Nikhil Lakhkar

Effect of structural design parameters on wafer level CSP ball shear strength and their influence on accelerated thermal cycling reliability

Ball shear testing is typically conducted in Wafer level chip scale package (WLCSP) fabrication to estimate the strength of the solder ball attachment. Generally, the solder ball shear strength is dependent on the solder ball size, pad size, solder/pad interface treatment, reflow temperature and time. Solder ball strength is also a function of ram speed and height at which the ball is sheared with respect to the wafer. Recent investigations suggest that ball shear test is being used as an indicator for board level reliability of assemblies. In current market lead time for launching a new product is very short. Unfortunately, it takes several weeks to qualify a new product by board level qualification process. If there is a methodology through which one can predict the board level performance by extrapolating the wafer level test, it will save great amount of resources in testing and millions of dollars worth of testing time. In the first part of this study, we conducted a wafer level ball shear test. A DOE was created for varying wafer level structural parameters like solder ball size and type. Ball shear tests and Accelerated thermal cycling have similar failure signatures of compression on inner side and tension on outer side. Thus, for specific cases there is a possibility of correlating the two failure methodologies based on their failure signatures. Strain rate for ball shear test was determined based on shear speed and solder pad diameter. Strain rate for accelerated thermal cycling was determined based on difference in CTE between board and package. In this paper, results from ball shear test and accelerated thermal cycling are compared to find correlations for specific cases. The correlations derived from this study are statistical and empirical.Copyright

CFD modeling of a thermoelectric device for electronics cooling applications

Increased power density in electronics systems is limiting the ability of air cooling to provide adequate cooling for the components in the systems. Thermoelectric cooling are solid state and highly reliable devices that can be used as either refrigeration or heat pump. The use of thermoelectric device for hot spot cooling has attracted significant research. Under specific conditions, a well-selected thermoelectric device can act as an enhancement for air cooling devices. A Thermoelectric device pumps heat from the cold to hot side, this direction is controlled by the voltage that is applied across the thermoelectric device. If the voltage is applied in opposite direction, the same thermoelectric cooler acts as a thermoelectric heater. In this paper, thermoelectric is used to study as a cooling device. A numerical test bench was set up in Icepaktrade. The power applied to thermoelectric, current supplied to thermoelectric and G factor were varied to find out the effect of each of these factors on the performance of the thermoelectric. The performance is evaluated by package temperature. In this study it was revealed that thermoelectric is not only used as a hot spot cooling solution but also can be used as a primary solution too. This study is mainly aimed for high end processors used in applications such as gaming.

Solar shroud design using Computational Fluid Dynamics

The Wireless Telecom Industry is moving into a new age wherein remote data transmission capabilities must meet the demands of versatile mobile devices. These needs will be met by powerful new innovations such as 4G and WiMax.

Impact of Area Contact Between Sensor Bulb and Evaporator Return Line on Modular Refrigeration Unit: Computational and Experimental

In the past, virtually all commercial computers were designed to operate at temperatures above the ambient and were primarily air-cooled. However, researchers have always known the advantages of operating electronics at low temperatures. This facilitates faster switching time of semiconductor devices, increased circuit speeds due to lower electrical resistance of interconnecting materials, and reduction in thermally induced failures of devices and components. Depending on the doping characteristics of the chip, performance improvement ranges from 1% to 3% for every 10°C lower transistor temperature can be realized. The IBM S/390 high-end server system is the first IBM design which uses a conventional refrigeration system to maintain the chip temperatures below that of comparable air-cooled systems, but well above cryogenic temperature. In previous work, the focus was to study the effect of variation of evaporator outlet superheat on the flow through thermostatic expansion valve at varying evaporator temperature. The effect of change in bulb location and effect of bulb time constant on the hunting at the evaporator has been reported. The effect of area contact on the stability of the system is been predicted theoretically. Mechanical analysis is performed in order to check the stresses induced. The evaporator return line and the sensor bulb are simply attached. The effect of area contact is further studied experimentally on an experimental bench.

Experimental and Computational Characterization of a Heat Sink Tester

With the continuing industry trends towards smaller, faster and higher power devices, thermal management continues to be extremely important in the development of electronics. In this era of high heat fluxes, air cooling still remains the primary cooling solution in desktops mainly due to its cost. The primary goal of a good thermal design is to ensure that the chip can function at its rated frequency or speed while maintaining the junction temperature within the specified limit. The first and foremost step in measurement of thermal resistance and hence thermal characterization is accurate determination of junction temperature. Use of heat sinks as a thermal solution is well documented in the literature. Previously, the liquid cooled cold plate tester was studied using a different approach and it was concluded that the uncertainty in heat transfer coefficient was within 8% with errors in appropriate parameters, this result was supported by detailed uncertainty analysis based on Monte-Carlo simulations. However, in that study the tester was tested computationally. In this paper, testing and characterization of a heat sink tester is presented. Heat sinks were tested according to JEDEC JESD 16.1 standard for forced convection. It was observed that the error between computational and experimental values of thermal resistances was 10% for the cases considered.Copyright

The impact of Area Contact Between Sensor Bulb and Evaporator Return Line in a Modular Refrigeration Unit - Part I, Computational Study

