Amid an ongoing tidal wave of mobile phone and portable Internet of Things (IoT) development (such as wearables), designers are being challenged to fit greater functionality into smaller form factors, with every square millimeter of a PCB becoming precious. Given that after years of miniaturizing passive devices still occupy the largest real estate section on a PCB – approximately 70% of board space – it makes sense to continue last month’s topic of embedding components, this time focusing not on a PSMA power source study, but on overall developments in embedded passives.
By embedding passives into the printed circuit board core the package footprint can be significantly reduced compared to designs employing surface mounted devices. Embedded passive components can either be formed by creating components directly during PCB manufacturing or, using currently existing or specially-designed components, by first inserting them into the inner layers of a board and then adding additional layers to the PCB.
Neither approach is new; embedding passives has been done for many years, although there is now more need for it due to the higher component densities of the newest designs. There are, for instance, an increasing number of application processors for mobile phones with embedded capacitors in the package. Among the numerous suppliers offering processes to embed passives into a PCB include, among others (and this list presents no more than a sample), embedded MLCCs from Ibiden, Shinko Electric’s MCeP (Molded Core embedded Package), TDK’s CeraLink technology (which embeds snubber capacitors in IGBT modules), Taiyo Yuden’s EOMIN (Embedded Organic Module Involved Nanotechnology, a process that can embed MLCCs and chip resistors), the Embedded Component Packaging (ECP) process from Austria Technologie & Systemtechnik Aktiengesellschaft (AT&S), ASE Group’s Embedded Chip Technology and Samsung Electro-Mechanical's EPS (Embedded Passive Substrate, where decoupling capacitors, normally used to stabilize the power supply voltage level, are built in the substrate to reduce the inductance of the power / ground network compared to SMT decoupling capacitors on the top or bottom package layers).
Reducing PWB size is only one of the benefits of embedding passives such as capacitors. Others include improved electrical performance. For one thing, more efficient power delivery is possible because in the case of the capacitors charge can come from directly underneath the device, reducing the current path and package inductance. Consider, also, parasitics such Equivalent Series Inductance (ESL). If the ESL of a decoupling capacitor is large, much of the high-frequency current discharged from the capacitor will be reflected and an insufficient high-frequency current will be conducted through the power supply or GND line, causing voltage fluctuations, generating logic errors or becoming common mode noise that is discharged outside the device resulting in electromagnetic interference. Due to their lower ESL characteristics embedded decoupling capacitors are employed today with high-speed application processors (APs) in mobile products.
Unlike their surface mounted cousins embedded passives also face fewer reliability issues through the elimination of solder joints. What’s more they offer simplified circuit routing by eliminating vias, traces and pads and they provide improved heat transfer ability.
In addition, eliminating some of the need to handle extremely small individual components reduces potential problems and can lower pick-and-place costs. In their white paper Ultrathin discrete capacitors for emerging embedded technology AVX’s Radim Uher and Tomas Zednicek point out that while the price of tiny ceramic capacitor components is low, “users especially in the consumer sector should pay attention to the total cost of the system as the pick-and-place costs can be three times higher than the component cost when placement time, yield and rework is considered.”
The industry has been making continuous advancements in embedded passive technology. For example, GaWon Kim, Max (Sunghwan) Min et. al. of Samsung Semiconductor System LSI SoC Bay Area R&D presented a paper entitled “Package embedded decoupling capacitor impact on core power delivery network for ARM SoC application” at the 2014 Electronic Components and Technology Conference (ECTC). In this paper, they introduced a BGA package having an ARM SoC chip, which has component-type embedded decoupling capacitors.
The authors noted that previous research mainly focused on thin-film embedded decoupling capacitor technology. However, the thin-film embedded decoupling caps needed additional layers, special dielectric material, and a new assembly process. On the other hand, they showed that a passive component-type embedded decoupling cap can be implemented using a general capacitor component and normal substrate with an extra hole and an SMT process, making for a more cost effective method for achieving better electrical performance.
To evaluate and confirm the impact of embedded decoupling capacitors on the core PDN (power distribution network), the researchers manufactured two different packages; one with and one without the embedded decoupling caps. They concluded there was clear improvement of system-level core PDN performance in the middle frequency range when the embedded decoupling caps were employed, noting that the overall system-level core PDN for an ARM SoC could meet the target impedance in frequency-domain as well as the target on-chip Dynamic Voltage Drop (DvD) levels by means of embedded decoupling caps.
The benefits of embedded passives are not limited to small form factor devices. Larger die and server products requiring high performance power delivery solutions can also benefit from embedded passives. For microprocessor applications, capacitors are used to reduce package electrical impedance and enable the system to maintain a near-constant voltage across all operating frequencies. That quality has attracted the attention of microprocessor giant Intel, which has collaborated with its suppliers to develop and commercialize an embedded capacitor technology that it claims provides significant power delivery benefits for high performance computer applications. In a paper entitled “Embedded capacitors in the next generation processor” (2013 Electronic Components and Technology Conference [ECTC]) authors Yongki Min, Reynaldo Olmedo, et. al. reported on the first commercialized embedded capacitor technology used by Intel.
The company has successfully taken a customized embedded capacitor from the proof of concept stage to an actual product implementation in a high performance server product. The researchers examined an ultra-low inductance embeddable capacitor placed under the die shadow to reduce the path to the die.
According to the authors, the embedded capacitor dramatically reduced impedance in the 100 MHz range by the reduction in package inductance. Also, an impedance drop in the 2-10 MHz range was observed since the embedded capacitor added to the total amount of package capacitance. The reduction of high frequency impedance resulted in significant reduction of power supply noise, and this in turn provided what was described as “improved server product performance.”
Intel’s embedded capacitors were fabricated based on a combined multi-layer ceramic capacitor (MLCC) and low temperature co-firing ceramic (LTCC) technology, taking into consideration compatibility with the microprocessor package structure and manufacturing process.
The authors do admit that while embedded technology demonstrated performance benefits, it did increase the package manufacturing cost, so they concluded that tradeoff studies are necessary for a given product design to understand if the improved performance and package form factor reduction warrant the additional cost.
For this and other reasons it will be years before embedded technology becomes predominate in assembly processing but manufacturers continue to move toward acceptance of the technology and advances can be expected to continue. We’ll continue to report on them when they do.