Victor Adrian Chiriac

Compact Thermal Resistor-Capacitor-Network Approach to Predicting Transient Junction Temperatures of a Power Amplifier Module

Junction temperature is an important issue for a semiconductor package, influencing the packages thermal, mechanical, and reliability performance. An accurate prediction of junction temperature provides informative guidance in design, development and operation of the package. A compact thermal resistor-capacitor (RC) network approach is presented in this paper to accurately predict transient junction temperatures. The thermal RC network in this approach is a nongrounded Foster network. This approach consists of extraction of the thermal Foster network and prediction of the transient junction temperature response to a given power input using the extracted network. The network extraction part is based on Kirchhoffs current law and Laplace transformation technique, and uses the Foster network to facilitate changes of the RC network structure. The temperature prediction part is a direct substitution-and-calculation process, and therefore is fast to carry out. Since Laplace transforms are directly or indirectly available for most power inputs, their transient temperatures may be predicted by the proposed approach. Superposition is employed in cases where the Laplace transform of a given power input is not directly found in Laplace tables, or where the junction temperature is affected by multiple heat sources. The proposed approach is demonstrated with a power amplifier (PA) module; predicted junction temperatures are accurate in both single and multiple heat source cases.

Novel energy recovery systems for the efficient cooling of data centers using absorption chillers and renewable energy resources

The study proposes an efficient energy recovery system for the cooling of a data center using renewable energy resources. The cooling system consists of an absorption chiller driven by the thermal energy recovered from the data center components and additional extra solar energy. The heat dissipated to the ambient by the data center racks is used to vaporize the cooling agents and thus activate the absorption Br-water refrigeration unit and cool the water which in turn will be used to cool the data center environment. When the thermal energy received from the data centers servers is not sufficient to activate the refrigeration unit, a dual system is proposed to heat the water needed for the absorption chiller, using solar energy or other available regenerative heat source. The current study will provide a solution for the optimal design of the data center/server cooling systems, the appropriate selection and the functional analysis of the absorption chiller together with the complementary heat exchangers required by the overall system. The efficiency of the system is defined in terms of the Coefficient of Performance (COP), the ratio between the recovered energy from the system and the total power dissipated by the system. The COP values can reach 0.33 for a typical system. The conclusions highlight the efficiency of the system and comparison to the classical solutions.

A figure of merit for mobile device thermal management

With mobile devices available in various sizes and shapes, it is challenging to think in a consistent and comparative manner about the effectiveness of the thermal management solutions that they employ. Here we define a universal thermal figure of merit - a dimensionless Coefficient of Thermal Spreading (CTS) - calculated using either numerical simulations or IR surface temperature imaging and can be used to compare the thermal design effectiveness of many mobile devices and power levels. Heat spreading at the mobile device surface becomes an important factor that impacts the overall device performance. In order to meet the various performance specifications (skin and junction limit temperatures), the processors have to be throttled to reduce the amount of power and avoid exceeding the temperature limits, which in turn reduces the device overall performance. The proposed CTS Figure of Merit quantifies the effectiveness of heat spreading within the device by means of the uniformity of the surface temperature, and addresses a long-time need to quantify the thermal design effectiveness of various mobile devices which are skin temperature limited. The CTS indicates how much a phone or other mobile device can be improved for the given shape and size/form factor. This paper includes details on the CTS Figure of Merit definition and governing equations, will quantify the design targets and how the CTS can be tested and improved.

