COSMOS helps analyze circuit board connectors

Company Samtec Industry Electrical/Electronics Location U.S.A. Product Used COSMOSWorks, COSMOSM Type of Analysis Nonlinear More Details

The Challenge
Samtec's LGAF and LGAM Series are connectors that terminate LGAs (Land Grid Arrays Devices) to a printed circuit board (PC board). The products are available on two centerlines - 0.050-in. (LGAF Series), and 1 mm (LGAM Series). This is a solderless system, although the socket can also be terminated to the PC board 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 PC boards and then removed without changing the pad layout on the PC board. These can be used in mezzanine and risercard applications, as well as to test emulation and debug applications.

Both connectors utilize Samtec's Z-Beam contact system, a beryllium copper compression contact. Designing the Z-Beam contact system was the most challenging part of developing the connector. An important characteristic of LGAs is that they concentrate a large number of connections (around 600) in a very small area, perhaps one square inch. This makes it necessary, for the contact system to be very small, in this case 0.003-in. thickness, (about the same as a human hair) and 1.5mm 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 between 132 PSI of pressure, more than a typical PC board is capable of withstanding.

The Solution
In order to meet the requirements of this application, Samtec engineers had to reduce normal force to the order of 40 grams at 0.010 in. deflection. This reduces the pressure generated by the contact system to about 52 PSI, which can be accommodated by the PC boards. Another important requirement of the contact system is not to take a permanent set when the pin is inserted.

The nonlinear analysis tool used in this application, the NSTAR module of COSMOSM, provided the ability to analyze nonlinear static and dynamic problems with geometric and material nonlinearity, hyperelasticity, creep, thermo-plasticity and viscoelasticity. The software also analyzed 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.

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 Analysis, such as Nonlinear Analysis and Dynamic Analysis. 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 is displaced downwards by 0.01-in. and then the block is returned to its original position. The surface-to-surface gap elements were automatically generated.