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PCBs with Buried Resistances
Posted on 26/10/2018

Advantages of Buried Resistors

The electronic industry is increasingly demanding faster processing times and improved performances. This has led to the generation of new challenges in both RF and digital circuit board designs. Replacing discrete and SMD resistors on the surface of electronic boards with embedded or buried resistors offer several advantages:

  • Reduced number of solder joints—improves reliability
  • Reduced crosstalk and noise—reduces EMI
  • Elimination of inductive reactance
  • Reduced series inductance
  • Shorter signal paths
  • Improved signal routing—lower number of vias
  • Increased active component density
  • Reduced form factors
  • Reduced cycle times in PCB assembly—shorter time to market
  • Laser trimming capabilities

Types of Resistors

The industry currently uses two types of buried resistances—thin film and thick film.

Thin Film Buried Resistances

Manufacturers use a thin-film vapor deposition technique to deposit a uniform resistor material layer on the PCB. The deposited resistive material is usually around 0.1 µm thick and an alloy of Nickel, Chromium, and/or Aluminum and Silicon. Different layer thicknesses produce a range of resistance values. As the layer is uniform and dense, fabricators use a subtractive process of laser trimming to increase the resistive path and achieve a specific resistance value.

Benefits of Thin Film

Thin film technology produces resistors generally known as metal film resistors, thin enough to bury within the inner layers of a multilayer PCB. With low thermal coefficient of resistivity, thin film foil resistors adapt well to lamination. Precision vacuum metallization techniques ensure better resistor tolerances. Good thermal dissipation properties offer excellent thermal stability, exhibiting excellent ductility at elevated temperatures to with stand stresses without cracking.

Thick Film Buried Resistances

This is a somewhat older technology for incorporating reliable carbon resistors printed on to PCBs using standard PCB processing steps. Fabricators use a novel hydrophobic polyimide resin as a printed resistor material to serve as a polymeric thick film resistor.

After printing the thick film resistor, the fabricator has to cure the film in a standard convection oven, a thermal belt furnace, or an infrared furnace. Once cured, the film is ready for laser trimming.

Benefits of Thick Film

In comparison to thin film embedded resistors, thick film buried resistors are cheaper, have lower TCR values, are less stable, and do not reach high tolerances. On the other hand, thick film resistors are able to handle higher power, provide a wider range of resistance values, and are capable of withstanding high voltage surge conditions.

Although the process burying resistors in PCB layers is more complex, it is more versatile than using SMDs. Buried resistors free up the surface space and allow designers to place additional active components, thereby increasing the functionality.

Application and Properties

Embedded passives, both resistors and capacitors, yield a more reliable PCB as they reduce the number of solder joints. This reduces rework on assemblies, while lowering the total system cost. As laser trimming produces specific values with close tolerances, designers can achieve higher performance and low signal losses, by placing the embedded passives more optimally in the circuits.

Embedding components within PCB layers achieves an additionally important function, that of preserving the intellectual property of the OEM and preventing it from being easily duplicated.

Conclusion

The team at PCB Global have the skills and expertise to assess and advise if buried resistors are more suitable or beneficial to your PCB design and applications. For more information or to speak to our team about the benefits of buried resistors, please don’t hesitate to contact us at sales@pcbglobal.com

Image Courtesy: http://ohmega.com/wp-content/uploads/IMS-2017-Workshop_r01.pdf