a frame with lid to terminate the device to the pc board.
A frame, made of a material such as anodized aluminum, is placed on the pc board. The LGAF socket is placed in the frame, and the LGA device is placed on the LGAF socket. A screw-down lid compresses the leads on the device to the top of the socket, 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 is the most challenging part of developing the connector," Vicich says. "An important characteristic of LGAs is that they concentrate approximately 600 connections into perhaps one square inch. This makes it necessary, of course, for the contact system to be very small, in this case 0.003-inch thickness, about the same as a human hair, 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 would require 100 grams of normal force, which in a 600-pin connector would amount to about 132 lbs per sq inch of pressure, more than a typical pc board can withstand."
In order to meet these requirements, Samtec engineers had to reduce normal force to something on the order of 40 grams at 0.010-inch deflection. This reduces the pressure generated by the contact system to about 52 psi, which can be accommodated by the PCB.
Another important requirement of the contact system is that it not take a permanent set when the pin is inserted. If a permanent set occurs, the socket could only be used once. These connectors must avoid a permanent set because they are largely targeted at test and measurement systems into which LGAs will be continually inserted and removed.
Nonlinear analysis. In order to address these concerns, Samtec engineers decided to use nonlinear finite element analysis as part of the process of designing their new LGAF and LGAM connectors. The key advantage of nonlinear finite element analysis is that it makes it possible to evaluate key design parameters, such as the spring rate of the connector system, 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 that costs well into five figures on an annual lease. The latest personal computer-based nonlinear analysis tools include many analysis capabilities that were previously limited to high-end software, yet they maintain the much simpler user interface and high level of integration with CAD software that is characteristic of PC-based analysis tools.
The nonlinear analysis tool used in this application, the NSTAR module of COSMOS/M, provides the ability to analyze nonlinear static and dynamic problems with geometric and material nonlinearities. These include large deflection, large strain plasticity, hyper-elasticity, creep, thermo-plasticity, and visco-elasticity.
The software also analyzes nonlinear contact problems involving surface interactions of 3D models with or without friction. The NSTAR module can virtually handle problems of any size with no limit on the mesh size. NSTAR incorporates a direct sparse solver that utilizes sparse matrix technology and advanced reordering techniques to speed up the force-stiffness solution. This new solver is far more efficient than the conventional direct methods. In addition, NSTAR includes a new iterative solver which is recommended for medium to large models. With this new solver, solutions to "large" nonlinear problems can be obtained orders of magnitude faster than direct solver procedures.
CAD integration. "We created a solid model of the proposed contact design in the SolidWorks CAD system," Vicich explains. "Using the Cosmos/Works module that integrates tightly with SolidWorks, preprocessing was performed within the CAD system. The contact part was inserted into a SolidWorks 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 material removed does 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 undergoes heavy bending deformation during compression and therefore, four elements are used through the thickness to obtain an accurate solution. The flat portion of the contact, where it is attached to the motherboard body, was constrained. The prescribed displacement of 0.01 inch was not applied on the block, in Cosmos/Works software, because this displacement needs to follow a loading curve and currently the loading curve can be defined only in the Geostar GUI. A very stiff material was assigned to the block while Samtec provided elasto-plastic material properties for the contact. This model was then exported from the Cosmos/Works interface into the Geostar interface.
The assembly consisting of the contact, the block, material properties, restraints and mesh was imported into Geostar, which is the GUI for activating controls for advanced finite element analyses, such as nonlinear analysis and dynamic analysis. Future releases of Cosmos/Works will have the ability to run advanced analysis, without having to use Geostar.
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 displaced downwards by 0.01 inch and then returned to its original position. The surface-to-surface gap elements were automatically generated. Engineers then ran the analysis on a Pentium II 500 with 512 Mbytes RAM. The nonlinear analysis took approximately 5 hours to complete.
The model stress and the element stress at the end of the analyses were found to match.
| Analysis results at total compression and at permanent set | |||||
|---|---|---|---|---|---|
| Vertical displacement at the top (inches) |
Horizontal displacement at leading edge (inches) |
Maximum stress (nodal & element stress average-ksi) |
Maximum strain (total) |
Forced load (lbf) |
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|
Compressed |
0.01 |
0.01988 |
2.08e5 |
0.026 |
0.107 |
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Permanent Set |
0.0013 |
0.0035 |
89,000 |
0.012 |
0 |
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