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|Design, Fabrication, Electrical Characterization and Reliability of Nanomaterials Based Embedded Passives|
|Keywords: nanocomposites, passives, interconnects|
|Passives account for a very large part of today's electronic assemblies. This is particularly true for digital products such as cellular phones, camcorders, computers and several critical defense devices. This paper presents an entire process from design and fabrication to electrical characterization and reliability test of embedded passives on organic multilayered substrates. A variety of thin film capacitor and resistors were utilized to manufacture high-performance embedded passives. The electrical properties of capacitors fabricated from polymer-ceramic nanocomposites showed a stable capacitance and low loss over a wide temperature range. We have designed and fabricated several printed wiring board (PWB) and flip-chip package test vehicles focusing on resistors and capacitors. Two basic capacitor cores were used for this study. One is a layer capacitor. The second capacitor in this case study was discrete capacitor. In both cases, capacitance values are defined by the feature size, thickness and dielectric constant of the polymer-ceramic compositions. Nanocomposite can be directly deposited either by liquid coating or screen printing. Alternatively, nanocomposite thin films can be laminated and capacitor laminate can be used as the base substrate for subsequent build-up processing. For example, Resin Coated Copper Capacitive (RC3) nanocomposites were used to fabricate 35 mm substrates with a two by two array of 15mm square isolated epoxy based regions; each having two to six RC3 based embedded capacitance layers. The total TV core consists of four to eight metal layers. Design features, including antipad diameters, internal plane pickups for vias, and core via pitch were varied within each 15 mm square region. High temperature/pressure lamination was used to embed 6 capacitance layers into the 8 layer internal core. The capacitor fabrication is based on a sequential build-up technology employing a first patternable electrode. After patterning of the electrode, RC3 nanocomposite can be laminated within PCB. An Impedance and network Analyzer was used for dielectric characterization of 8 layers core. Cores are showing high capacitance density ranging from 15 nF to 30nF depending on Cu area, composition and thickness of the capacitors. When the capacitor is embedded in the substrate, the impedance from the active device to the supporting capacitor can be much lower than with a discrete SMT capacitor. Therefore, a much lower capacitor value can provide the required filtering. In another design, we have used eight layer high density internal core and subsequent fine geometry 3 buildup layers to form a 3-8-3 structure. The eight layer internal core has two resistance layers in the middle and the 6 capacitance layer sequentially applied on the surface. This allows multiple capacitance layers in a thin total structure. The resin coated copper capacitive nanocomposite layer does not need to supply any structural support; it can be very thin and achieve high values of capacitance per unit area. Also, since it is not structural, the material choices expand significantly. The structure with small vias allows the vias to thread through the legs of the serpentine resistors and significantly improves z-directional communication. This is especially important when there are multiple voltages that are supported by the capacitor layers. The overall approach lends itself to package miniaturization because capacitance can be increased through multiple layers and reduced thickness to give the desired values in a smaller area. These layers can be accessed because the laser drilled small holes (about 50 um diameter) do not consume large amounts of capacitive area. The study also evaluates the resistor materials for embedded passives. Resistors are carbon based pastes and metal based alloys NiCrAlSi. Embedded resistor technology can use either thin film materials, that are applied on the copper foil, or screened carbon based resistor pastes that can achieve any resistor value at any level. For example, combination of 25 ohm per square material and 250 ohm per square material enables resistor ranges from 15 ohms through 30,000 ohms with efficient sizes for the embedded resistors. Similarly, printable resistors can be designed to cover the resistance in the range of 5 ohms to 1 Mohm. The embedded resistors can be laser trimmed to a tolerance of <5% for applications that require tighter tolerance. A z-interconnect RF substrate with embedded resistors was built using 0S2P/0S1P and 2S2P/2S1P building blocks. Reliability of the test vehicles was ascertained by IR-reflow, thermal cycling, PCT (Pressure Cooker Test ) and solder shock. Embedded discrete capacitors were stable after PCT and solder shock. Capacitance change was less than 5% after IR reflow (assembly) preconditioning (3X, 245 oC) and 1400 cycles DTC (Deep Thermal Cycle). Detailed electrical characterization and reliability evaluations are in progress.|
|Rabindra N. Das, Sr. Advisory Technologist
Endicott Interconnect Technologies, Inc.