High-Speed Boards with Buried Resistances

With the advancement of modern technology and scientific research, newer digital gadgets are arriving in the market. Most of the new arrivals are multi-functional, but also highly miniaturized and portable. As a consequence, the printed circuit board or PCB?inside these tiny gadgets are also miniaturized, with high component density, and operating at high speeds.

Typically, electronic PCB assemblies have numerous resistances spread out on the top surface, using up much of the available surface area. This is an impediment to further miniaturize the gadget, as is necessary for the new generation of miniature electronic high-speed digital transceivers. For such miniature mobile devices, Rush PCB recommends advanced high-speed PCB technology like buried resistances.

Advantages of Buried Resistances

Burying the resistances within the PCB layers has two major advantages apart from allowing further miniaturization. One is it improves the dependability and reliability of the board, and the other is it helps to preserve intellectual property.

There are other secondary advantages of using buried resistances also. For instance, buried resistance technology:

●????Improves reliability by eliminating solder joints.

●????Increases density of active component while reducing form factor of PCB.

●????Improves signal routing by eliminating SMT vias.

●????Eliminate the inductive reactance of the SMT device,

●????Improves line impedance matching.

●????Reduces EMI, crosstalk, parasitic effects, and noise.

●????Shortens signal paths, and reduces series inductance.

●????Shortens cycle times in PCB assembly

With buried resistances, the track length connecting them to the rest of the circuit reduces, and this contributes to an increase in the efficiency of the circuit board. The total number of parts in the Bill of Materials or BOM reduces significantly, improving the efficiency of material handling and inventory management. Assembling the board requires the pick and place machines to handle a substantially lower number of components, thereby improving the throughput. As a lower number of components need soldering, the opportunities for failure also reduces substantially.

Typically, with an increase in functionality, the number of Inputs/outputs rises, and the related ICs become bigger, while the number of passive components increases proportionately. With buried resistances, the designer can make better use of the free surface area. They can use components with larger number of inputs/outputs, improving the functionality of the device, while the number of passive components reduces on the surface of the board.

Using modern technology, it is possible to incorporate resistances into the structure of the high-speed PCB to reduce the PCB size or increase its functionality. Buried resistances not only enhance the reliability of the assembly, but also reduce the chances of bad soldering. The reduction in track length improves the integrity of the circuit while improving its operational efficiency. Overall, this leads to a lowering of the PCB cost.

Types of Buried Resistances

At present, PCB manufacturers use two types of buried resistance technology. One method is to integrate a buried resistance by pasting the resistance into the internal layer of the board, similar to placing a regular SMT resistor. The other method introduces a resistance via printing, engraving it and then etching it to the desired value. In both cases, the thickness of the incorporated resistance is low enough to not change the thickness of the layer.

Manufacturers of buried resistors use a thin film resistor electrodeposited on copper NIP metal alloy, forming the resistor material. They laminate this on a dielectric material and process it subtractively to produce a planar resistance. As this resistor is a thin film, it is possible to bury it within PCB layers without the board thickness increasing. Burying the resistance within a layer means it does not occupy surface area like a discrete resistor would.

These thin-film buried resistors have a low resistance temperature coefficient of less than 500 ppm, and are stable over a large frequency range beyond 20 GHz. These resistors generally have a long life of over 100,000 hours tested at 110 °C.

Thermal Considerations

Since resistors dissipate heat, designers must consider effective thermal management, since heat dissipation by convection is not possible for buried resistors. To reduce stress due to heat build-up, designers must consider providing thermal or mechanical isolation from a plated through via or surface mount pad during soldering via reflow or hot air.

The mechanical isolation is also necessary to prevent changes in the value of the buried resistance during via drilling, or movements of the PCB in any direction.

Rush PCB recommends using thermal isolation distances of 10 mils from the plated through hole to the resistor element, and 5 mils from laser drilled microvias.

Specifying a Buried Resistor

Schematic: A buried resistor in a schematic capture appears just like any other resistor does. The resistor value appears below the resistor shape. The difference is in the naming convention—using BR rather than R—followed by the resistance value, and the resistance material type.

Layout: It is necessary to turn on a reference layer to show the position of the buried resistance in a specific layer. The designer must turn on the composite layer and the reference layer before generating the Gerber files.

Modern CAD PCB software offer an effective platform for designing embedded passive components like resistors, capacitors, inductors, and transformers. They do not require a separate library for buried components, as the software can synthesize the component values usually available as properties in the schematic.

For a successful synthesis, the designer must specify the nominal value, trimmable range, power rating, tolerance, geometry, and aspect ratio. Once the software generates the buried resistor, the designer can interactively edit it with dynamic feedback.

DRC for Buried Resistors

Regular design rule check or DRC does not incorporate checking for buried resistors, as embedded passives are not standard components. Rather, they appear as DC short circuits. Therefore, designers must change DRC rules to allow effective differential recognition of DC short circuits and AC short circuits. Modern CAD PCB software has online verification of clearance rules, signal integrity rules, trace resistance, and impedance and propagation delays.

Conclusion

Rush PCB recommends using embedded resistors in PCBs for the benefits listed above. It is possible to use buried or embedded resistors in any type of board like rigid, flex, rigid-flex, and HDI.