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PRINTED CIRCUIT DESIGN is published monthly by: UP Media Group Inc. 2018 Powers Ferry Road, Ste. 600 Atlanta, GA 30339 Tel (678) 589-8800 Fax (678) 589-8850 All material published in this file and in PRINTED CIRCUIT DESIGN is copyrighted © 2002 by UP Media Group Inc. All rights reserved. Reproduction and distribution of material appearing in PCD and on the www.pcdmag.com Web site is forbidden without written permission from the editor. For information on obtaining reprints go to: http://www.pcdmag.com/mag/reprints.html COVER STORY Embedding Resistors and Capacitors Putting passives in the board saves surface area, shrinks the board and cuts weight and thickness. So what’s the hold up? By Rolf Funer icrovias have been ments in power distribution systems are starting material is a copper clad laminate called the third needed. More passives are needed. This is with a very thin dielectric layer. This is fab- revolution in printed why the use of passives is increasing at a ricated into a power/ground pair. The very circuitry. Are embed- greater rate than that of active components. thin dielectric results in a very close spacing ded passives – resistors Placing passives inside the board is one between power and ground. This capaci- Mand capacitors buried in the board – the big advantage, but there is another. Surface tance can be accessed with conventional fourth, one with even greater potential to solder joints produce inductance. plated holes. Figure 1 shows a conventional change the face of design? While microvia Embeddeds eliminate the solder joints, thus board redesigned with buried capacitance. circuits enable higher densities, lighter lowering inductance, which reduces power Basically, this builds a big parallel plate weight and better performance, the board system impedance. So, embedding resistors capacitor into the board. remains at its core just a bunch of wires. and capacitors saves precious surface area, The level of capacitance achieved depends With passives built in, the board is a very shrinks the board and cuts weight and on the thickness and dielectric constant of different entity indeed. thickness. And by eliminating solder joints, the dielectric layer and the size of the board: Embedded passives are a relatively new reliability is improved (solder joints are the concept.Why embed them? Lack of space on leading failure mode of assembled boards). C = ADkK/t the board surface. In a typical assembly,com- Embedded passives permit shorter leads and where C = total capacitance ponents that cost less than 3% of the total closer component placement, thus enhanc- A = area price can take up 40% of the real estate! And ing electrical performance. Dk = dielectric constant it’s getting worse. As we design boards with K is a constant more functions, higher clock rates and lower Plane Capacitance T = thickness voltages, more power and higher currents The most common method for using are required. Noise budgets go down with embedded capacitance involves a concept The only true commercial embedded the lower voltages, and significant improve- called distributed or plane capacitance. The product is BC 2000 (Sanmina-SCI Corp.). While capacitance is low, about 0.5 nF/in2, it has found good use filtering capacitance. Buried capacitance Plane capacitance is particularly valuable in Standard PCB 6 layer 8 layer conversion high-frequency situations where the induc- Signal 1 Signal 1 tance of conventional discrete capacitors Power 2 Ground 2 Standard core Power 3 ZBC-2000 increases (see Figure 2). Signal 3 Signal 4 Several companies are developing new Standard core Thinner core Signal 4 Signal 5 products with higher capacitance. Some are Standard core Ground 6 Ground 5 distributed capacitor types; others are dis- Power 7 ZBC-2000 crete embedded caps. DuPont and 3M are Signal 6 Signal 8 developing distributed capacitance products with much higher levels of capacitance by Figure 1 – Adding capacitance with thin dielectric power/ground planes filling the dielectric with barium titanate, a 8 PRINTED CIRCUIT DESIGN www.pcdmag.com high dielectric constant material. 