Land Grid Array Sockets Research Articles

Contact Resistance Behavior of Land Grid Array Sockets at Cryogenic Temperatures Required for Quantum Measurements

The current state of quantum device maturity proposes the use of removable, reusable interconnection means between such device and their classical microelectronic measurement system. One such means is a land grid array (LGA) socket, used pervasively in microelectronics but relatively untested at cryogenic temperatures necessary for most quantum technologies. This article investigates two distinct commercially available LGA socket technologies with respect to contact resistance performance under cryogenic temperature measurement conditions. Test methodologies were successfully developed for both sockets, demonstrating predictable and repeatable contact resistance values during multiple excursions down to 1.5 K and a single excursion to 10 mK. The experimental data show that contact resistance of individual interconnection sites decreased from 30 to 40 $\text{m}\Omega $ at room temperature to 14-17 $\text{m}\Omega $ at 1.5 K and to 6–8 $\text{m}\Omega $ at 10 mK, thus supporting their reliable use in cryogenic measurement applications. Theoretical calculations based on the construction of the microspring socket validated that measured values behaved in a manner that was consistent with temperature-dependent changes in resistivity of the metals comprising the socket interconnection. Spring deflection allowances compensated for any temperature-driven movement of components within the assembly, thus enabling predictable resistance behavior. Regarding the elastomer socket, the high coefficient of thermal expansion (CTE) of the elastomer bulk material induced large variations in socket compression across the temperature range investigated. Sufficient compression to provide consistent contact resistance values at cryogenic temperatures imposed the need for very high compression at room temperature, which may place into question the longer term integrity of this solution. Nevertheless, a preliminary investigation (five cryogenic cycles and subsequent construction analysis) did not reveal any degradation in either socket type.

Structural Design of Land Grid Array Loading Mechanisms for Intel Central Processor Unit Stack Retention

For more than a decade, land grid array (LGA) has been one of the main central processor unit (CPU) packages developed at Intel and AMD, and widely used in different computer systems. LGA loading mechanism has become more critical to achieve mechanical, thermal, and electrical functions with the increasing retention force requirement. During the development of the loading mechanisms for LGA packages and sockets, socket pin contact to LGA pad under retention load, solder joint reliability under shock load, socket pin fretting under vibration, and load degradation are some of the key structural risks. This paper reviews the structural designs of different loading mechanism solutions systematically and summarizes the key structural concerns and advantages. While the finite element analysis (FEA) was used to guide the design options in early platform architectural definition, this review discusses the evolution of Xeon LGA loading mechanisms developed at the Intel Data Center Group.

High Current Testing and Simulation for Land Grid Array Sockets

Abstract Land grid array (LGA) sockets are commonly used for industry standard and custom microprocessors to meet the increased performance challenges for a variety of server applications. Along with the need for increased high speed signaling capabilities comes the challenge to support lower voltages and higher currents. Typical testing that is conducted by the LGA socket suppliers does not provide an accurate assessment of the maximum current capabilities in a real product application due to the test card design and construction limitations. Typical test card designs use daisy chain connections to wire multiple LGA socket contacts in series. The daisy chain wiring in the test card adds to the resistive heating and results in an inaccurate maximum current rating. Also, the test cards typically do not have a cross section construction that is representative of a real product application with multiple ground planes that provide improved thermal dissipation of the heat generated by the LGA socket interface. Hardware testing was conducted to better understand the performance limitations for a new product application. The test card was designed to use multiple voltage and ground planes in the circuit card cross section to provide a low impedance path for current flow and a low voltage drop through the LGA socket interface. In addition to the test card construction, the test hardware included a special test module with a shorted chip to provide a more accurate power distribution path through the socket and processor package. The test variables included different plating metallurgy options for the LGA socket and the processor module along with different configurations for the voltage supply and ground return contacts. Electrical and thermal modeling techniques were used to simulate the test hardware configuration with good correlation between the hardware and modeling results. Based on the positive correlation results, additional modeling was conducted to simulate the worst case power mapping conditions for the processor chip along with a more accurate power distribution. The additional modeling results provided further insights into the maximum current capabilities for the LGA socket based on the temperature increase from the resistive heating in the socket contacts.

Prediction enhancement of the J-lead interconnection reliability of land grid array sockets

The solder joint is a key component in land grid array (LGA) sockets. A simplified solder joint has been widely used in finite element model (FEM) computations because the J-lead interconnection solder joint is relatively complex. Therefore, there are discrepancies between the physical phenomenon and FEM simulations. In this study, an alternative method to simulate the J-lead interconnection solder joint through an interface program using surface evolver software is presented. Simulations of the J-lead interconnection solder joint were improved to reduce the mismatch between the actual physical shape and the simplified finite element models that are typically used to predict component reliability. To perform these simulations, an interface program capable of simulating solder interconnections for twelve different pad–solder combinations was developed. Predictions of J-lead interconnection solder joints were carried out using the interface program. Geometric comparisons between experimental data and predictions showed good agreement, with the exception of wetting height. To evaluate the prediction accuracy of the simulated J-lead solder joints, FEM analysis was performed for the static load and the thermal cycle.

Reliability Assessment of Land Grid Array Sockets Subjected to Mixed Flowing Gas Environment

Land grid array (LGA) sockets are used as separable interconnects between printed circuit boards, and high I/O integrated circuit components. Because the socket contacts with the component and the circuit board on both sides, low and stable contact resistances among these interfaces are desirable for the reliable electrical connection of the LGA package. This paper compares the contact resistance degradation of two LGA socket designs, silver particles in an elastomer column, and silver-plated-nickel in an elastomer film, subjected to accelerated corrosion environment, or so-called mixed flowing gases (MFG). Increased contact resistance was observed in the sockets due to corrosion products over the contact surface after metal corrosion. Corrosion products were analysed using environmental scanning electron microscopy (E-SEM), energy dispersive spectroscopy (EDS), and x-ray photoelectron spectroscopy (XPS). AgCl, Aga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O, and Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> SO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> were identified after silver corrosion; NiCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> SO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , and MS were found after Ni corrosion. The nickel corrosion product bridged the adjacent contacts as whiskers with time. The lifetime can be estimated by an acceleration factor: two day MFG Battelle Class II exposure equals one year of field service. The shelf life of conductive elastomer LGA sockets is expected to be longer than five years.

Contact Resistance Estimation for Time-Dependent Silicone Elastomer Matrix of Land Grid Array Socket

A land grid array (LGA) socket provides high-density, electrically-separable interconnect between a component and a printed circuit board. The LGA socket must not be susceptible to creep and stress relaxation, which can lead to loss of contact force and consequently the degradation of contact resistance. In this paper, the time-dependent contact resistance failure mechanisms were experimentally assessed and theoretically analyzed for silicone elastomer matrix socket interconnects. A time-dependent physics-of-failure model was developed for contact resistance simulation

Land grid array sockets for server applications

The design and performance of land grid array (LGA) sockets are discussed with respect to high-performance server applications. Motivations for the use of this technology are presented, and the specific challenges associated with its application are discussed from a mechanical perspective. A variety of mechanical performance considerations are identified for LGA socket technologies, and a detailed evaluation of competing socket-actuation designs is presented using finite element structural analysis. Some design approaches are shown to suffer from excessive mechanical flexure, which can consume the allowable range of motion of the contact, resulting in excessive contact load variation within the LGA. Statistical considerations for the mechanical deflections and tolerances that contribute to range-of-motion consumption are developed using Monte Carlo techniques, and a parametric study of the mechanical design variables that influence contact load variation within the LGA is presented.