Thin laminates as embedded capacitance for PDN designs

The function of power distribution networks (PDNs) is to provide clean power to electronics, which is a mandatory requirement for power integrity and to provide a solid return path for signals as an option for signal integrity [1].

Clean power distribution in electronic devices requires a system design that makes best use of on-board, on-package and on-chip capabilities. Power decoupling from on-board capacitors is best suited for lower frequencies and mid-frequency power decoupling is best accomplished by in-package capacitors, while on-chip capacitors satisfy high frequency power decoupling (see Figure 1). 

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Figure 1. Power distribution model – using planar capacitors in the package in close proximity to the IC reduces loop inductance and overall circuit impedance [2]

 
 
 

Power-distribution applications in telecom and networking industry demand ever-denser IC designs, smaller chip geometries and higher device speeds, boosting the current drawn by the microprocessor from the PDN. The increase in power dissipation places rigorous demands on noise reduction, indicating many more decoupling capacitors must to be added to the PDN to reduce the source of noise. However, it causes routing problems and changes the predicted performance of a capacitor because of the increased inductance.

Use of thin and very thin copper-clad laminates as embedded capacitance (i.e., a pair of power/ground layer pairs) is an approach that eliminates many decoupling capacitors from the board – not because of the increased capacitance of the thin laminate, but rather due to its lower inductance [3].

The impedance plot in Figure 2 shows impedance magnitude of a square pair of planes with various dielectric thicknesses. With 0.25mm (10-mil, 250µm) plane separation, the impedance swings are substantial at the resonance frequencies, and as thinness goes to 25µm or below, we see a significant reduction. The PDN impedance is approximately proportional to the laminate thickness. As the laminate gets thinner, suppression of modal resonances can be achieved.

Figure 2. Self-impedance magnitude with different laminate thickness – thin laminates help to reduce high-frequency impedance through their inductance proportional to plane spacing [4]

 
 
 

Interra® HK04 Thin Laminates

DuPont provides Interra HK04 thin laminates as embedded capacitance. They can be used as one of the PDN design options. The use of both 25µm and 12µm dielectric laminates shows the benefits of each – for example, reducing the number of decoupling capacitors, freeing up design space and reducing noise, resulting in higher board reliability. 

Interra HK04 delivers excellent capacitance stability over a range of frequencies, bias voltages and temperatures. The homogeneity of the dielectrics is compatible with rigid board manufacturing processes, showing excellent chemical resistance, especially in dry film processes, and is less susceptible to delamination. In addition, with their high pliability, the laminates can be made thinner without compromising mechanical properties, which improves handling for fabricators and boosts yields. Figure 3 shows a microsection of a 25µm HK04J film. 

 
 
 

Figure 3. Interra HK04J microsection – all-polyimide between copper planes provides dielectric film strength to achieve mechanical reliability as well as electrical performance. 

 
 
 

The newest addition to our Interra HK04 family is HK04M, which further improves on our longtime Interra HK04J. Key features of this new laminate include:

  • High modulus polyimide core to better handle 0.5-mil dielectrics
  • Optimization for low-profile copper option
  • Ability to help yields in the inner layer process

Interra HK04M delivers the same performance benefits as HK04J, with further improvements to mechanical strength to provide reduced laminate thickness. Interra HK04M laminates ensure their ability to deliver a high degree of quality, performance and stability in the PCB manufacturing process. Table 1 shows the typical laminate properties of 25µm and 12µm HK04M together with 25µm HK04J.

 
 
 

Table 1. Typical properties of Interra HK04M laminates

 
 
 

Interra HK04M is available today. DuPont will work with customers to implement HK04M for their new designs as well as to optimize existing designs.

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References

[1] Istvan Novak and Jason R. Miller, Frequency-Domain Characterization of Power Distribution Networks, Artech House, 2007.

[2] Bill Borland, “Decoupling of High-Performance Semiconductors Using Embedded Capacitor Technology,” 2006 15th IEEE International Symposium on the Applications of Ferroelectrics.

[3] Istvan Novak, “SUN’s Experience with Thin and Ultra-Thin Laminates for Power Distribution Applications,” TechForum TF-THA2, DesignCon 2006.

[4] Istvan Novak, “Quiet Power: Causal Power Plane Models,” PCB Design Magazine, Oct. 2017.

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