The combination of increased power dissipation and increased packaging density has led to substantial increase in junction temperature, at both the chip and module level in computers and especially at the high-end. In the past, virtually all-commercial computers were designed to operate at temperatures above the ambient and were primarily air-cooled. However, researchers have always known the advantages of operating electronics at low temperatures. This facilitates faster switching time of semiconductor devices, increased circuit speeds due to lower electrical resistance of interconnecting materials and reduction in thermally induced failures of devices and components. Depending on the doping characteristics of the chip, performance improvement ranges from 1 to 3% for every 10°C lower transistor temperature can be realized. The paper addresses improving the cooling of IBM’s high-end server unit, which uses a conventional refrigeration system to maintain the chip temperatures below that of comparable air-cooled systems, but well above cryogenic temperature. The IBM S/390 high-end server system is the first IBM design that employed refrigeration cooling. Advantage of using refrigeration unit is improvement in reliability, and performance improvements related to the lower operating temperature. In previous work, the focus was to study the effect of variation of evaporator outlet superheat on the flow through the thermostatic expansion valve at varying evaporator temperature. Also the effect of change in bulb location and effect of bulb time constant on the hunting at the evaporator has been reported. Currently, the evaporator return line and the sensor bulb are simply attached with a clip, with no thermal consideration. In the present study, the performance that results from varying the area of contact between the evaporator return line and sensor bulb is discussed. Subsequently, the effect of various interface materials on the performance is examined.Copyright

Multi-Objective Parameterization of Graphite Heat Spreader for Portable Systems Applications

The advancement made in portable electronic systems has primarily been due to miniaturization of electronic systems. This in turn leads to an increase in power density which leads to higher temperatures and formation of hot spots. There is a temperature specification of system surfaces for human comfort (such as the surface close to a keyboard on laptops). Challenge in cooling portable devices is that there is not enough room to accommodate heat sinks. It is therefore important to have heat spreaders that can transfer the heat from critical devices to regions where cooling is available. Traditionally, copper has been the best heat spreader due to its high thermal conductivity. However, copper has a relatively high density and correspondingly high weight. Graphite is a suitable alternative. Recent advances in graphite technology have resulted in fairly high conductivity in the planar directions. In spite of these advances, the cost of graphite is an issue. In this paper, a multi-objective parameterization is utilized that considers weight and temperature distribution of the graphite heat spreader as objective parameters. The data is then compared to published data that utilizes graphite in a laptop.Copyright

Process development and die shear testing in MOEMS packaging

Micro-Opto-electro-Mechanical Systems (MOEMS), which are MEMS integrated with photonics, share the traditional challenges of MEMS packaging with the additional issues of optical interconnects and optical surface contamination. Optoelectronic systems require signals through a package using fiber-optics, coaxial or other interconnection approaches. Precise optical component alignment and accurate thermal management is critical to achieve component and system performance capabilities. As the requirements for higher signal speeds and higher data rates grow, and as operating frequencies move to the higher GHz regions and beyond, the choice of packaging technology becomes critical. Optical MEMS packages not only have to provide electrical connections but also support high precision optics and mechanics, vital for operation of optical MEMS devices. Analogous to MEMS, packaging is also a critical path in reliability and cost of MOEMS devices. One of the commonly observed failures is the de-lamination between the chip (die) and the die attach. The focus of this paper is the development of a reliable fluxless die attachment process suitable for MOEMS assemblies with long operational life time. The reliability assessment was carried out at the Texas Microfactory™ at UT Arlington according to the MIL-STD-883F for die shear reliability.Copyright

Mechanical characterization of Sn-Ag-Cu solder with gold addition under tensile loading

Lead (Pb)-free implementation for electronic assembly has created a stir in electronic packaging industry during the last five years since Europe and eastern Asian countries decided to restrict the use of lead in electronic packages. Lead (Pb) content in solders is mandated to be less than 0.2 wt % (USA) and 0.1 wt % (EU). Sn-Ag-Cu (Tin, Silver, and Copper) solder is selected as one of the options to replace tin-lead solders. This solder is a preferred option as it comes closest to tin-lead solder in terms of parameters such as melting temperature (∼217°C), wettability, cost, availability, and reliability. Various agencies like NEMI, JEIDA and IDEALS recommend Tin-rich Lead (Pb) free solders as the possible alternatives to Pb-Sn solder [1]. Addition of elements like Au, Co, Fe, Ni, etc in small quantities can affect the properties of Sn-Ag-Cu solder. It has been reported that the addition of Au in small quantities improves the properties of lead-free solder. Au has very high reactivity with Sn and also improves the wettability of solder. Au forms a β-phase with Sn at the interface. This phase is considered beneficial in terms of improving fatigue life and fracture toughness as this β phase acts as a crack arrester thereby improving its strength Addition of Au also reduces the liquidus temperature to 204 °C. In this paper we tested and compared the strength of pure Sn-Ag-Cu solder and Sn-Ag-Cu solder with Au addition and it was concluded that the strength of material increases from 50 MPa to 70 MPa under tensile loading.Copyright

Design of a cold plate tester for accurate measurement of thermal performance

With the continuing industry trends towards smaller, faster and higher power devices, thermal management has become an extremely important element in the development of computer products. The primary goal of a good thermal design is to ensure that the chip can function at its rated frequency, while maintaining its junction temperature below the specified limit, to ensure reliable operation. The use of a heat sink or cold plate to manage the external thermal resistance has been well documented in the literature. However, the measurement of thermal performance of today state-of-the-art cold plates is challenging because of the low value of thermal resistance that they offer to heat dissipation. In this paper, the design of a tester apparatus for such high performance cold plates is presented. The expected performance of the tester is modeled numerically for a heat flux of 250 W/cm2 , and for a range of footprint areas of 100-400 mm2 . The analysis study is supported by a detailed uncertainty analysis that utilizes a Monte Carlo simulation approach. It was observed that the sum of random and repeatable errors could be controlled to within ±7.5% even for a very high performance cold plate with an effective heat transfer coefficient of 200,000 W/m2 -K dissipating 250 W/cm2 , with assumed errors in other relevant parameters.Copyright