Electrical device

An electrical ceramic capacitor, in particular a multilayer ceramic capacitor, comprising a ceramic body including a stack of parallel metallic layers of opposing polarity separated by a dielectric material arranged in an active zone of the ceramic body enclosed between outer surfaces, wherein at one or more surfaces a shock-absorbing region is arranged.

TECHNICAL FIELD

The present invention relates to an electrical device and, in particular, to an electrical ceramic capacitor.

BACKGROUND

Ceramic capacitors are well known in the art. Multilayer ceramic capacitors (“MLCC”) are frequently used as surface mounted devices (“SMD”) because of their high package density. Typically, ceramic capacitors are manufactured by assembling a stack of green compact plates and conducting metal layers alternatingly, followed by sintering the stack to a compact sintered body. A first set of metal layers are electrically contacted by a first end terminal covering one side wall of the sintered body, and a second set of the metal layers are electrically contacted by a second end terminal covering another side wall, for instance, a side wall opposing the first side wall. The end terminals are intended as electrodes having different electrical polarities. The metal layers are arranged in an interdigital structure so that a metal layer contacting the first end terminal is facing a metal layer contacting the second end terminal, the metal layers being spaced apart by the dielectric ceramic plates. The metal layers spaced apart by the ceramic plates form an active zone of the capacitor with a well-defined capacitance.

However, capacitors made of ceramics are prone to cracks caused by mechanical shocks, and thermal shocks as well, which may destroy the device or alter the electrical properties of the capacitor. Often, cracks emerge at the outer surfaces and propagate to the active zone of the device. Thermal shock cracks and cracks due to flexing of a printed circuit board (“PCB”) to which the capacitor is mounted are two of the extrinsic defects commonly occurring in multilayer ceramic capacitors. In addition, mechanically induced multiple cracks may occur in multilayer ceramic capacitors. These types of cracks have the same defect mechanisms in common. Firstly, mechanical stresses or forces are coupling through the ductile metal layers, and secondly, cracks occur in the underneath brittle ceramic capacitor layers. When these cracks propagate and cut through the active zone of the capacitor they may lead to electrical failures of the device.

In U.S. Pat. No. 8,576,537 a ceramic capacitor is described, where crack mitigation void patterns are provided close to the surface of the ceramic body. The crack mitigation void pattern is intended to channel emerging cracks, which originate from the surface and propagate into the ceramics body in a safe zone outside the active capacitor zone inside the ceramic body.

The present invention is directed toward overcoming one or more of the above-mentioned problems.

SUMMARY

It is an object of the present invention to provide a ceramic capacitor where crack propagation is strongly reduced or minimized.

At least this object is achieved by the features of the independent claim(s). The other claims, the drawings and the specification disclose advantageous embodiments of the present invention.

An electrical ceramic capacitor is proposed, in particular, a multilayer ceramic capacitor, comprising a ceramic body including a stack of parallel conducting layers of opposing polarity separated by a dielectric material arranged in an active zone of the ceramic body enclosed between outer surfaces, wherein at one or more surfaces a shock-absorbing region is arranged.

Advantageously, the top and/or bottom regions of the ceramic capacitor serve as a mechanical shock or stress absorber to stop crack propagation into the ceramic layers in the active zone of the capacitor body. In case mechanical stress is coupled through the end termination metal layer, a crack would be started at the interface of metal and these top and/or bottom regions and the mechanical stress would be absorbed and diminished in these regions. The shock-absorbing region is configured to crack easily, thus stopping crack propagation into the ceramic body.

Favorably, current ceramic capacitor manufacturing processes can be easily adapted, with only minor changes in the formulation of the ceramic green parts or ceramic green tapes before the ceramic body is sintered to form a monolithic capacitor body.

According to an advantageous embodiment, the shock-absorbing region may comprise one or more shock-absorbing layers. In particular, the one or more shock-absorbing layers may be are arranged as top surface layers. Favorably, at one or more surfaces a stack of at least two shock-absorbing layers may be arranged. Favorably, the shock-absorbing layer can be manufactured having a thickness in the range between 2 μm to 15 μm, preferably 5 μm to 10 μm, for example.

