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Vijay K.,Indium Corporation
2012 4th Electronic System-Integration Technology Conference, ESTC 2012 | Year: 2012

The use of High Power devices are increasing exponentially given the need for increased current switching and conversion rates. High power devices are used in diverse end applications including traction (trains, powertrain management in an Electric Vehicle), LEDs (high brightness, high power), Diode Pumped Solid State Lasers, computing and graphics. Higher power translates to more heat being generated, and this heat needs to be removed from the power die and dissipated out of the package. Keeping the power die cooler is the key to increased functionality of the device and extending the life of the device. The choice of the right Thermal Interface Materials (TIMs) used to transport heat away from the power source is therefore crucial in preventing the power die from overheating. This paper discusses the work done on Solder-TIMs that have been developed for high-power applications and how they compare to thermal grease. Solder-TIMs as the name indicates are solid solders (typically 100% Indium / Indium-containing alloys) that are very soft versus typical thermal greases that are silicone-based with metal (Ag) particles). With higher power and higher heat dissipation needs, regular grease is not able to make the cut and superior performing Solder-TIMs are better suited to take the heat away from the die due to the following reasons: (a) Thermal grease has a low bulk thermal conductivity of 3-12 W/m.K compared to 100In that has a very high bulk thermal conductivity of 87 W/m.K; (b) During device usage over time, thermal grease tends to pump-out and migrate away from the center of the die (due to the diaphragm effect) and this means that the center of the power die gets hotter and this could lead to premature failures. On the other hand, there is no pump-out with a Solder-TIM. 100In is extremely soft (4X softer than lead) and this softness helps fill the interface gaps thus reducing thermal interface resistance. In addition, over time, the malleability of the solder helps fill the interface gaps even better. So thermal interface resistance with a Solder-TIM decreases over time as opposed to thermal grease where the thermal interface resistance increases over time; (c) Over time, grease tends to bake-out and dry (becomes powdery), thereby increasing thermal resistance and reducing heat-dissipation effectiveness. With Solder-TIMs, there is no bake-out; (d) Grease is messy when applying versus a solid solder that can be packaged in tape & reel and picked & placed. The Solder-TIM tested was the Heat-Spring® which is a foil made of In/In-containing alloys, with a proprietary altered surface for reduced thermal interface resistance. The Heat-Spring® needs only compression force, does not need to be melted/reflowed, does not need a flux and therefore eliminates voiding associated with flux and reflow, does not need any special substrate metallization. The Solder-TIM was compared to industry-used thermal greases. Testing regimes included (i) Bake test: at 90 deg C for 1500, 3000 hours; (ii) Power Cycling: 1000 cycles, 0-50W; (iii) Change in interface resistance and solder thickness for (a) T=1000 hours; (b) Thermal Cycling- 1000 cycles, -10/+95C; (c) HAST: 85C/85% RH for 1000 hours. In all the tests, Solder-TIMs consistently outperformed thermal grease by achieving low thermal interface resistances especially over time and prevented the power die from overheating. Source


Lasky R.C.,Indium Corporation
SMT Surface Mount Technology Magazine | Year: 2012

Busy booths, quality attendees, and high numbers at the workshops impressed Dr. Lasky this year. Source


Vijay K.,Indium Corporation
2012 4th Electronic System-Integration Technology Conference, ESTC 2012 | Year: 2012

Current technology drivers span a broad spectrum that include miniaturized hand-held devices where real estate is at a minimum as well as high density server assemblies that have high-reliability requirements. SMT (Surface Mount Technology) assembly consequently involves overcoming challenges that include-complex components such as sub-0.4mm CSPs, 01005s, LGAs; Area Ratios of 0.6 and below; Stencil apertures sub-250 microns dia, Stencil thicknesses between 75-110 microns; Solder-Mask vs Non-Solder-Mask defined pads; varied surface finishes such as OSP, ImAg; and long reflow profiles with reflow in air. With miniaturization and smaller stencil apertures, some of the key manufacturing challenges include (a) achieving a good print brick deposit, maximizing paste transfer, minimizing print-to-print deviation; (b) eliminating HiP (head-in-pillow) which is the biggest issue facing the industry; (c) achieve complete solder coalescence and prevent clumpy/grainy solder joints for the small paste deposits that see longer reflow profiles; (d) achieve low voiding even for micro via-in-pad designs. These conditions impose heavy demands on the solder paste flux chemistry. To add to the complexity, halogen-free flux requirements in a Pb-free process are pretty much becoming the norm. Pb-free solders mean higher temperatures and therefore the flux needs to do more work to reduce oxide. This is more difficult for the smaller paste deposits because of the higher surface area to flux volume ratio. Halogen-free fluxes mean that the fluxes have lesser "juice" when compared to normal halide-containing fluxes, but still need to address higher temperatures associated with a Pb-free process. This study details the development of a flux technology platform to (a) achieve print consistency (maximize paste transfer, minimize deviation) for small apertures (sub-300 microns diameter, area ratios of 0.5-0.6); (b) eliminate head-in-pillow with an enhanced oxidation barrier approach as opposed to just making a flux more active; (c) achieve complete solder coalescence and prevent clumpy solder joints; (d) achieve very low voiding. The flux in the solder paste is a complex optimized chemistry and is truly the "secret sauce" that helps maximize the process window (print, reflow) towards achieving high yields. Source


Briggs E.,Indium Corporation
SMT Surface Mount Technology Magazine | Year: 2014

The explosive growth of personal electronic devices, such as mobile phones, tablets, and personal music devices, has driven the need for smaller and smaller active and passive electrical components. Not long ago, 0201 passives were seen as the ultimate in miniaturization, but now we have 01005 passives with rumors of even smaller sizes not far behind. For active components, array packages with 0.4 mm pitch are virtually a requirement for enabling the many features in modern portable electronic devices. To meet the challenge of stencil printing smaller stencil apertures, there is an increased interest in using finer particle-sized solder pastes to improve transfer efficiency. The smaller particle size results in a large surface area-to-volume ratio that challenges the solder paste's flux to effectively perform its fluxing and oxidation protection action. The potential resulting surface oxidation can lead to voiding, graping, head-inpillow, and other defects. The combination of higher lead-free process temperatures, smaller print deposits, and temperature restraints on electrical components has created several challenges. Two in particular are obtaining consistent volume in the printed solder paste deposit and minimizing the oxidation of the solder powder in the small deposit during reflow. Solder pastes comprised of finer particle solder powders may help with stencil printing, but the increased surface oxide associated with finer powders may also reduce the reflow process window. The focus of this paper is to provide a statistical comparison of the transfer efficiency of different solder powder particle sizes, specifically types 3, 4, 5, and 6, and to visually observe post-reflow results in both optimal and harsh conditions. Source


Patent
Indium Corporation | Date: 2014-01-16

Methods and apparatus are provided for attaching a heat spreader to a die and includes disposing a solder thermal interface material between a first surface of a die and a first surface of a heat spreader without disposing a liquid flux between the die and the heat spreader to form an assembly, wherein at least one of the first surface of the die and a first surface of the heat spreader have disposed thereon a metallization structure comprising a transition layer and a sacrificial metallization layer, the sacrificial metallization layer disposed as an outer layer to the metallization structure adjacent the solder thermal interface material; and heating the assembly to melt the thermal interface and attach the die to the heat spreader.

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