Building up, not out: miniaturisation of MLCCCs
28 July 2017
Engineers face a constant battle to reduce the size and weight of electronic components. Moore’s law supposes that the number of transistors in a dense integrated circuit doubles approximately every two years.
This article originally appeared in the July 2017 issue of Electronic Product Design & Test; to view the digital edition, click here – and to register to receive your own printed copy, click here.
This article originally appeared in the July 2017 issue of Electronic Product Design & Test – to view the digital edition, click here – and to register to receive your own printed copy, click here.
If available space is likely to decrease then it follows that component size must also dramatically reduce. Jonathan So, Applications Engineer at Knowles Capacitors, explains how its StackiCap development is one solution.
Unlike semiconductor wafer processing, the practicalities of handling and mounting discrete components mean there is a rapidly approaching baseline. There have, however, been improvements in manufacturing and materials technology over the past few decades, allowing for miniaturisation of Multilayer Ceramic Chip Capacitors (MLCCCs). Volume manufacturers have pioneered these developments, with the current commercially available pinnacle being a sand grain-like MLCCC, measuring only 16 thousandths of an inch by 8 thousandths (EIA size 01005).
The focus of this miniaturisation has broadly been in the field of base (or noble) metal electrode technology, as well as improvements in other processes, such as tape casting. However, they generally apply to low voltage capacitors – up to 100Vdc-rated parts in a small form factor, such as 0402-size. A few suppliers offer parts up to 250V-rated using Class I material, but these put a limitation on the capacitance range. For higher voltage, large capacitance values and high power applications, there is still a need for larger-sized components; however, miniaturisation can still provide significant benefits.
The space and aerospace industries are major beneficiaries of component weight reduction. According to NASA, it costs $10,000 to put a pound of weight (454g) into space – $22 a gram – so any drop in weight can lead to significant savings. The drive towards greener technology means that weight saving in any transportation system is also of benefit. With the increase in development and production of electric and hybrid electric vehicles, the use of high voltage, high capacitance MLCCs are on the rise. Despite this demand, the rate of miniaturisation has not occurred at anything close to that of smaller low voltage components. Whilst there are technical issues to overcome, it may also be that designers are tied to MIL specifications that dictate a certain part, or range of parts, that must be used.
Dielectric imperfection problem
Historically, mid-high voltage ceramic capacitor development has been limited to some extent by its failure modes, categorised as intrinsic and extrinsic. Intrinsic defects are linked to the raw materials or manufacturing process, whilst extrinsic imperfections are attributed to mechanical and thermal cracking during usage. Intrinsic type defects inside the ceramic element, such as a void or delamination (Figure 1), can lead to a short leakage current path, resulting in degraded chip heat resistance and increased susceptibility to cracking. Early MLCC design was limited by the quality and purity of the dielectric materials themselves, which made it difficult to fit in the growing number of layers within the chip, while at the same time reducing the thickness of those layers. Therefore, to increase the overall overlap area of the electrodes and achieve higher capacitance, the capacitor tended to have more point defects, reducing overall reliability.
To address and improve dielectric processing capability, Knowles has driven the development of high reliability, high voltage MLCCCs. As the capacitor increases in capacitance value, the number of electrode layers and/or the overall chip thickness grows. It therefore becomes dependent on the dielectric strength of the material itself, in terms of maintaining the reliability – without dielectric breakdown (Figure 2) that may lead to short defects or insulation resistance degradation.
The electrostrictive phenomenon of the ceramic dielectric material causes the chip to stretch and contract in its layer-stacking direction when voltage is applied, which can lead to cracking. This is a common failure mode for capacitors made with high dielectric constant material, such as Barium Titanate. Figure 3 shows a typical example of the tendency of change in strain of the piezoelectric material of X7R under various bias voltages applied: the strain percentage increase will cause degradation of the dielectric material over time and induce failure of the device.
The crack typically runs through the centre of the component, along one or two dielectric layers as shown in Figure 4. This is more prominent for larger capacitor chips, such as 1210-size and above, and with higher voltage ratings, such as 200V or more. To alleviate this piezo cracking due to the electrostrictive phenomenon, manufacturers have devised various solutions. One commonly-known design method involves stacking capacitors together, with metal terminals attached to the chip’s external electrodes; it is then attached to the circuit board via these metal terminals. This kind of design tends to have a thicker component profile to reach the high capacitance. Another design approach is to use special dielectric formulations, but this can be at the expense of a lower dielectric constant, thus limiting capacitance range.
StackiCap background development
To address these drawbacks, Knowles Capacitors developed its StackiCap technology, based on a patented built-in stress relieving layer (Figure 5) to buffer the strain of expansion and contraction in response to the electrostrictive effect on the dielectric material. The stress relieving layer is positioned where friction is greatest, to optimise the mechanical decoupling of the capacitor package’s multiple component layers – dramatically improving its integrity and reliability. Furthermore, it exhibits the same electrical and physical behaviour found when using multiple capacitors, but with the benefits of reduced size. To further improve reliability, StackiCap incorporates a silver epoxy polymer termination layer to prevent the core of the capacitor from cracking: FlexiCap. This layer absorbs the stress associated with the expansion or shrinkage of the solder joint – brought on by thermal shock, alongside other factors, such as flex stress from the substrate. Components terminated with FlexiCap can withstand a greater level of mechanical strain when compared with sintered terminated components.
StackiCap enhanced C/V values
Capacitors that employ StackiCap technology benefit from an increase in volume efficiency compared with products with conventional structures, such as stacked leaded components. For example: a 1 kV 330nF MLCCC can be replaced by a 2220 1kV StackiCap, reducing volume by a factor of 4.5 times and weight to only 1/5. Referring back to the NASA example, this would result in a $56 saving per component just in payload costs. This type of component is often used multiple times in each application, so the size, weight and cost savings can mount up significantly.
StackiCap technology can also dramatically reduce board area when replacing non-stacked alternatives. This is useful as designers are often restricted in the X-Y planes, but not Z; and in extreme cases an 8060 size can be replaced with a single 2220. See Figures 6 and 7.
The StackiCap range offers significant reductions in ‘PCB real estate’ for equivalent capacitance value when board space is at a premium. The development of StackiCap has paved the way for the wide adoption of high voltage, large-size chip capacitors for a wide range of applications: this includes switch-mode power supplies for filtering, tank and snubber, DC-DC converter, DC block and voltage multipliers. Moreover, it provides huge benefits in applications where size and weight is critical – such as the growing electric vehicle market, where the StackiCap range has AECQ qualification, making them ideal for EV applications.
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