The Microelectronics Cooling Impact and Comparison of 2D and 3D Unsteady Effects of a Pair of Opposed Confined Angled Impinging Air Jets

The unsteady laminar flow and heat transfer characteristics for a pair of angled confined impinging air jets centered in a channel were studied numerically. The time-averaged heat transfer coefficient for a pair of heat sources centered in the channel was determined, as well as the oscillating jet frequency for the unsteady cases. The present study is a continuation of the authors’ previous investigations, identifying the similarities and differences arising from the expansion to the third dimension. It examines the interaction between the angled jets and the associated impact on the cooling of the heat sources placed on the board at a jet Reynolds number of 100 and 600. Maintaining the inlet jet width, W, at 1 cm, as in the previous studied cases, the interaction between the 45° angled jets leads to the formation of unsteady symmetrical jets that impinge on the two heat sources placed on the board at a Reynolds number of 100. A second case investigates the hydrodynamic interaction between the 45° angled jets at a Reynolds number of 600. In this case the jets interact and form a region of unsteady shear causing the jets to sweep the target board and the heated components placed on it. The nature of this unsteadiness depends on the proximity of the jet inlets, the channel dimensions and the jet Reynolds number. The jet unsteadiness causes the stagnation point locations to sweep back and forth over the impingement region causing the jets to “wash” a larger surface area on the target wall. The relevant trends for the 2D and 3D jet hydrodynamic and thermal fields are further documented by comparing the field plots and the Nusselt numbers on the target walls for the cases under evaluation. Although similar in nature, the unsteady 3D opposite jets produce results that deviate from the 2D unsteady opposite jets. The complex vortex patterns resulting from the jet interaction at various jet inlet locations, as well as the velocity, vorticity and temperature fields for both 2D and 3D cases are thoroughly evaluated.Copyright

Sequential Reflow-Process Optimization to Reduce Die-Attach Solder Voids

Solder voids are detrimental to the thermal, mechanical, and reliability performance of integrated circuit (IC) packages and must be controlled within certain specifications. A sequential method of optimizing solder-reflow process to reduce die-attach solder voids in power quad flat no-lead (QFN) packages is presented. The sequential optimization consists, in turn, of theoretical prediction, heat transfer comparison, and experimental validation. First, the theoretical prediction uses calculations to find the optimal pause location and time for a lead frame strip (with dies bonded to it by solder paste) to receive uniform heat transfer during the solder-reflow stage. Next, reflow profiles at different locations on the lead frame strip are measured. Heat transfer during the reflow stage at these locations is calculated from the measured reflow profiles and is compared to each other to confirm the theoretical prediction. Finally, only a minimal number of actual trials are conducted to verify the predicted and confirmed optimal process. Since the theoretical prediction and heat transfer comparison screens out most of the unnecessary trials which must be conducted in common design of experiment (DoE) and trial-and-error methods, the sequential optimization method saves significant time and cost.

Heat transfer enhancement by low amplitude forcing in unsteady confined impinging jets

A finite-difference model, derived using a control-volume approach, was used to compute the flow and heat transfer characteristics in a two-dimensional confined laminar air jet impinging on an isothermal surface. Several cases were studied using Reynolds numbers of 650 and 750 with a nozzle-to-plate spacing, H/W, of 5. The behavior of the jet and the corresponding heat transfer from the target wall were investigated when the jet was forced by fluidic excitation at the nozzle exit. At a Reynolds number between 585 and 610, the unforced jet exhibits a transition to an unsteady regime, leading to asymmetric vortex shedding and jet flapping. An investigation of the velocity spectra found distinct dominant modes; the lowest frequency is associated with the jet flapping while the highest frequency is associated with the asymmetric vortex formation that causes buckling of the jet column. As a result of the two combined modes, the peak heat transfer is enhanced and the extent of the lateral cooling is broadened. The jet was subjected to forcing by the introduction of numerical excitation on each side of the jet. This was used to simulate fluidic excitation with the jet being forced on both sides at the exit. Both in-phase and out-of-phase modes were considered. At a Reynolds number of 750, forcing with an out-of-phase mode near the highest frequency leads to a complete stabilization of the jet. The forcing suppresses the low-amplitude, low-frequency flapping mode leaving only a high-frequency vortex formation mode. The suppression of the jet flapping leads to a decrease in the peak heat transfer, but because separation is suppressed, the average wall heat transfer is enhanced.