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Photo Defined, Electroless Platable Vertical Resistors Whose Resistivity Is Controlled by Top Connector Area

IP.com Disclosure Number: IPCOM000004827D
Original Publication Date: 2001-Jun-20
Included in the Prior Art Database: 2001-Jun-20
Document File: 2 page(s) / 32K

Publishing Venue

Motorola

Related People

Min-Xian Max Zhang: AUTHOR

Abstract

Photo Defined, Electroless Platable Vertical Resistors Whose Resistivity Is Controlled by Top Connector Area

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Photo Defined, Electroless Platable Vertical Resistors Whose Resistivity Is Controlled by Top Connector Area

Min-Xian Max Zhang

BACKGROUND

In an electronic device such as a cellular phone, there are over 200 resistors ranging from a few ohms to several megaohms. Those resistors are typically SMT discretes, which occupy a significant portion of the valuable PWB surface area and add assembly cost.

Currently 0402 (40 mil by 20 mil) chip resistors are used in volume production. Further size reduction to 0201 (20 mil to 10 mil) components and beyond will results in a price premium due to higher component cost and lower assembly yield.

PROBLEM

One approach to reduce board size and SMT assembly cost is to embed a portion or all of the resistors into the printed circuit board (PCB). Currently there are two existing technologies to embed resistors into a PCB: Ohmega-Ply and Polymer Thick Film (PTF) resistor. Ohmega-Ply is a Ni-P thin film plated on Cu foil with limited sheet resistivity (25 ohm/sq to 100 ohm/sq) and is high cost. PTF screen printed resistor is low cost, but its application is limited where tight tolerance control is required.

Since each PTF ink can cover a narrow range of resistance, multiple PTF resistor ink printing and curing processes are required to fabricate resistors in a wide range of resistance.

SOLUTION

This publication discloses a new resistor geometry, called vertical resistor, that can form very low value resistors from a high resistivity ink. The resistor material, which is photo definable and electrolessly platable, is described by authors in U.S. Patent 62250351. The vertical resistor is similar to a capacitor in cross section, except it is a resistive material instead of a dielectric material sitting between the top and bottom electrodes. The resistor top electrode is electrolessly plated over the resistor body, and its dimension is precisely controlled by a photo definable and electrolessly plateble photo-sensitive resin. The resistance of the vertical resistor is determined by the top electrode area instead of the length and width of the resistor print area.

Figure 1 shows a resistor body in the shape of a square with L as the square length and d as the resistor material thickness. If current is allowed to flow from left side to right side, as a normal resistor shown in Figure 1(a), the material resistivity can be expressed as ? Rsqd where Rsq is the sheet resistivity. If the current is allowed to flow from top to bottom, as in a vertical resistor shown in Figure 1(b), the resistance can be expressed as Rv Rsq (d/L)2 where Rv is the vertical resistor resistance. As d is much smaller that L, Rv can be much smaller that Rsq by a factor of (d/L)2, allowing us to make a low value resistor from a high sheet resistivity ink. Table 1 shows the calculated resistor size L for certain Rv values assuming Rsq 20 k? and d 6 ?m.

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