Designers Notebook: The 'New and Growing' Embedded Resistors

Why is embedded resistor technology considered to be “new” and “growing” despite decades of history?

In fact, a broad number of established PCB fabricators are knowledgeable about the materials and processes for embedding resistor elements but not all may be prepared to alter procedures established for their more conventional multilayer circuit board customer base. 

The next question is, “What is the motivation for embedding resistors?”

  • PCB densification—a primary driver
  • Functionality—a contributor
  • Performance—an enhancement
  • PCB assembly—simplified

The primary benefit of embedding resistor elements within the layers of the multilayer PCB is the ability to more efficiently arrange and interconnect the primary active components placed on the outer surface of the circuit boards surface. Furthermore, embedding most of the passive components can contribute to the development of a more robust PCB assembly, one that will not be physically impacted by environmental extremes or when the end product is exposed to excessive vibration and shock. Additionally, incorporating these passive resistor elements within the circuit board structure simplifies the logistics required for procurement, stocking, and assembly processing for multitudes of damage-prone ceramic-based components.

A key issue facing the PCB designer is to determine which resistors will be more suitable for embedding within the circuit board’s structure and which resistor elements are more appropriate for placement onto the circuit board’s outer surface. The printed board designer really acts as the facilitator and is rarely the sole decision maker for embedding components. Because of the potential cost impact on the printed circuit board, the decision to embed resistors is more likely an engineering and management level issue, typically justified by the restricted surface area reserved for mounting passive components and/or the potential for enhanced performance of the finished product.

In preparation for implementing embedded resistor technology the designer and/or program manager must first seek an experienced supplier company that can furnish practical guidance in selecting a process (thick-film or thin-film) that will meet both technical and budgetary (cost) goals established for the end product.

Formed resistor elements may be furnished as a printed thick-film composition or an imaged and chemically etched thin-film process.

  • Thick-film resistor materials are formulated to furnish a wide range of primary values and have been successfully used for a broad number of commercial applications. The resistor formulations are based on carbon-filled polymer chemistry that enables screen printing or deposition to form elements directly onto pre-patterned termination lands furnished on a designated circuit board layer.
  • Thin-film resistors are formed using copper foil material that is pre-coated with resistive material. The resist layer is deposited onto the copper sheet material using vapor disposition that provides uniformity of the resistor base value across the entire sheet. Thin-film resistor materials are supplied in a variety of base values using Grade 3 copper foil. The copper sheets developed for the thin-film resistor forming process are available in thicknesses of 18 µm (0.5 oz) and 35 µm (1 oz).

The information furnished in this installment of this series has been prepared to provide guidance to the circuit board design professional considering the implementation for embedding “thick-film” resistors.

When identifying candidate resistors for embedding, the designer must consider both resistor value range, the allowable tolerance bandwidth, and application. The thick-film resistor forming process is generally employed where tolerances are less critical, primarily used in digital and analog circuit applications for terminating resistors, current limiting, transistor biasing as well as for pull-up/pull-down resistors where precise value tolerances are not critical.

Resistor Functionality
Termination resistors are placed at the end of an electrical transmission line or when working with differential pair signals. Pull-up and pull-down resistors on, the other hand, are commonly used in logic circuit applications. For example, the function of the pull-down resistor is to hold the logic signal near to zero volts when no other active device is connected. The pull-up resistor’s function is to ensure that the voltage between power and ground cannot be directly connected. Depending on the circuit logic type, typical values selected for termination, pull-up and pull-down resistors can vary in values that range from 500 ohm to 10K ohm and may tolerate value tolerance limits as high as ±20%. Resistor elements designated for “current limiting” are used for setting an upper limit to the amount of current that flows through a component while “transistor biasing” resistors are commonly used in combination with transistors and semiconductor components.

The overall performance of the thick film resistor materials is related to the optimized circuit design and fabrication process. The materials and process parameters of polymer thick-film resistor must be considered in order to successfully achieve the performance requirements of circuit designs. For example, the decision on what landsize-to-aspect-ratio to use for a particular resistor element depends on a number of factors. These include target resistance values, electrical considerations, available resistivity values, trimming requirements, and the distribution of resistances of all the resistors present on the same layer of the board.

