A novel fluxless soldering process is presented, that enables lead-free soldering o f semiconductor die-bheat spreader (and heat spreader-tu-heat sink structures) at room temperature. The process is based on the use of reactive multilayer foils to locally melt the solder interface. Silicon-copper samples joined with indium solder are thermally characterized for a range of die sizes and bond line thicknesses. The thermal resistance of the solder joints is found to be an order of magmtude lower than for conventiond t h e d interface materials (TIMs), with good thermal fatigue resistance. The predicted thermo-mechanical behavior of the solder interface in a central processing unit (CPU) application indicates that such joints would survive application environments without causing die cracking. The soldering technology employed could greatly enhance the thermal performance of power IC packages such as CPUs, by enabling the adoption of a solder-based TIM between the die and integrated heat spreader.

Reactive NanoTechnologies (RNT) has developed a new platform joining technology that can form a metallic bond between a chip package and a heat sink and thereby offer a thermal interface resistance that is up to ten times lower than current thermal interface materials (TIM). The joining process is based on the use of reactive multilayer foils as local heat sources. The foils are a new class of nano-engineered materials, in which self-propagating exothermic reactions can be initiated at room temperature with a hot filament or laser. By inserting a multilayer foil between two solder layers and a chip package and heat sink, energy generated by a chemical reaction in the foil heats the solder to melting and consequently bonds the components. The joining process can be completed in air, argon or vacuum in approximately one second. The resulting metallic joints exhibit thermal resistances up to an order of magnitude lower, than current commercial TIMs. We also demonstrate, using numerical modeling, that the thermal exposure of microelectronic packages during joining is very limited. Finally we show numerically that reactive joining can be used to solder Si dies directly to heat sinks without thermally damaging the chip.

Reactive NanoTechnologies (RNT) has developed a reactive bonding technology to directly bond silicon dies to heat sinks with indium solder using a reactive multilayered foil. In this new method of bonding, heat is generated locally by exothermic mixing within the multilayered foil. This heat is used to melt indium solder layers to join the dies to the heat sinks. The measured thermal resistance of the resulting solder bond is 4 to 5 K mm2/W (0.006 to 0.008 K in2/W). In addition, the reactive foil also localizes the heat to the interface, thus minimizing residual stress and thermal damage in the components. In this paper we discuss the thermal performance and reliability test results for reactive multilayer bonding with different bond line thicknesses. We also present detailed comparisons of thermal performance between reactive multilayer bonding and other current Thermal Interface Material (TIM) solutions, including polymer-based greases, phase change materials, and low melting metallic alloy. Benchmark tests were done using the graphics processor on an operational video card as a test vehicle. The test results show that the introduction of a reactive multilayer bond as an interface material between the graphics processor and the thermal management device demonstrates significant performance advantages over any of the other current commercially available TIM solutions.

The thermal behavior of a reactively soldered interface between a Cu heat sink and a Si die is analyzed experimentally. Two aspects are addressed, thermal fatigue of these joints, and the survival of the circuitry on the die during joining. In the reactive soldering process a multilayered reactive NanoFoil™ acts as a heat source for melting the solder during joining. The foils consists of hundreds of alternating nanoscale layers of Ni and Al, and supports controlled self-propagating reactions that can be initiated in air and at room temperature. The nanostructured reactive multilayered foils (NanoFoils) are designed to minimize thermal exposure of the components as well as residual stress.To study thermal fatigue of reactively soldered Cu-Si joints three populations of samples are studied, one fabricated using conventional soldering techniques, and two reactively soldered populations fabricated using different solder thicknesses. Both the conventionally and reactively soldered samples consist of a stack composed of two Cu blocks with a Si die soldered between them. The difference between these two types of samples is that for the conventionally soldered joints the heat source used for melting the solder is an oven, while for the reactively soldered joints a nanostructured reactive multilayered foil is used. We show that neither the conventionally soldered samples nor the reactively soldered populations exhibit significant degradation of thermal performance after 1000 cycles between 25 °C and 125°C. In addition we show that a commercially available bare-die chip package can be successfully reactively bonded with no degradation in the performance of the chip, but does offer significant reduction in die operating temperature during normal operation.