3M’s prod- Embedded Resistors the copper and the nickel, patterns of nickel uct, C-Ply, has an epoxy binder, while Two methods are in use for burying resistors. resistors with copper terminations are DuPont’s HiK uses a polyimide binder. Ohmega-Ply has been around for two formed. These are laminated into an internal Discrete embedded capacitors are created decades. It is a bi-metal foil – copper foil with layer. by printing a high dielectric constant paste a thin plating of a nickel alloy that becomes Ohmega-Ply has a limited resistance range – onto one plate (usually a square of etched the resistor element.This is laminated resistor 25 to 250 ohms/square – but it is possible to copper foil), curing it, and then printing or side down to the substrate. Then by etching make higher-value resistors by making long plating another plate on top. In one novel concept, DuPont prints and fires a high tem- perature capacitor material onto copper foil in the sizes and locations needed.1 This is 3 Ideal plane Discrete capacitor capacitor laminated into a circuit and the excess cop- per etched away. Because these dielectrics Zo Noisy at 2 higher frequencies have such high dielectric constants (impedance) (1,000+), high capacitance densities of 100 to 180 nF/in2 are possible. Shipley’s product has an even higher dielectric constant. Some 1 Quietest of these newer materials and their properties frequency (resonance) Remains quiet are shown in Table 1. Not all will make it to µ Typically .1 F 0 and 3nH commercial use, but some will. 0 10 20 30 40 50 60 70 80 The effect shown on powerbus noise for Frequency (MHz) a test circuit for no caps, discrete decoupling caps and several different distributed cap materials is shown in Figure 3. Note that Figure 2 – Resonant discretes vs. nonresonant plane bypass capacitor impedance discrete capacitors lose effectiveness as fre- quency increases while the embedded ones 5 1 MHz to 1 GHz remain effective up to 5 GHz. 0 1 GHz to 3 GHz Motorola employs buried capacitance in an 3 GHz to 5 GHz -5 interesting way.2,3 It uses “mezzanine” capaci- tors made from an epoxy resin paste contain- -10 ing barium titanate. This material is coated -15 onto the board, then discrete capacitors are dBm made by photoimaging. Substrate cost is -20 increased, but assembly cost is reduced so that -25 final module cost is 12 to 14% less. Buried -30 resistors are also used. The revised design is smaller and lighter in -35 weight, occupying 43% less space than its sur- -40 Courtesy: 3M face mount counterpart. The technology to No Caps (FR-4) Decoupling BC2000 EmCap HiKi (DuPont) C-Ply (3M) Caps (Sanmina-SCI) make embedded passive circuitry is licensed Material to three PCB fabricators: AT&S, Wus Circuits and Ibiden. Motorola has built millions of Figure 3 – A comparison of powerbus noise for a test circuit with no caps, discrete GSM modules with embedded passives. decoupling caps and several different distributed capacitance materials Table 1 – Embedded material types and properties Manufacturer Sanmina 3M DuPont DuPont Shipley and licensees Product BC2000 C-Ply Interra HiKI/HiKII InSite Type Plane Plane Plane Plane Discrete or discrete Dielectric FR-4 BaTiO3 Sintered BaTiO3 in Ceramic in epoxy ceramic polyimide on foil on foil film Capacitance 0.5 - 0.9 5 - 30 150 - 600 1 - 14 >1000 (nF/in2) DECEMBER 2002 9 COVER STORY meander patterns (high length to width). The are made with plated holes or microvias. Pastes corrosion at the copper/carbon interface. material has been used in communications have been around for a long time and were Motorola developed a stability promoter that equipment such as satellites, base stations, generally limited by high tolerances and poor reduces resistance drift to less than 10% after medical electronics, avionics and computers. environmental resistance, which limited them exposure to 85%RH/85˚C for 500 hrs. This Like embedded capacitors, the resistors save to consumer applications.A new generation of change is reversible when baked, so it is not space and weight, and can reduce size.They can paste products with better properties has been indicative of device performance in real operat- also improve electrical performance. For exam- developed, however, and are finding their way ing conditions. The resistors are stable to high ple, see Figure 4. This probe card has over 100 into more sophisticated applications. temperature as well. Exposure to 5X reflow resistors and six different values. Note that they One example is Siemens’ Simov, a car- (peak temperature: 220˚C) followed by 500 are placed directly under the component leads bon-based resistor paste with a resistance cycles of liquid-to-liquid thermal shock results to reduce the signal path to the resistors. Figure range of 20 to 150 ohms/square and a tol- in less than a 4% change. Motorola uses these 5 depicts a camera circuit in which the resistors erance of +/-25% at values under 100 ohms resistors in GSM phone modules. form a rheostat type of pattern. What is note- and a tolerance of +/-40% at values greater Pastes have made progress but still have worthy about this application is that each resis- than 100 ohms. It has been used for auto- wide tolerances. Are better systems on the tor changes value gradually and, for precision, motive applications. It is produced by horizon? Several emerging materials and they are laser-trimmed to +/-1%. InBoard, a board fabricator owned jointly by processes are in various stages of develop- A second way to embed resistors uses resis- Siemens and Sanmina-SCI. ment (Table 2).