Embedded Capacitors and Resistors in PCB

Integration of Embedded Capacitors and Resistors in Multi-Layer Printed Circuit Board (PCB)

Olome Baudouin Ekute 

EEF714: PCB DESIGN AND FABRICATION

Faculty of Engineering and Technology, University of Buea

 

I.                   I. Introduction

The rapid development and high demands for more efficient, miniature electronic devices as well as increases reliability, have led researchers to develop new technologies in manufacturing printed circuit boards. In 1956 the first PCB was patent in the US which began with a single side on the PCB, then later the double sided PCB was developed forming connections between the components using the surface mount technology and the hole technology [1]. Due to high functional demand, the multi-layer printed circuit board was developed with smaller component sizes. In order to achieve this, passive components like capacitors and resistors which are components necessary in almost every electronic circuitry are embedded in the multi-layered PCB. It is essential to note that over 70% of circuit components in a typical PCB is made of these passive components (resistors, capacitors and inductors). As such, these components sizes are reduced making it less voluminous and possessing a higher density of connection of the PCB, thereby highly reducing parasitic losses and also minimizing cost. The miniaturization of this PCB permits it to be most conveniently used in mobile phones designs, air planes, military applications, etc. because of small size requirements in such applications. 

II.                II. Multi-layer PCB formation and embedding Materials

There are quite a number of available materials for embedded passives manufacturing and this keeps on growing day by day as a large number of manufacturers keep on with technological development. Beforehand, the multi-layered PCB consist of different layers containing different materials as seen in the figure below.

 

Figure 1: A view of the various layers in a multi-layered PCB 

In multilayer PCB, each inner layer can also be used to place passive components [6]. Prepreg (binding element) is sandwiched between two cores or between a core and a copper foil in the PCB to provide the required insulation. The prepreg most commonly used is prepreg 106 which contains high resin content (69%). Designing a PCB with two prepreg layers permits to embed into the inner layers electronic components with a thickness up to about 75 μm. Chemical solutions are used to the oxide treatment of copper surface before the lamination in order to increase its adhesion to the dielectric layers. Usually quite aggressive and strong oxidizing acid or alkaline solutions are used. They cause the formation of strongly adherent oxide layer increasing the contact surface of the dielectric to the copper by micro etching and simultaneous oxidation. Thermo-mechanical exposures occurring during the lamination process causes the resin polymerization and durable connection of all PCB layers. The lamination occurs at a pressure and temperature of 30 bars and 180°C respectively. Several materials (alloys), characterized by a relatively good chemical resistance and constant surface resistivity (dependent on the layer thickness and composition) are used. The following ones can be mentioned among them:

Table 1: Embedded PCB components, characteristics and the material design [1, 2].

S/N	Component	Characteristics	Materials for embedded passives manufacturing
01	Thin-film resistors materials	Thickness   Highly resistant to chemical   Constant surface resistivity	NiP alloy electroplated onto the copper foil e.g OhmegaPly through subtractive process
			NiP plated directly onto the inner layer through an additive process
			NiCr alloy
			NCAS-Aluminium Sil-icon
			CrSiO Doped-platinum deposited on copper foil
02	Thick-film resistors	Thickness Resistance range from several 𝛀 to M𝛀.	Polymer pastes with carbon e.g. Electra Polymers, Carbon-silver nanopowders
03	Composites for embedded capacitors	Thickness of dielectric Thickness of one or two layers of copper	modified epoxy or modified epoxy-filled ceramic between Cu layers or on one CU layer
			Thin film paraelectrics SiOx
			Anodized Ta2O5
			BaTiO3 thin film on copper
			High Tg (170C) epoxy resin with 106 fiberglass
			Polymer film with copper deposited thereon

 

I.                   III. Embedded Passive Technologies

Different technologies in embedding resistors and capacitors are:

III.1. Chip embedded within the Substrate - placing chip modules as a component

Connection between the substrate and components can be by copper or by conductive adhesive instead of nickel and tin soldering

Figure 2: Component embedded within the substrate

III.2. Embedded subtractive fabrication technology - thin film

A)    Thin Film Resistor

This technological process involves imaging, electroplating and Etching. Copper laminate is used as a basis for electrical interconnection, with the copper foil sheet having a thin resistive layer [3]. The resistance of the sheet material is defined in ohms/square. Sheet resistivity is stated in Ohms per square.

