High Tolerance, Micron Scale, Inline Embedded Resistors using Thin Film Additive Manufacturing

Embedded passive components have certain advantages over surface mount components. These advantages include: Increased circuit density through real-estate savings, decreased end product weight, improved electrical properties through additional termination and filter layout options, shortening of electrical connections, increased product quality through elimination of incorrectly attached devices and improved reliability through the reduction of solder joints [1]. The disadvantages to their current implementation include a limited range of resistance values and broader resistance tolerances [2]. In this paper, we discuss an additive print technology for custom embedded resistors with tolerances better than 10% and feature size geometries below 1 mil (25 μm). This technology produces a wide range of custom resistance values on typical PCB substrate materials thus combatting the noted disadvantages. The resistors are printed in-line on the signal layer eliminating the need of additional PCB resistor layers. Manufacturing challenges and design rules of this process are discussed and the DC and RF performance extrapolated to 15 GHz are presented.

Effects of MnSO4 on microstructure and electrical resistance properties of electroless Ni−P thin-films and its application in embedded resistor inside PCB

Purpose – Nickel phosphorus (Ni−P) thin-films have been electrolessly deposited in an acid-plating bath with the addition of manganese sulfate monohydrate (MSM) to achieve higher resistance for the application of embedded resistor with value beyond 10 KΩ. As this material is being used for fabricating embedded resistors under the addition of MSM, its resistance properties including effects of MSM concentration and plating time on resistances, temperature coefficient of resistance (TCR), and resistance tolerance of embedded resistor were investigated. The paper aims to discuss these issues. Design/methodology/approach – The structure of fabricated Ni−P film was detected by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The properties of substrate, including the surface morphologies, glass transition process and boundary of copper pad and substrate surface, were performed by SEM, dynamic mechanical analysis and optical microscope, respectively. The resistance tolerances of embedded resistors were elaborated from the cases of Ni−P thin-film resistance tolerance and the size effects of resistors, respectively. Findings – The fabricated film was found to be constructed with numerous Ni−P amorphous nanoparticles, which was believed to be the reason of increasing thin-film resistance. The Ni−P thin-films presented over one magnitude order of resistance increasing in the case of MSM concentration varied from 0 to 40 g/L. For the case of TCRs, Ni−P thin-films deposited with 20 g/L MSM exhibited low TCRs of within ±100 ppm/°C Before TR at temperature elevating from 40 to 160°C, indicating that this Ni−P thin-film belongs to the constant TCR materials according to the military standard. For the tolerance of embedded resistor, the tolerance contributed by Ni−P thin-film was obtained to be 9.8 percent, whereas the geometry tolerances were in the range of 0-20 percent according to the geometries of embedded resistor. Originality/value – For Ni−P thin-film without MSM, its low resistance with around 100 ohm/sq. limit the values of resistor few KΩ and restricted its widespread application of embedded resistor with higher resistance beyond 10 KΩ. The authors introduced MnSO4 in Ni−P electroless plating process to improve the low resistance of Ni−P thin-film. The resistance was increased over one order of magnitude after added with 40 g/L MnSO4. Due to the specific structure, as this material is being used for fabricating embedded resistors, the electrical properties and its application properties to verify its appliance in embedded resistor were systematically investigated by means of SEM, TEM, XRD characterizations, TCRs, resistance tolerance analysis, respectively.

Environmental tests of embedded thin- and thick-film resistors in comparison to chip resistors

