Embedded passives

Embedded passives

Keyword: Embedded passives

The number of surface mount passive components (resistors, capacitors and inductors) used in electronic assemblies has increased dramatically during the last few years in line with the increased functionality of all electronic devices. These passive components now account for the majority of parts on a board and occupy a surface area almost equal to that required for the active components within the design. Typically, there are nine times more capacitors than resistors on a design and the majority of capacitors are used for decoupling functions.

Clearly, there is need to reduce the amount of space on a board occupied by passive components, whilst simultaneously enhancing electrical performance. One way of achieving this is to remove the components from the surface and to embed them within the board itself using Embedded Passive Technology – the subject of this special themed edition of Circuit World.

Embedding passives technology has the potential to reduce form factors and/or increase functional density, improve electrical performance and reduce the number of solder joints giving enhanced overall reliability. High-density and high-speed applications are now creating a market pull for this technology since it is needed to provide a cost effective solution for the emerging electronic equipment.

Embedding passives, i.e. removing surface mounted devices and defining them on the inside of the electronic substrate is, however, not new; the technology has been used in ceramic (hybrid) circuits for decades. For organic substrates such as printed circuit boards (PCBs), both buried (embedded) resistor (e.g. Ohmega-Ply™) and capacitor (e.g. ZBC2000™) technologies have been available for 10 years or more, yet they have been limited to a niche market due to cost of ownership, manufacturability, capability and the continual miniaturization of conventional surface mount passive devices.

The choices available for embedding passives are currently many. For capacitors, the key performance indicators are the use of lower dielectric thicknesses and higher dielectric constant materials in order to minimise the area they occupy. Also, the number of capacitance layers required drives the choice of materials along with the required operating frequency range,temperature and voltage performance.

Compatibility with established PCB manufacturing techniques is key to the use of these thin materials with much focus being put on having the simplest, most robust process routes to achieve high yields and therefore make embedded capacitance cost effective. The most likely market growth area is for power supply decoupling where buried passives can deliver lower parasitic inductance and where there is a requirement for distributed rather than singulated capacitance.

For resistors, the key is to target the best ohmic range (Ω/) to capture the majority of resistor values needed and to achieve the best tolerance possible with acceptable life cycle reliability.

There are both additive (plating, inorganic and organic thick film) and subtractive (thin film) products with varying ohmic values, performance and cost characteristics. Tolerances of better than 10 per cent are the current benchmark and ohmic values of 250-1,000 Ω/ are the key areas of development for thin films. The drive with the additive systems (excluding plating) is for better stability and tolerance to take advantage of their lower cost of use.

It is worth noting, however, that barriers still exist to the large-scale implementation of embedded passives technology in the areas of design automation, cost, testing, PCB manufacturing infrastructure and reliability data. Conservative estimates suggest that embedded passives may account for 5 per cent of the total discrete passives market by 2005, which does not sound much until one considers that the total market is for 1,000 billion passive devices with a value of $1-2 billion per annum.

The papers presented in this issue of Circuit World have been selected to illustrate the breadth of research and applications engineering work ongoing in the field of embedded passives.

Tarja Rapala-Virtanen reviews the current status and trends in several areas of PCB advanced manufacturing technology including embedded resistors, the use of resistively heated presses for HDI PCBs and micro-via materials.

Richard Ulrich gives a comprehensive review of capacitor materials in terms of their characteristics and embedded capacitor performance requirements and confirms the evolutional nature of this technology.

Peter Sandborn et al. detail the cost aspect of embedding passives,showing how cost modelling of both product and life cycle can be used to assess the case for adoption of embedded passives using an avionics example board.

John Davignon examines the key areas for the use of lasers in trimming of embedded resistors. This method allows suppliers to achieve the higher tolerances required in some applications and will ultimately help to make embedded passives available to a larger market.

Percy Chinoy et al. discusses the properties, processing and properties of a high ohmic value thin film resistor material deposited by a novel Combustion Chemical Vapor Deposition technique.

Swapan Bhattacharya et al. detail three methods for producing high dielectric constant capacitor materials with the focus on them being PCB processing compatible.

John Andresakis et al. describe the influence of copper foil topography, dielectric type and dielectric thickness on capacitance performance and material reliability in a non-reinforced "flex" type embedded capacitor material.

Martin Goosey gives the recently published book "Integrated Passive Component Technology" by Richard Ulrich and Len Schaper a deserved high rating and recommendation – it should become one of the future reference books for embedded passives technology.

As is demonstrated by the papers presented here, elsewhere, and at the series of conference sessions dedicated to embedded passives, Embedded Passives Technology is an industry "hot topic".

There are a many potential materials available with varying characteristics and some consolidation will be necessary in order to allow designers to have multiple sources of similar value materials. There also remains much work to be done to convince the OEMs that embedded passives are a cost effective and reliable technology. Ultimately, however, and probably in the near future,embedded passives will play a greater role in electronic devices, as electrical performance will demand their use.

I am pleased to have been invited to introduce this special issue of Circuit World on Embedded Passives Technology and trust that you will find the papers herein as interesting and informative as I have.

Tony RidlerJuly 2003