In general, on most printed circuit board designs, resistor value distribution will vary between 1W at the low end and 10MW at the highest. Selecting the most practical composition for the thick-film resistors, the PCB designer should consider the most prominent base-value usage and select a material that facilitates the lower end of the value range. From a statistical standpoint, the greater number of resistors in a digital or analog circuit will likely fall into a range between 10W and 10KW (Table 1). With that in mind, selecting the 10W material as the base value will provide greater flexibility in expanding the resistor geometry to accommodate a wide range of finished resister values.

Solberg_Table_1_cap.jpg

Geometry Principles
The geometry of the resistance material can be as simple as a square or rectangle, or for more complicated resistor values, a serpentine shape designed to maximize resistor element length while minimizing area. The values provided are based on the resistance measured between opposite edges of a square. For example, a single square of 1K material printed or deposited between two copper lands will provide a 1K resistor element while a pattern that is twice the length, or two squares, furnishes a 2K resistor.

The rectangular “bar” geometry (Figure 1) is most common for resistors with values close to the basic thick film composition selected while the serpentine geometry is employed when resistor values are significantly greater than the thick film materials base value. The “top-hat” shaped resistor geometry is commonly applied for elements that will likely require extensive laser trimming to reach their target value.

Solberg_Fig1_cap.jpgSuppliers recommend that designers furnish resistor widths and lengths greater than 0.25 mm (0.010 in). Larger resistor dimensions will reduce the reliance on the print variations or accuracy of the copper etching processes. Regarding terminating the resistor elements, the land pattern geometry provided for the resistor termination should allow for a nominal 0.25–0.50 mm overlap of the thick-film resist material and consider allowances for printing process variables.

As noted, the thick-film resistor ink formulations are based on a carbon-filled polymer chemistry. By adjusting the ratio of carbon content within the polymer medium, the material can be formulated to furnish a wide range of primary values. Following printing or deposition of the resist compound the circuit boards are transferred to an oven for curing at temperatures in a range between 150–250°C. Five commercial sources for printable or deposited thick-film resistive materials and the base value range that they offer are furnished in Table 2.

Solberg_Table_2_cap.jpg
When the application requires a value modification or a tolerance that is better than that noted in Table 2, laser trimming systems can be employed to make the necessary adjustments. The examples shown in Figure 2 illustrate thick film resistors that have been modified using laser technology to achieve a specific value or tolerance target.

Solberg_Fig2_cap.jpg
Laser trimming systems developed for high volume PCB fabrication are equipped with multiple flying probe contactors that are preprogrammed to reach any component location, size, orientation and layout within the board or multi-unit panel. The probe contactors sweep across the board’s surface contacting the embedded component lands or dedicated test point locations to measure and transmit the resistor value and tolerance as printed to direct the laser in making the required cut (Figure 3). Automated calibration routines ensure cut placement accuracy within 15 microns. Cut widths are typically in the 10–50-micron range.

Solberg_Fig3_cap.jpgThose considering thick-film resistor technology must understand that the process requires precise imaging and consistent material density to ensure that the printed image will meet the target resistor value range. And due to the printing and curing complexity for printed thick-film resistors, the printed board fabricator will prefer applying only one resistor base value material onto a single substrate layer. If the fabricator can use an inkjet-type of deposition process, however, they will have greater latitude in applying two or more base value resistive ink compounds onto a single circuit layer.

Although the thick-film resistor forming process has a long history and remains a popular, low-cost solution for embedding, many PCB fabrication companies may not offer this capability. This is because thick-film resistor forming is considered a wet process requiring controlled storing, careful mixing, printing and curing operations. PCB fabricators that do offer embedded resistor capability will often prefer to adopt alternative thin-film processing solutions.

In Part 2, I will review the materials, design rules, and process parameters for embedding formed thin-film resistors.

This column originally appeared in the April 2021 issue of Design007 Magazine.

