This article will describe the design, construction, and performance benefits of AVX’s proprietary FLEXITERM termination layer technology, and how that technology is enabling advanced new passive components, like FLEXITERM and FLEXISAFE MLCCs, the design and construction of which enable significantly enhanced safety and performance capabilities in applications that regularly experience intense mechanical flexure and thermal cycling.
Since their market introduction in 2004, FLEXITERM capacitors have proven to be so robust that the results of the stringent automotive testing they have endured are worth sharing with the greater electronics industry, as these devices stand poised to improve many applications outside of the automotive industry, including those within the industrial, solar inverter, base station power supply, space, and aerospace markets, amongst others.
This article will also explain how theoretical and practical design enhancements made to FLEXISAFE capacitors led to an increased DC Voltage capability, and thus a reduced risk of failure due to any electrical overstress.
The thermal and mechanical stresses that MLCCs with a FLEXITERM termination layer can absorb without failure is supported by the results of rigorous reliability testing programs that were required by top automobile manufacturers whose test standards exceed general automotive industry standards.
Numerous data tables, graphs, and cross-section images (view in gallery above) will provide evidence of how this high-quality product line can satisfy the most stringent test regimes and prove beneficial when employed in a variety of other applications in which safety and reliability are at the forefront of the design cycle.
Electronics Growth; Its Impact on Passive Components
Over the past 10 to 20 years, the electronics industry has experienced phenomenal growth and development across all aspects of hardware, software, and the human computer interfaces (HCIs) that allow us to use the multiplicity of electronic devices and products currently available on the market.
Electronic devices are an inextricable part of modern life, especially in the areas of communications (e.g., mobile phones and tablets), travel (e.g., automotive electronics), and entertainment (e.g., TV, PC, video games), and this proliferation of electronic devices as daily objects has led to steady growth in the demand for enabling components.
As such, the passive component industry has also experienced steady growth and development, especially in the areas of downsizing, as demonstrated by smaller capacitors, which are now available in ultra-miniature 0105 case sizes; higher capacitance values, which are now available in the µF range; and capacitors with lower inductance values, which support the faster signal processing required by high-speed processors.
The passives industry has also experienced significant growth in component manufacturing, and now produces billions instead of millions of parts per day to satisfy demand for the dense population of miniature components necessary to imbue PCBs with the increased functionality demanded of today’s electronic devices.
In addition to the demand for higher volume production, the quality and reliability demands placed on the electronic components industry have also intensified over time, requiring manufacturers and suppliers to ensure that their devices can satisfy ever-higher quality requirements and perform as intended over an established life cycle. These standards range from ISO international standards to the automotive AECQ200 quality system, and the very high reliability demands for military, aerospace, and space flight applications.
Ceramic Capacitor Technology
Traditional multilayer ceramic capacitors (MLCCs) have long experienced much greater demand than most other component types. As such, MLCC suppliers have had myriad opportunities to experiment with and refine the material sets for both ceramics and electrodes. The resulting design revisions, such as the widespread use of smaller particle sizes and various formulation adaptations, have subsequently led to many major developments in the materials and processing technologies necessary to realize the higher values and smaller sizes the industry demands.
For example, in the early 2000s, the predominant MLCC chip sizes were 0805, 1206, and 1210. In many markets, including consumer electronics, these have since been replaced with 0201, 0402, and 0603 sizes, which are considerably smaller and thinner, but with comparable or even extended capacitance values, thanks to MLCC formula revisions like the addition of rare earth elements, which allow manufacturers to achieve the greater volts-per-micron capability necessary to both create thinner dielectrics and add more dielectric layers to each MLCC to achieve higher capacitance values.
Similarly, although the industrial and hi-rel automotive markets often still utilize 0805 to 1210 MLCCs, current iterations of these devices have much higher capacitance values than any of those sizes could offer even a decade ago: typically in the range of 0.1 to 10µF with a rating of 16V to 100V.
Larger styles and values continue to dominate in other high-reliability markets as well, due to both power (V/I) demands and capacitance values that frequently extend into the µF range. Although, some communications and processing technologies are now moving towards the 0402 sizes and specific low inductance capacitors, such as interdigitated capacitors (IDCs), which are offered in 0306 and 0508 chip sizes.
Ceramic Capacitor Failure Types and Recent Trends
In the 1990s, MLCCs made a notable shift from leaded to surface mount technology, and thermal cracking and mechanical flexure defects became the dominant failure modes.