Purpose – The purpose of this paper is to investigate the influence of parameters of embedded resistive elements manufacturing process as well as the influence of environmental factors on their electrical resistance. The investigations were made in comparison to the similar constructions of discrete chip resistors assembled to standard printed circuit boards (PCBs). Design/methodology/approach – The investigations were based on the thin-film resistors made of NiP alloy, thick-film resistors made of carbon or carbon-silver inks as well as chip resistors in 0402 and 0603 packages. The polymer thick-film resistive films were screen-printed on the several types finishing materials of contact terminations such as copper, silver, and gold. To determine the sensitivity of embedded resistors versus standard assembled chip resistors on environmental exposure, the climatic chamber was used. The measurements of resistance were carried out periodically during the tests, and after the exposure cycles. Findings – The results show that the change of electrical resistance of embedded resistors, in dependence of construction and base material, is different and mainly not exceed the range of 3 per cent. The achieved results in reference to thin-film resistors are comparable with results for standard chip resistors. However, the results that were obtained for thick-film resistors with Ag and Ni/Au contacts are similar. It was not found the big differences between resistors with and without conformal coating. Research limitations/implications – The studies show that embedded resistors can be used interchangeably with chip resistors. It allows to save the area on the surface of PCB, occupied by these passive elements, for assembly of active elements (ICs) and thus enable to miniaturization of electronic devices. But embedding of passive elements into PCB requires to tackle the effect of each forming process steps on the operational properties. Originality/value – The technique of passive elements embedding into PCB is generally known; however, there are no detailed reports on the impact of individual process steps and environmental conditions on the stability of their electrical resistance. The studies allow to understand the importance of each factor process and the mechanisms of operational properties changes depending on the used materials.

Thermal effects in embedded thin- and thick-film resistors in comparison to chip resistors

Purpose – The purpose of this paper is to investigate the thermal behaviour of thin- and thick-film resistor with different dimensions and contacts embedded into printed circuit board (PCB) and compare them to the similar constructions of discrete chip resistors assembled to standard PCBs. Design/methodology/approach – In investigations the thin- and thick-film embedded resistors with the bar form in different dimensions and configurations of contacts as well as rectangular chip resistors in package 0603 and 0402 were used. In tests were carried out the measurements of dissipated power in temperature of resistor about 40°C, 70°C and 155°C. The power dissipation was calculated as a multiplying of electrical current flowing through the resistor with voltage across the resistor. The dissipation of heat generated by electrical current flowing through resistors was examined by means of the FLIR A320 thermographic camera with lens Closeup×2 and the power source. Findings – The results show that, in case of chip resistors, the intensity of heat radiation strongly depends on dimensions of copper contact lands and also depends on the dimensions of the resistor. In case of embedded resistors, with comparable dimensions to chip resistors, they have lower ability to power dissipation, as well as the copper contact lands dimensions have lower influence. The thermal radiation through resin material is not as effective as it is in case of resistors assembled on PCB. However, the embedded thick-film resistors, especially made of paste Minico M2010, have already the similar parameters to 0402 chip resistors. Research limitations/implications – Research shows that embedded resistors can be used interchangeably with SMD resistors it allows to open up space on the surface of PCB, but it should be taken into account the lower energy dissipation capabilities. It is suggested that further studies are necessary for accurately determining the thermal effects and investigate the structures of embedded passive components that allow for better heat management. Originality/value – Thermal stability of embedded resistors during operation is a critical factor of success of embedded resistor technology. The way of power dissipation and heat resistance are one of the important operating parameters of these components. The results provide information about the power and the energy dissipation of embedded thin- and thick-film resistors compared to the standard surface mount technology.

Design, Fabrication, Electrical Characterization and Reliability of Nanomaterials Based Embedded Passives

This paper discusses thin film technology based on barium titanate (BaTiO3)-epoxy polymer nanocomposites. In particular, we highlight recent developments on high capacitance, large area, thin film passives and their integration in System in a Package (SiP). A variety of nanocomposite thin films ranging from 2 microns to 25 microns thick were processed on PWB substrates by liquid coating or printing processes. SEM micrographs showed uniform particle distribution in the coatings. The electrical performance of composites was characterized by dielectric constant (Dk), capacitance and dissipation factor (loss) measurements. 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. 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. Cores are showing high capacitance density ranging from 15 nF to 30nF depending on Cu area, composition and thickness of the capacitors. In another design, we have used eight layer high density internal core and subsequent fine geometry n (1 to 3) buildup layers to form a n-8-n structure. The eight layer internal core has two resistance layers in the middle and 2 to 6 capacitance layer sequentially applied on the surface. 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. 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 °C) and 1400 cycles DTC (Deep Thermal Cycle).