According to an advantageous embodiment, the shock-absorbing region is configured to comprise a higher porosity than the active zone. Preferably, the shock-absorbing layers of the shock-absorbing region are configured to comprise a higher porosity than the dielectric layers in the active zone. For instance, a higher porosity may be generated by a higher concentration of an organic binder agent in the formulation of green part or green tapes before sintering the material. Favorably, the shock-absorbing region may be manufactured from a green part comprising a higher concentration of organic binder agent than the active zone dielectric ceramic materials below the shock-absorbing region or between the shock-absorbing regions. Preferably, the shock-absorbing layers are manufactured from a green tape comprising a higher concentration of organic binder agent than the dielectric layers in the active zone below the shock-absorbing region or between the shock-absorbing regions comprising the shock-absorbing layers. In particular, the concentration of organic binder agent in a green part or green tapes forming the shock-absorbing region may be at least 2 wt. %, and preferably at least 3 wt. %, greater than the concentration of organic binder agent in a green part or green tapes forming the active zone.

According to an advantageous embodiment, the shock-absorbing region may be configured to be more brittle than the active zone, which may comprise dielectric ceramic materials. For instance, an increase in brittleness may be achieved by adding glass powder in the formulation of ceramic green part or green tapes before sintering the material. Favorably, the shock-absorbing region or shock-absorbing layers may be manufactured from a green part or a green tape comprising a higher concentration of glass powder than the active zone or the dielectric layers in active zone below the shock-absorbing region or between the shock-absorbing regions. In particular, the concentration of glass powder in a ceramic green part or green tape forming the shock-absorbing region or shock-absorbing layers may be in a range of 5 wt. % to 55 wt. %, and preferably at least 10 wt. % to 50%. For instance, the glass powder may contain at least one of iron oxide, chromium, manganese oxide and cobalt. Adding glass powder to the ceramic green part or ceramic green tape is only a minor change to the formulation of the ceramic green part or ceramic green tape and does not overly alter the manufacturing process.

According to an advantageous embodiment, the shock-absorbing region or shock-absorbing layers may be configured to comprise a distinct color. This may be achieved by adding colorized glass powder. Green glass powder contains iron oxide and chromium. Black glass powder contains manganese dioxide. Blue glass powder contains cobalt. Beneficially, by using colorized glass powder to the ceramic green parts or ceramic green tapes forming the shock-absorbing region or shock-absorbing layers, the top and bottom surface of the ceramic capacitor may be easily distinguishable from the front and back surfaces, in particular, when the top and bottom surfaces are manufactured with different colors than the front and back surfaces. In automated processes where the capacitors are mounted to a substrate by pick and place machines or the like, automated optical identification is possible via optical means such as, for example, a camera. Placement of the capacitor with the shock-absorbing layers in correct orientation can be ensured, thus protecting the capacitor from external thermal mechanical shocks and/or stress induced by printed circuit board (“PCB”) flexing.

It is of advantage to combine an increase in porosity and increase of brittleness for forming the top and bottom shock-absorbing regions in the capacitor. The quality and shock tolerance of the resulting capacitor can be increased further.

According to an advantageous embodiment, the shock-absorbing regions, which are preferred at the top or the bottom of the capacitor, may have a thickness in the range of 5 μm to 10 μm, for example.

The present invention allows for providing high quality capacitors for small packages, for instance, for 0201 packages (inch code, i.e., 0603 in metric code) or even smaller. The inventive capacitors are immune to the failure modes described previously, virtually without increasing manufacturing costs. DC leakage current and other failures (due to cracks) can be reduced resulting in reduced rework and scrap costs, and increased reliability.

Further features, aspects, objects, advantages, and possible applications of the present invention will become apparent from a study of the exemplary embodiments and examples described below, in combination with the Figures, and the appended claims.

DETAILED DESCRIPTION

In the drawings, like elements are referred to with equal reference numerals. The drawings are merely schematic representations, not intended to portray specific parameters of the present invention. Moreover, the drawings are intended to depict only typical embodiments of the present invention and therefore should not be considered as limiting the scope of the present invention.

