Published Review Articles
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This review article briefly introduce a few assembly techniques, such as microrobotic assembly, guided self-assembly, additive manufacturing, and transfer printing and summarizes that for accurate and high-yield approach for heterogeneous integration, transfer printing is well suited for high-volume manufacturing. Additive manufacturing creates a structure by forming successive layers of materials under computer control. With computer-aided design, structures of nearly any shape or geometry can be manufactured toward rapid prototyping. Stereolithography, 3D magnetic printing, inkjet printing, laser sintering, fused deposition modeling, direct laser writing, and screen printing are some techniques of additive manufacturing which are or can be can be reconfigured for heterogeneous integration.
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This review article briefly introduce a few assembly techniques, such as microrobotic assembly, guided self-assembly, additive manufacturing, and transfer printing and summarizes that for accurate and high-yield approach for heterogeneous integration, transfer printing is well suited for high-volume manufacturing. The technique of transfer printing is commonly explored to pick each material component from the growth substrate via a stamp and then to deliver it onto the target of interest. Transfer printing is a two-step process: pickup and printing, both of which can be programmed into a deterministic manner for material assembly and micro/nano-device fabrication. The pickup step refers to the retrieval of material or structure (termed as “inks”) from the growth substrate onto the elastomeric stamp; the printing step delivers the retrieved inks onto the target substrate.
Citation:
Gao Y, Cheng H. Assembly of Heterogeneous Materials for Biology and Electronics: From Bio-Inspiration to Bio-Integration. ASME. J. Electron. Packag. 2017;139(2):020801-020801-16. doi:10.1115/1.4036238.
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With the increase in use of mobile devices such as laptops, tablets or mobile computers, the thermal interaction with human skin can pose major problems and skin related syndromes. It has become essential to study the thermal discomfort caused due to these devices with the human skin. A few standards are widely used as a reference to prevent the risks of burns; the upper limit is defined as 45˚C for prolonged contact for most standards. the standards also provide information about the temperature limits with different materials, including coated and uncoated metal, wood, plastic and ceramics materials. Erythima abigne or toasted skin syndrome is caused by the repetitive exposure to mild heat that is either cutaneous or radiation. The typical cases for laptop-induced erythema are reported mostly for young adults. To study the human-computer interaction, researchers have found an interaction effect of surface temperature of a tablet computer and the ambient temperature on local thermal sensation and comfort on hands. The above figure demonstrates the experiment carried out by researchers to study thermal sensation and thermal comfort on fingers.
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Very few research has been conducted on the quantitative modelling on device related thermal discomfort. The researchers have developed a gel finger and a multi-layer heating surface to simulate the process of human hand in touch with a mobile phone. Aluminium and polycarbonate were used to measure the change of temperature on the device surface. The correlation between the experimental data and simulation results proved that gel finger was able to quantify the temperature change and required power for the heated simulated phone surface. More work can be done to correlate the temperature change with human perception directly. With the ever reducing package size and increasing processing power, heat dissipation posses a major challenge for computing in small form factors. To tackle these challenges, research opportunities are available in correlating thermal sensation and comfort between quantitative modeling and objective testing devices; how thermal comfort can be affected by smaller packaging form factors; and how to dissipate heat more efficiently for higher performance computing technologies.
Citation:
Zhang H, Hedge A. Overview of Human Thermal Responses to Warm Surfaces of Electronic Devices. ASME. J. Electron. Packag. 2017;139(3):030802-030802-9. doi:10.1115/1.4037146.
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The popular laser-based packaging technologies currently employed by electronic and MEMS packaging industries are summarized in this article. Value addition by various researchers is also discussed here. The laser drilling method is the leading technology in creating blind micro-vias for devices which incorporate high-density interconnections requiring multilayer PCB with electrical connections. Currently CO2 drilling machines are broadly used in the processing of interlayer micro-vias. The laser excise is a depaneling procedure to extract parts from a bigger sheet of both rigid and flex circuits. The excise method utilizes a technology where a circuit cutline can be made by a computer controlled highly focused laser beam. Large wafers are being made so that more die can be fabricated. Also with the increase in die functionalities, wafers itself are made thin to have a small footprint package. The laser dicing and singulation method is employed for removal of chips with increased speed and high yield of fabrication.