 

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2021

Designers Notebook: The 'New and Growing' Embedded Resistors

04-19-2021

Why is embedded resistor technology considered to be “new” and “growing” despite decades of history? In fact, a broad number of established PCB fabricators are knowledgeable about the materials and processes for embedding resistor elements but not all may be prepared to alter procedures established for their more conventional multilayer circuit board customer base.

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Designers Notebook: Developing Panel Level Semiconductor Packaging

02-22-2021

While semiconductor packaging has traditionally utilized a narrow strip of organic copper-clad organic-based laminate and wire-bond processing for the single-die BGA. Companies furnishing devices for high-volume markets are now implementing very fine-pitch alloy bumped flip-chip package technologies that enable face-down interface.

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2020

Designers Notebook: Panel-level Semiconductor Package Design Challenges

05-15-2020

Semiconductor package specialists continually work to improve high-volume manufacturing process efficiencies while reducing manufacturing costs. A majority of the commercial semiconductors are built-up on the surface of a circular-shaped silicon wafer with metalized terminal features at their perimeter to accommodate wire-bond interface with a lead-frame or package substrate. Vern Solberg explains.

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Designers Notebook: Design Challenges for Developing High-density 2.5D Interposers, Part 2

01-29-2020

In Part 2 of his column series on design challenges for high-density 2.5D interposers, Vern Solberg discusses primary base materials for 2.5D interposer applications, design guidelines, technical challenges, and key planning issues.

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Designers Notebook: PCB Design and HD Semiconductor Packaging

01-15-2020

To better meet their performance and miniaturization goals, manufacturers are looking for higher functionality for their semiconductor packages. For that reason, many manufacturers will rely heavily on more innovative IC package solutions, often integrating a number of already proven functional elements within a single-package outline. Vern Solberg covers how this and more impact PCB design and HD semiconductor packaging.

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2019

Designers Notebook: Focus of Interest at SMTAI 2019—Low-temperature Solder

10-03-2019

Both suppliers and users of solder materials participated in discussions at SMTAI 2019 related to low-temperature solder (LTS). The solder supply companies present had a wide range of material compositions that employed elements of bismuth or indium to reduce the liquidus temperature of the alloy during the joining process. Key issues that user companies are concerned with are the lower-temperature alloys selected must be reliable and exhibit shear strength, creep resistance, and resistance to thermal fatigue for the duration of the product’s life cycle.

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Designers Notebook: Embedding Components, Part 7—Semiconductor Placement and Termination Methodologies

03-11-2019

Progress in developing high-density embedded-component substrate capability has accelerated through the cooperation and joint development programs between many government and industry organizations and technical universities. In addition to these joint development programs, several independent laboratories and package assembly service providers have developed a number of proprietary processes for embedding the uncased semiconductor elements.

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Embedding Components, Part 6: Preparation for Active Semiconductor Elements

01-10-2019

Designers are well aware that a shorter circuit path between the individual die elements, the greater the signal transmission speed, which significantly reduces inductance. By embedding the semiconductors on an inner layer directly in line with related semiconductor packages mounted on the outer surface, the conductor interface distance between die elements will be minimized.

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2018

Embedding Components, Part 5: Alternative Termination Methodologies and Surface Plating Variations

12-19-2018

Because they are furnished with a very thin profile, resistor and capacitor components with different values can be mounted directly onto land patterns on a subsurface layer of the printed circuit structure. However, handling and placing of these small components requires systems with a high level of positional accuracy. Interconnection can be accomplished using either deposited solder paste and reflow processing or applying a conductive polymer material. Due to the extremely small land pattern geometries required for mounting the miniature passive components, companies commonly rely on precision dispensing these materials.

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Embedding Components, Part 4: Passive Component Selection and Land Pattern Development

11-29-2018

As noted in Part 3 of this series, a broad range of discrete passive component elements are candidates for embedding, but the decision to embed these component elements within the multilayer circuit structure must be made early in the design process. While many of these components are easy candidates for integrating into the substrate, others may not be suitable, or they are difficult to rationalize because they involve more complex process methodology.