FIG. 1depicts an embodiment of the present invention in a cut view of a multilayer ceramic capacitor10mounted as an SMD capacitor to a substrate60such as a printed circuit board. The bottom side of the capacitor10is attached with its metallic end terminals20,30to conducting pads62by solder64. The end terminals20,30cover side walls of the capacitor10and grip around the edges of the ceramic body18of the capacitor10, thus covering edge regions on the top side12and bottom side14.

The capacitor10comprises a body18having a top shock-absorbing layer52and a bottom shock-absorbing layer56forming shock-absorbing regions50at the top side12and the bottom side14. An active zone16is arranged between the top and bottom shock-absorbing regions50separated from the shock-absorbing regions50by dielectric42at the top and at the bottom of the ceramic body18. The active zone16is composed, as is well-known in the art, by a stack of alternating metal layers22,32connected to the first end terminal20and second end terminal30, respectively. The metal layers22,32serve as electrodes of the capacitor10. Each metal layer22connected to the first end terminal20is followed by a metal layer32connected to the second end terminal30in stack direction70and separated from each other by a dielectric40. Only a few of the metal layers22,32and the dielectric40are denoted with reference numerals for clarity reasons.

The dielectric42between the shock-absorbing regions50and the active zone16may be the same material as for the dielectric40.

The shock-absorbing layers52,56of the shock-absorbing regions50are configured to be prone to cracks. The layers52,56may be prepared with a high porosity or with a high brittleness, or with a high porosity and a high brittleness. These layers52,56serve as mechanical shock or stress absorbers and crack stoppers.

In case mechanical stress is coupled through the metal layers of the end terminals20,30, a crack would start at the interface between metal and the shock-absorbed in the top or bottom shock-absorbing region50, i.e., in the top or bottom shock-absorbing layers52,56, and then propagate into these top or bottom layers52,56. As a result, the mechanical stress is relaxed and diminished, and the crack is contained or stopped at these top or bottom layers52,56.

The current ceramic capacitor manufacturing process known in the art can be maintained with only minor changes in the formulation for the top and bottom layers52,56, such as higher concentration of binder agent for a more porous material, or adding of glass powder for increasing brittleness, thus making the layers52,56porous and brittle and, hence easy to crack.

The shock-absorbing layers52,56of the shock-absorbing region50can be manufactured in a range of thickness between, for example, 5 μm to 10 μm. Such shock-absorbing regions50can easily be added to a ceramic capacitor design having a ceramic layer thickness of 50 μm on both sides of the active zone16of the capacitor10. This allows for application of the inventive shock-absorbing regions50in highly miniaturized capacitors10such as so called 0805 type SMD capacitors, having EIA-standard dimensions of a length of 0.079±0.006 inches and a width of 0.050±0.006 inches.

FIG. 2illustrates an embodiment where two shock-absorbing layers52,54and56,58are arranged at the top and the bottom side12,14each of the capacitor body18. The bottom side14of the capacitor10may be attached with its metallic end terminals20,30to a printed circuit board (not shown), such as displayed inFIG. 1. The end terminals20,30cover side walls of the capacitor10and grip around the edges of the ceramic body18of the capacitor10, thus covering edge regions on the top side12and bottom side14.

The capacitor10comprises a body18having two top layers52,54and two bottom layers56,58forming shock-absorbing regions50at the top side12and the bottom side14, each of the layers52,54and56,58being spaced from each other by a dielectric44.

An active zone16is arranged between the top and bottom shock-absorbing regions50and separated from the shock-absorbing regions50by dielectric42at the top and at the bottom of the ceramic body18. The active zone16is composed, as is well-known in the art, by a stack of alternating metal layers22,32connected to the first end terminal20and second end terminal30, respectively. The metal layers22,32serve as electrodes of the capacitor10. Each metal layer22connected to the first end terminal20is followed by a metal layer32connected to the second end terminal30in stack direction70and separated from each other by a dielectric40. Only a few of the metal layers22,32and the dielectric40are denoted with reference numerals for clarity reasons.

The dielectric44, as well as the dielectric42between the shock-absorbing regions50and the active zone16, may be the same material as for the dielectric40.