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Extremely small electronic components and heat sensitive equipment can be soldered effectively with the laser soldering process for highly integrated packages. The small spot size, high power density laser beam make way for effective soldering of scaled down subassemblies and sensitive components. An electro-less metallization process, LDS(Laser Direct Structuring) combined with current 3D printing technology allows the reduction of size and weight of the component and parts to be fabricated. This method is specifically applied on injection molded doped thermo-plastic components. Any changes in the circuit layout of an LDS part can be easily done by modifying the laser program. The use of laser in MEMS packaging is limited. One major application is the manufacturing of pressure sensor for mechanical application. A combination of 3D-LDS and laser welding/joining process ensures stacking of various parts at different levels of an MEMS package. The low dissipation energy and short processing time while joining of sensitive electronic parts will prove to be an additional benefit for the MEMS packaging industry.
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Growing demand in automotive electronic applications with more emphasis on electronic control and operability, integrated packaging approach may give rise to issues such as size limitations, wire harnessing complexity, connector size and MEMS sensor packaging. Laser technology will provide substantial solutions to all the packaging complexities. In future, laser technology for MEMS packaging will be TGV, TSV, thin-film capping, active capping, dicing and singulation. The LDS technology along with 3D printing technology has tremendous potential to affect the scaling down of electronic components. With the increasing use of thin wafers to reduce the fabrication cost and increase the number of dies, laser based wafer dicing method is finding growing interest.
Citation:
Rahim K, Mian A. A Review on Laser Processing in Electronic and MEMS Packaging. ASME. J. Electron. Packag. 2017;139(3):030801-030801-11. doi:10.1115/1.4036239.
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Many polymeric and inorganic materials have densely packed structure. Still water absorption takes place in mold compound, die attach, underfill, solder resist etc., which are polymeric in nature. The voids generated in the ‘free’ volume region comprise up to 2.5% of total volume. The absorbed moisture leads to pop corning, interfacial delamination and degradation of adhesion strength. This article gives the measure of diffusivity, solubility and hygroscopic swelling coefficient to critically assess the moisture behavior during solder reflow after preconditioning. The figure a) indicates weight gain histories of two mold compounds at three different temperatures and RH 75%. It is also observed that diffusivity and solubility of the same test mold compounds followed Arrhenius relationship over the entire temperature range of the experiment.
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The diffusion of water molecules leads to increase in inter segmental hydrogen bond length and results in swelling. The hygroscopic strain developed has a linear relationship with moisture concentration. There are some methods to determine the coefficient of hygroscopic swelling; bending cantilever method, a micrometer, Michelson interferometry and thermo mechanical analyzer combined thermo gravimetric analyzer. In this review article , moire interferometry and digital image correlation have been utilized for experimental procedure. Fringe patterns were obtained for null field, zero and interval of 40 h.
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The internal cavity of MEMS devices needs to be maintained at ambient conditions. In compliance to the performance and reliability standards, good hermiticity is the measure of maintaining stable and ambient conditions in the cavity. The commonly used sealing materials are Au-Sn, Au-Si and other tin-based alloys. More recently polymeric seals such as BCB, parylene and polyimide are used due to their advantages. The fictitious polymer with a very large diffusivity and an equivalent solubility are modeled to ensure proper vapor pressure within the cavity.
Citation:
Han B, Kim D. Moisture Ingress, Behavior, and Prediction Inside Semiconductor Packaging: A Review. ASME. J. Electron. Packag. 2017;139(1):010802-010802-11. doi:10.1115/1.4035598.
