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New for July 2021 Embedded resistors on organic boards (printed circuit boards) are the equivalent of thin-film resistors on ceramic substrates. They are a convenient way of providing resistors without attaching components. In high-frequency applications, embedded resistors will perform better than surface mount technology (SMT) resistors, as most of the parasitics are eliminated.
Tom Sleasman, manager of sales and marketing at Ticer Technologies (now part of Quantic), provided the content below. He also contributed an embedded resistor calculator spreadsheet which you can fine in our download area, here.
Thin-Film Embedded Resistor Foil
Thin film embedded resistance foil consists of a low profile (PCB Grade 3) copper foil with a thin layer of resistive alloy sputtered or plated onto the matte side of the copper. When this material is patterned and selectively etched, the resistive layer serves as the embedded resistor element.
The need for high performance and low signal losses in electronic devices continues to drive the development of electronic systems with passive devices embedded into multilayer PCBs. Including embedded resistors interior to the PCB design allows these resistors to be placed more optimally in the circuit. This shortens routing lengths and lowers overall inductance. The number of SMT devices can also be reduced, improving routing efficiency and allows for more outer layer space for active devices.
Integrated thin-film resistor foils are produced in a wide range of standard Ohms per Square values (10, 25, 50, 100, 250, 1000) using nickel phosphorus, nickel chromium and other resistive alloys.
The formation of the thin film embedded resistor element is done in a series of printing and etching steps to define the resistors length and width. These dimensions are calculated to correspond to the finished ohmic value of the desired element based upon the base materials Ohm per square (OPS) values. PCB fabricators with experience in creating ER elements will adjust artwork, etching speeds, etc. to build high quality and acceptable tolerance resistor elements.
Resistor foils have been characterized in terms of their nominal surface resistance values as ohms per square. When designing circuits with these resistive foils, a simple relationship can be applied to determine the design resistance value of a planar resistor: the length is divided by the width and then multiplied by the surface resistance of the resistor foil material, where the length and width are the dimensions of the planar resistor to be used in the design.
Embedded resistor technology has been used reliably in the defense, aerospace, high speed digital, RF, cellular and satellite communications industries for over 40 years. Embedding resistors and other passives (capacitors) can increase board functionality without increasing board size. It provides better signal performance, reduced parasitic and cross talk. Reliability is increases by eliminating many surface mount devices and the defects associated with assembly processes such as placement and soldering.
Below are some images we added for September 2021, thanks again to Tom. The first image shows where the resistor layer is stacked, below the copper foil, on the top side of an organic circuit board.
Here's some images of how embedded reistors are processed. On the upper left is the starting material, with the resistor laminate below the copper foil. Next, photoresist is used to define all features that are either resistors or conductor strips in the other images in the left column. Then, moving down the right column, a second photoresist step removes copper where the resistors are needed. Note that having the resistor material under the conductors has a minimal effect on RF transmission line loss, as the skin depth in the resistor material is much greater than its thickness, so the RF fields penetrate into the copper.