The shock-absorbing layers52,54and56,58of the top and bottom shock-absorbing regions50are configured to be prone to cracks. The layers52,54and56,58may be prepared with a high porosity or with a high brittleness, or with a high porosity and a high brittleness. These layers52,54and56,58serve as mechanical shock or stress absorbers and crack stoppers.

In case mechanical stress is coupled through the metal layers of the end terminals20,30, a crack would start at the interface between metal and the shock-absorbed in the shock-absorbing region50, i.e., in the top or bottom shock-absorbing layers52,56, and then propagate into these top or bottom layers52,56. The additional layers54,58, sandwiched between the outer shock-absorbing layers52,56and active zone16inside the ceramic body18, provide additional protection for the capacitor in case a crack may nevertheless propagate beyond the outer shock-absorbing layers52,56.

As a result, the mechanical stress is relaxed and diminished, and the crack is contained or stopped at these top or bottom layers52,54and56,58.

The current ceramic capacitor manufacturing process known in the art can be maintained with only minor changes in the formulation for the top and bottom layers52,54and56,58, such as higher concentration of binder agent for a more porous material, or adding of glass powder for increasing brittleness, thus making the layers52,56porous and brittle and, hence easy to crack.

The shock-absorbing layers52,54and56,58of the shock-absorbing region50can be manufactured in a range of thickness between, for example, 5 μm to 10 μm. Such shock-absorbing regions50can easily be added to ceramic capacitor design having a ceramic layer thickness of 50 μm on both sides of the active zone16of the capacitor10. This allows for application of the inventive shock-absorbing regions50in highly miniaturized capacitors10such as so called 0805 type SMD capacitors, having EIA-standard dimensions of a length of 0.079±0.006 inches and a width of 0.050±0.006 inches.

In the embodiments depictedFIGS. 1 and 2, the capacitor10is shown with a body18having rectangular edges. It should be understood that in other embodiments the edges of body18may be rounded instead of rectangular.

FIG. 3illustrates an assembly of a multilayer ceramic capacitor before sintering the assembly to form a monolithic body to which end terminals are attached at side walls. In general, manufacturing processes can be used which are well known in the art in accordance with standard practice for MLCC manufacture. By way of example, basic manufacturing steps are elaborated in U.S. Pat. No. 8,576,537. However, other standard manufacturing methods can be used as well.

The reference numerals refer to the components of the capacitor10inFIG. 1, although the ceramic components are green parts or green tapes before a sinter step.

Along a stacking direction70, a shock-absorbing layer56(green tape) is covered with a dielectric layer42(green tape) on top of which a metal layer32is placed. A green tape of dielectric40is mounted on top of metal layer32, followed by metal layer22. Metal layer32is intended to connect to an end terminal (not shown) at one side wall of the capacitor body after sintering (at the left side in the Figure). Metal layer22is intended to connect to an end terminal (not shown) at an opposing side wall of the capacitor body after sintering (at the right side in the Figure). Both metal layers22,32overlap except for a region at the outer edges of the layers22,32. On top of metal layer22a dielectric layer42is arranged and topped by a green tape shock-absorbing layer52.

The metal layer22, dielectric40and metal layer32serve as active zone in the sintered capacitor. In the Figure, only two metal layers22,32spaced by dielectric40are shown. It is to be understood, that a multitude of such metal layers22,32and dielectric40can be provided depending on a desired capacitance of the capacitor.

These shock-absorbing layers52,56are preferably a dielectric ceramic having a higher content of organic binder agent in order to achieve a porous layer after the sintering step. Alternatively or additionally, a glass powder can be added to achieve a brittle material after the sintering step.

Dielectric42may be the same material as dielectric40. Metal layers22,32may comprise nickel alloy, silver, platinum and the like.

Besides the higher content of binder agent and/or additional glass powder, the composition of the base layers52,56may be the same as for the dielectric40,42. Favorably, the content of organic binder agent in the base layers52,56is at least 2 wt. %, and preferably at least 3 wt. %, higher than for the dielectric layers40,42. For glass powder, 10 wt. % to 50 wt. % of glass powder, such as regular soda-lime-silica glass is preferred.

Advantageously, the glass powder can be colorized which allows an easier alignment of the capacitor as well as an automated optical detection of top and bottom side of the capacitor.