However, when wave soldering, which was often responsible for thermal damage to the devices, began to be replaced by reflow soldering, which uses a gentler temperature profile, in the early 2000s, thermal cracking reduced considerably, and mechanical cracking became the dominant failure mode.
From 2005 to 2010, mechanical cracks were the most common type of MLCC defect. Often so small that they’re invisible to the naked eye, mechanical cracks can result in a range of physical defects that will inevitably cause an electrical failure at some stage in the product lifecycle.
Crack-related failure modes normally cause low insulation resistance within the device, and this can lead to a short circuit, which can cause the PCB to heat up, damage components, and even become a fire hazard if there’s enough energy in the circuit to create thermal runaway in the device (see Figure 3).
An estimated 70% to 80% of all MLCC failures reported by customers during this five-year span resulted from these types of cracks. As such, many customers and PCB assemblers conducted extensive investigations into root cause analysis and subsequently made several substantial process improvements to reduce mechanical stresses, and especially flexure-related stress.
Ceramic Capacitors With Flexible Terminations
These efforts weren’t able to fully eliminate mechanical stresses, though, and — during this same period — PCBs, capacitors, and other components were rapidly diminishing in both size and thickness to suit smaller device applications, which made solving the flexure problem even more critical. In response, several MLCC manufacturers began reviewing new design and material options that would allow them to create a more robust capacitor better equipped to survive the high-flexure stresses that continued to be encountered in a variety of applications and industries.
One solution, proposed, developed, and tested by AVX, was to add a flexibility-enhancing termination layer to the ends of the capacitor. The primary termination for standard MLCCs is a layer of copper material that’s electroplated with a thin layer of nickel (typically measuring 1 – 3µm) and finished with a layer of 100% tin plating (typically measuring 4 – 6µm). To achieve MLCCs with more flexible terminations, AVX added an extra layer of proprietary termination material (FLEXITERM) around the copper/base termination.
Especially designed to compensate for the stresses encountered when a PCB flexed beyond normal allowable limits — which, according to the AECQ200 flexure test pictured in Figure 4, are up to 2mm standard component flexure over a 90mm span of PCB — FLEXITERM material (Figure 5) exhibits conductive properties and flexible buffer stress-relief capabilities, enabling both excellent electrical contact with the copper undercoat and enhanced component flexibility to effectively prevent failures resulting from mechanical and thermal PCB flexure.
Data generated when comparing standard X7R MLCCs to X7R MLCCs with FLEXITERM-coated terminations shows that the latter increases the mechanical flexibility of a typical X7R part by two to three times, allowing for at least 5mm deflection, instead of the standard 2mm, before there’s any discernable change in performance (Figure 6). This data also shows that MLCCs with FLEXITERM layer technology increase thermal cycling performance by a factor of at least three, enabling up to 3,000 thermal cycles to be achieved across temperature spanning -55°C to +125°C (Figure 7).
Enhanced Reliability Testing
Since FLEXITERM capacitors were especially developed to deliver the more flexible and more robust MLCC solutions widely needed throughout the electronics industry, a number of the major automotive manufacturers, including several in Germany, began evaluating and qualifying the components using large sample sizes and elaborate test sequences purposely designed to overstress them well beyond any normal AEC-Q200 test requirements.
The testing program described in Figure 8 was designed by German automotive manufacturer, and involved two separate 5mm bend tests, 1,000 thermal cycles, and 1,000-hour temperature humidity bias (THB) tests in a specific sequence.
At each key stage in these extremely harsh test sequences, engineers measured electrical parameters — including insulation resistance, capacitance, and dissipation factor — for 0603, 0805, 1206, 1210, 1812, and 2220 chip size FLEXITERM X7R MLCCs (see Figures 9 and 10), and then cross sectioned samples to conduct a destructive physical analysis (DPA) designed to identify any evidence of internal ceramic cracking, but the FLEXITERM parts proved exceedingly stable throughout (see Figure 11).
As several independent and in-house tests have now proven, when used according to specifications, the enhanced performance capabilities of these flexible-termination MLCCs essentially eliminate low insulation and short circuit failures due to mechanical flexure. Consequently, FLEXITERM capacitors were embraced by the American and European automotive customer base and their tier-one and tier-two suppliers shortly after their market release. Since 2010, they have also experienced tremendous growth (40% year-on-year) in the Asian markets, and especially China and Korea, due to the regions’ steadily increasing local demand for cars and subsequent focus on high-quality automobile manufacturing.