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In this article, a review of fundamental experimental studies on flow boiling in plain and surface enhanced microgaps along with new results of subcooled flow boiling of water through a micropin-fin array heat sink with outlet pressure below atmospheric are presented. Water is one of the most attractive coolants used in flow boiling experiments because of its large-specific heat and latent heat of vaporization, which enables the absorption of considerable amount of heat during both sensible heating and boiling. However, saturation temperature of water at atmospheric pressure is 100 °C, which may be unacceptably high for continuous operation of complementary metal oxide semiconductor (CMOS) electronic devices.
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In plain microgaps, Flow patterns such as bubbly, intermittent, wavy, and annular are observed, and gap height is found to impact flow patterns and heat transfer characteristics. As gap height decreased between 2 mm to 0.2 mm, annular flow is dominant, and intermittent and wavy flow diminished. Generally, the two-phase heat transfer coefficients are higher in shorter microgaps than in taller ones, ranging from 10 kW/m2 K to 7.5 kW/m2 K for 110 μm and 500 μm microgaps, respectively. For uniform heating, at the same mass flux of 690 kg/ m2 s and heat flux range from 0 to 60 W/cm2, microgap cooled device demonstrated a smaller temperature gradient and smaller amplitude of pressure and temperature oscillation than microchannel. Also for microgap heights smaller than 500 μm, confined slug flow is the dominant flow pattern at low heat flux, while confined annular flow is the dominant flow pattern at higher heat flux; for microgap heights larger than 700 μm. It is concluded that confinement occurred in microgap heights smaller than 500 μm, and effect of confinement is negligible for microgap heights larger than 700 μm.
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In Microgaps with Micropin-Fin Surface Enhancement, which contained an array of 20 x 12 or 13 micro channels (in tandem) staggered hydrofoil pin fins with a wetted perimeter of 1030 μm and chord thickness of 10 μm. The heat transfer coefficient is found to increase with increasing heat flux until a maximum is reached and then decreased monotonically with increasing heat flux until CHF for heat flux range between 19 to 312 W/cm2 and mass flux between 976 to 2349 kg/m2 s. The increasing trend at low quality is ascribed to nucleate boiling, and the decreasing trend at high quality to dominance of convective boiling heat transfer mechanism. The two-phase heat transfer coefficient is moderately dependent on mass flux, and independent of heat flux, for the range of mass flux between 346 kg/m2 s to 794 kg/m2 s and heat flux between 20W/cm2 to 350W/cm2 tested.
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Results and discussion: The flow boiling experiment are conducted for 3 different operating conditions by changing parameters such as inlet temperature, vapor quality, heat flux, mass flux etc. Two-phase heat transfer coefficients for conditions where boiling is in the pin fin array are shown in Fig. 7(a), it is found out that the decreasing trend of two-phase heat transfer coefficient becomes less dependent on heat flux as heat flux increases. Figure 7(b) is a boiling curve and 7(d) represents the graph of exit quality versus effective heat flux, which shows that the wall superheat and estimated exit quality increases almost linearly with increasing heat flux. Also, from figure 7(c), it is concluded that the pressure drop increases with increasing heat flux because increase in pressure drop due to vapor phase is more prominent than decrease in pressure drop in single phase region due to decrease in viscosity.
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Flow instabilities –
Flow instabilities are observed at high heat flux during the flow boiling experiment. The wall temperatures and pressure drop, for the mass flux 1351 kg/m2 s at 267 W/cm2, fluctuated over a large range, as seen in Fig. 8. This happened when vapor pressure increased enough to overcome inlet pressure and induced reverse flow. Liquid is pushed backward and not enough liquid flow occurred in the heat sink, causing dramatic increase in temperature. As liquid accumulated at inlet, and pressure is sufficient to overcome pressure drop across the pin-fin arrays, the liquid is forced into the microheat sink again, and the temperature decreased.
Citation:
Han X, Fedorov A, Joshi Y. Flow Boiling in Microgaps for Thermal Management of High Heat Flux Microsystems. ASME. J. Electron. Packag. 2016;138(4):040801-040801-12. doi:10.1115/1.4034317.