This is specifically for the Bar type resistor pattern. Other patterns do exist such as the: Serpentine resistor type, Circular resistor and Arc resistor

B)    Thin film capacitors

Embedded formed capacitors are generally formed by the careful selection of dielectric material, dielectric thickness, and plane area. They typically have thin laminate.  Sheets of Cu-clad laminate recommended. Plates are set very close together and ideal for high frequency decoupling. The capacitance (C) is gotten from:

Where A is the area of capacitor plates, is the dielectric constant, d is the thickness between capacitor plates, is the permittivity of air.

Figure 3: Resistor and capacitor formation

C)    Resistor and Capacitor (RC) laminate-3M

In this case, the manufactured PCB will contain 1 substrate (core) instead of 2, with the copper clad laminate and can embed the resistor, capacitor and the RC network.

Figure 4: RC laminate-3M configuration

Below is a block representation of an embedded subtractive technology

Figure 5: Embedded subtractive fabrication technology-thin film [4]

III.3. Embedded Additive Technology-Thick film

Screen printing is an additive technique used to apply materials (conductors, resistors, dielectrics) in paste form to a substrate in defined patterns on the circuit board layer. The printed components are described as a ‘thick-film’ composite.

No imagining, electroplating and etching. The metal is only pasted where needed. This process reduces the number of steps in PCB manufacturing.

Figure 6: Embedded Additive Technology-Thick film [4]

II.                IV. Structure of a multi-layered passive Embedded PCB

Embedded resistors should be applied in rigid portions of PCBs. It is also essential to note that components embedded into the multi-layered PCB must be thinner than a distance between adjacent PCB layers. This is to ensure that it does not increase the overall thickness of the printed circuit board.

Figure 7: A cross-sectional view of an embedded a multi-layered PCB [5]

The IPC-7092 standard defines the following six embedded component structure designs:

-          Type A base-core consist of components, on one side of a mounting base.

-          Type B base-core consist of components, on both sides of a mounting base.

-          Type C base-core configurations consist of either active or passive components, or both, on one side of a mounting base that contains formed passive components.

-          Type D base-core configurations consist of either active or passive components, on both sides of a mounting base that contains formed passive components.

-          Type E base-core configurations consist of a pre-manufactured mounting-base that contains formed passive components unto which additional layers have been added in order to produce an HDI multilayered base-core. The process sequentially adds RCC layers and microvias for interconnection.

-          Type F Embedded Core process is really considered a ‘coreless’ assembly variation based on a ‘face down’ component mounting technology.

Figure 8: Embedded component PCB structure designs [6, 3]

 

III.             IV. CONCLUSION

Embedding of passives components into PCB allows further miniaturization, which potentially minimization of parasitic effects and reduce cost. Moreover components embedded into the PCB should have a reliable operation and stability parameters due to the impossibility of replacing defective parts or components. Also, the miniaturization of PCB has environmental benefits as this help minimize the amount of waste in electronic disposal. Looking at the considerable potential and advantages of embedded passives it seems that this technology will be in the near future more frequently used since it increase functionality, without increasing the area of the substrate freeing the surface by 30% - 50% [1].

 

REFERENCES

[1]	D. Andrzej, S. Jarosław and M. Piotr, "Structure and Properties of Resistors and Capacitors Embedded into Printed Circuit Board," p. 02, February 2014.
[2]	V. Bensman, "The Technology of Embedded Components for PCB," Eltek Ltd., Hamerkaz (Central) District, Israel, June 2021.
[3]	S. Vern, "Embedded Passive and Active Component: PCB Design and Fabrication Process Variation," in IPC APEX EXPO Conference Proceedings, Saratoga, California USA, 16 June 2016.
[4]	A. Mizrachi, "Embedded Passive Components Trend and Application," Orbotech, Isreal, 2001.
[5]	G. R David, "Embedded Passive Technology," Jet Propulsion Laboratory - California Institute of Technology, California, July 2005.
[6]	V. Solberg, "Design and Assembly Process Implementation for Embedded Passive and Active Components," in IPC-7092 APEX EXPO, 2015.