Additionally, many industrial applications now specify FLEXITERM capacitors in their designs, especially in the 25V to 100V ranges where there’s a higher V/I power risk to the product, for example: battery charger and docking stations, power supplies operating in the 12 – 48V range, and solar energy converters operating at around 48V.
High-Reliability Qualifications for Space
The most recent endorsements for FLEXITERM technology are evidenced by qualifications and design-ins achieved within the European and American aerospace industries. FLEXITERM layer technology was adopted as a standard feature for MLCCs intended for use by the European Space Agency (ESA) in 2015 and the National Aeronautics and Space Administration (NASA) in 2016, and these devices are now fully approved on the qualified parts listings (QPLs) for both agencies.
As part of the testing for the ESA and NASA QPL approvals, FLEXITERM MLCCs were subjected to rigorous long-term reliability testing, including up to 4,000 hours of life testing at 125°C and two times rated voltage, in addition to 10,000-hour life testing evaluations. Like those before it, these tests also proved the parts to be flexible, robust, stable, and reliable, and, as a result, FLEXITERM MLCCs are currently designed into some of the Orion space program equipment, launch vehicle power supplies, space docking camera guidance systems, and various satellite projects.
As FLEXITERM technology continued to be proven through vigorous testing conducted across a variety of markets, these extremely robust ceramic capacitors began to be employed in applications in which mechanical stresses on specific areas of the PCB layout were difficult to eliminate, such as along the edges where connectors are typically located, to proactively mitigate the risk that parts could crack and fail. Safety-critical applications, such as automotive airbags, saw a complete design switch to FLEXITERM MLCCs in recent years, and many of the new battery management systems (BMS) also design the product in with higher voltage ceramic capacitors 450 – 2,000V.
Automakers were especially concerned about the direct 12V battery connection in cars, due to the fact that some previous capacitor failures had caused fires, and had subsequently mandated a design restriction that required designers to use two capacitors in a series, placed at right angles, wherever there was a direct connection between the battery and ground in order to prevent circuit malfunction even if one of the capacitors failed due to mechanical stress.
In response to these concerns and the resultant two-part mandate, which both added costs and reduced available board space, AVX developed a new MLCC solution: a single discrete FLEXITERM capacitor with an internal structure consisting of two series capacitors (See Figures 13A and B). This device, the FLEXISAFE MLCC, effectively replaces the two discrete devices and provides all of the benefits of the FLEXITERM layer for mechanical robustness. As with the FLEXITERM product, several tier-one automotive suppliers requested additional testing beyond the AEC-Q200 test regimes, including 2,000 thermal cycles followed by life and THB testing, and even 9,000-hour life and THB testing (see Figure 14).
These tests proved that FLEXISAFE MLCCs were just as flexible, robust, and reliable as the FLEXITERM devices had proven to be. Consequently, nearly all of the major automakers in Europe and America, and, more recently, even those in the Chinese and Korean markets, have begun implementing FLEXISAFE capacitors in their designs.
Electrical Performance Safety Enhancements
One of the primary advantages of the FLEXISAFE design, especially with respect to electrical overstress and any risks associated with voltage spikes, is that, thanks to the two internal capacitors in series, each individual part is rated for 100V, but, in theory at least, is actually capable of 200V, which all but eliminates the risk of device failure due to overvoltage spikes. Since each internal capacitor (C1 and C2) is also designed for 100V, FLEXISAFE MLCCs are a significant achievement in the journey to achieve the ultimate zero-risk capacitor; as, even if one capacitor in the design fails, the other is still capable of handling 100V.
Originally designed to solve mechanical flexure challenges in automotive applications, FLEXITERM technology has proven especially beneficial for critical designs in which near-zero risk of failure is difficult to achieve due to PCB layout and/or mechanical stresses encountered during the assembly process and use.
The long-term reliability and robustness of both FLEXITERM and FLEXISAFE parts have been rigorously tested using many independent quality systems and technical evaluations, and have recently received space approvals for use in both ESA and NASA aerospace applications.
Due to its exceptional reliability with regard to thermal cycling and mechanical stress, FLEXITERM technology has also been adopted in several less critical applications throughout the electronics industry, and continues to proliferate, especially in power and energy applications.