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Embedding Components, Part 3: Implementing Discrete Passive Devices

11-15-2018

Most of the passive components used in electronics are discrete surface mount components configured to mount onto land patterns furnished on the surface of a PC board. Designers have several choices for providing passive functions in a system design, such as discrete surface-mounted passives, array passives or passive networks, integrated (Rs and Cs) passive devices, and embedded discrete passive components.

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Designers Notebook: Strategies for High-Density PCBs

01-01-2018

As hand-held and portable electronic products and their circuit boards continue to shrink in size, the designer is faced with solving the physical differences between traditional printed board fabrication and what’s commonly referred to as HDI processing. The primary driver for HDI is the increased complexity of the more advanced semiconductor package technology. These differences can be greater than one order of magnitude in interconnection density.

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2017

Strategies for High-Density PCBs

11-27-2017

As hand-held and portable electronic products and their circuit boards continue to shrink in size, the designer is faced with solving the physical differences between traditional printed board fabrication and what’s commonly referred to as high-density interconnect (HDI) processing.

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Embedding Components, Part 2

07-30-2017

Technology and processes for embedding capacitor and inductor elements rely on several unique methodologies. Regarding providing capacitor functions, IPC-4821 defines two methodologies for forming capacitor elements within the PCB structure: laminate-based (copper-dielectric-copper) or planar process and non-laminate process using deposited dielectric materials.

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Embedding Components, Part 1

06-30-2017

The printed circuit has traditionally served as the platform for mounting and interconnecting active and passive components on the outer surfaces. Companies attempting to improve functionality and minimize space are now considering embedding a broad range of these components within the circuit structure. Both uncased active and passive component elements are candidates for embedding but the decision to embed components within the multilayer circuit structure must be made early in the design process.

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2016

Specifying Lead-Free Compatible Surface Finish and Coating for Solderability and Surface Protection

07-06-2016

A majority of the components furnished for electronic assembly are designed for solder attachment to metalized land patterns specifically designed for each device type. Providing a solder process-compatible surface finish on these land patterns is vital...

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Flexible and Rigid-Flex Circuit Design Principles, Part 6

05-26-2016

The designer is generally under pressure to release the documentation and get the flexible circuit into production. There is, however, a great deal at risk. Setting up for medium-to-high volume manufacturing requires significant physical and monetary resources. To avoid potential heat from management, the designer must insist on prototyping the product and a thorough design review prior to release.

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Flexible and Rigid-Flex Circuit Design Principles, Part 5

04-27-2016

The outline profile of the flexible circuit is seldom uniform. One of the primary advantages of the flexible design is that the outline can be sculpted to fit into very oblique shapes. In this column, Vern Solberg focuses on outline planning, physical reinforcement, and accommodating bends and folds in flexible and rigid-flex circuits.

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Flexible and Rigid-Flex Circuit Design Principles, Part 4

03-30-2016

All of the design rules for the glass reinforced-portion of the board (land pattern geometry for mounting surface mount devices, solder mask and the like) are now well-established. One unique facet of fabricating the rigid-flex product is how the flexible portion of the circuit is incorporated with the rigid portion of the circuit. As a general rule for multilayer PCB design, furnish a balanced structure by building up the circuit layers in pairs (4, 6, 8 and so on).

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Flexible and Rigid-Flex Circuit Design Principles, Part 3

03-02-2016

This column focuses on methods for specifying base materials, and also address copper foil variations and fabrication documentation. It is important to research the various products in order to choose the one that best meets the design requirements.

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Flex and Rigid-Flex Circuit Design Principles, Part 2

02-19-2016

Flexible circuits are commonly developed to replace ordinary printed circuit board assemblies that rely on connectors and hardwire for interconnect.

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Flex and Rigid-Flex Circuit Design Principles, Part 1

01-27-2016

Flexible circuits represent an advanced approach to total electronics packaging, typically occupying a niche that replaces ordinary printed circuit board assemblies and the hard-wire interface needed to join assemblies.

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2005

PCB Designers Notebook: Flexible Circuit Design

01-03-2005

The flexible circuit was originally used as a conductive element for interfacing signals from one electronics assembly to another.

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