Embedded Component Technologies: Worth Another Look
If you’ve ever downsized your house you have a good idea of what designers of smartphones and tablets face every day. Moving to a smaller place entails finding room for drawers full of clothing, suddenly too-big furniture and items that you’ve had for years but are impossible to part with.
Similarly, ever-smaller tablets and smartphones are driving the need for further miniaturization of components such as inductors, capacitors, power amplifiers, SAW filters, duplexers, etc. Traditionally, passive and active components are mounted on the surface of the PCB using SMA technology. But while the component size, ball pitch and line width of the PCB has been decreasing rapidly, going forward it will be much more difficult to increase packaging density by any substantive amount. With designs running out of space in which to place components there is once again renewed interest in embedding passive and active components inside an organic substrate, that is, embedded in the printed circuit board.
Embedded technologies have been under development and test for about two decades, with some success. Murata, for instance, commercialized its GRU Series line of monolithic ceramic capacitors embedded in PCBs. AVX also has successfully embedded 0.15mm thickness 100nF 6.3V MLCC capacitors within the electronic circuit.
Recent embedded chip efforts are being pushed by LTE (Long-Term Evolution) 4G phone manufacturers looking to integrate additional functions without influencing the size and, most importantly, the thickness of the handset. TDK’s SESUB (semiconductor embedded in SUBstrate) is a technology where semiconductor chips, which are thinned down to as low as 50 μm are embedded in the substrate. The whole thickness of the substrate including the integrated semiconductor chips is just 300 μm. Using SESUB technology TDK has developed a power module for smartphones and what is said to be the world’s smallest Bluetooth Low Energy module. The power module contains two embedded dies which control all the power functions of the phone: a power management chip and a 16-bit RISC controller, as well as passive SMD components assembled on top. The compact module has a footprint of just 11 mm × 11 mm, which according to TDK is 60 percent smaller than a comparable discrete solution. Its slim insertion height of 1.63 mm, including shielding, is in line with the need for low-profile designs in smartphones. Similarly, the 5.6mm x 4.8mm x1.0 mm Bluetooth Low Energy module is said to have a 64% smaller mounting footprint compared to an equivalent modular construction using packaged ICs.
The compact construction of SESUB also delivers excellent thermal attributes due to the fact that the IC is completely embedded. All surfaces of the chip are in full contact with the laminate, which optimizes the heat transfer from the semiconductor into the substrate layers. These layers themselves contain the copper interconnection grids, which provide for efficient heat dissipation. This thermal performance is important for applications in the area of power management, transceivers, processors, and for the power amplifier – or all the main components of a smartphone. According to TDK comparing discrete-packaged power semiconductor chips with the same IC embedded in SESUB results in much lower chip surface temperature.
The company further claims that embedding the chips also leads to improved EMC performance due to the shielding effect of the metal layers inside the SESUB substrate. The compact design of the SESUB module and the shorter line connections within the substrate layers also lead to improved parasitics and thus better system performance.
Earlier this year at ECTC (May 2013, Las Vegas, NV) Intel’s Yongki Min, et al. presented a paper entitled “Embedded Capacitors in the Next Generation Processor.” The paper discussed Intel’s collaboration with its suppliers to develop and commercialize in a high performance server product an embedded capacitor technology that is claimed to provide significant power delivery advantages. The customized embeddable capacitor is said to have a similar structure to a standard MLCC, and is stacked alternatively with hi-k dielectric ceramic layers and patterned metal layers. Unlike simple two terminal ceramic capacitors, however, this embeddable capacitor has through-hole-vias and numerous top and bottom surface electrodes resulting in inherently low inductance (~ 1.5pH). The array capacitor is physically placed in the substrate core, and a polymer molding material is filled between the embedded capacitor and core material.
Intel reports that the embedded capacitor dramatically reduces the impedance in the 100 MHz range by a reduction in package inductance. Also, an impedance drop in the 2-10 MHz range was observed by the researchers since the embedded capacitor adds to the total amount of package capacitance.
The Intel authors admit, however, that embedding the ceramic capacitor into the thick server substrate core brings many concerns and issues to overcome such as the coefficient of thermal expansion (CTE) difference of ceramic capacitor, substrate core, molding polymer, and substrate build-up materials as well as substrate warpage due to the embedded capacitors.
The low inductance embeddable capacitor has been fabricated based on standard MLCC and LTCC (low-temperature-co-firing-ceramic technology) processes taking into consideration the compatibility with the microprocessor package structure and manufacturing process. Intel noted that the surface mounting of discrete capacitors is part of the backend process, but the embeddable capacitor is integrated at the initial substrate manufacturing step.
Unlike traditional surface mounting, embedded components are placed in cavities in the substrate. Components placed into the substrate can have very narrow tolerances of 20 microns or less. Traditional SMD placement is done by placing a component on solder paste. Small inaccuracies of the pick and place equipment process can therefore be overcome by the strong self alignment effect of the solder during reflow. Embedding in cavities has no self-alignment effect and must be placed with higher accuracy. High placement quality is a must because any undetected errors will end up in the final substrate and there is no chance of repair.
These and other fabrication-related reasons—embedded components increase the package manufacturing cost, for example-- are why some suppliers are choosing to sit this one out. But there are alternatives: Integrating multiple circuit elements to eliminate components and the interconnections between them is another way to generate a large net saving in PCB area. STMicroelectronics, for instance, has developed a portfolio of IPAD products integrating passive components and active functions such as ESD protection as well as smart antenna tuners that integrate components and functions such as EMI and common-mode filtering, ESD protection, and integrated RF passives.
By combining in a single-package the matching, filtering and protection functions usually implemented with discrete components, ST provides engineers with the flexibility to expand the number of functionalities of the end product or to create a smaller device. Designers can use the newly freed-up space to increase functionality by designing-in extra ICs, or they can by simplifying a PCB layout and shorten time-to-market for new products. For example, ST’s BAL-NRF01D3 is a miniature balun designed for low-power wireless solutions (the matching impedance has been customized for Nordic Semiconductor circuits) and is said to enable savings of up to 90% of the PCB space occupied by discrete antenna-matching and harmonic-filtering components, according to the company. It is also said to decrease the BOM count by 80%, from five components to one component.