Patent Application
20230395342
Embodiments of the present disclosure relate to thermal cut-off (TCO) devices, and, more particularly, to a TCO device suitable for surface mount reflow processes.
Commonly, electrical devices include components that are designed to protect against certain fault conditions, such as an overcurrent condition, overvoltage condition, or excessive temperature. Thermal Cut-Off (TCO) devices, for example, respond to overtemperature conditions that may damage circuitry within the electronic device. TCOs may feature bimetal components and positive temperature coefficient (PTC) devices, for example.
Bimetal components consist of two metallic components having different thermal expansion coefficients that bend when subjected to a change in temperature. When included in an electrical device, the bimetal component activates when the electrical device reaches an abnormal, excessive temperature, such as due to excessive current. The bending of the bimetal component acts as a switch to interrupt the current flow through the electrical device. Once the temperature drops below the excessive temperature, the bimetal component returns to its original shape, allowing current to again flow through the electrical device.
Bimetal components have low resistance at the point of contact, good current carrying capability, and rapid response to changes in temperature. Bimetal components are sometimes combined with PTC components, where the PTC component acts as a heater to improve latching of the bimetal component. The bimetal component is disposed in series with the circuit of the electronic device being protected, while the PTC component is disposed in parallel to the bimetal component. When the bimetal component is activated, the current flowing through it is diverted to the PTC component, causing the PTC component to heat up, where this heat is transmitted to the bimetal component, causing the bimetal component to remain activated.
Existing TCO devices include the Metal Hybrid Protection (MHP) devices manufactured by Littelfuse®. The MHP devices include MHP-TA (thermal activation) devices, such as the MHP-TAM, which provides battery cell protection for high-capacity Lithium Polymer and prismatic cells used in notebook PCs, ultra-book, tablets, and smart phones, MHP-TAT, which are additionally used in gaming PCs, and MHP-TAC devices, which are additionally used in E-cigarette and battery-powered portable devices, due to their relatively smaller size.
These devices are currently used for lead attached applications to provide overcurrent and overtemperature protection for the attached electrical device. However, they are not designed for surface mount applications.
It is with respect to these and other considerations that the present improvements may be useful.
This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended as an aid in determining the scope of the claimed subject matter.
An exemplary embodiment of a thermal cut-off (TCO) device in accordance with the present disclosure may include a base portion having a base mold and a base terminal. The base terminal includes first and second pads. The TCO device also includes a bimetal disc and an arm terminal to electrically connect to the base terminal. The first and second pads of the TCO device are attachable to a substrate using a surface mount reflow process.
Another exemplary embodiment of a TCO device in accordance with the present disclosure may include a cover plate, an integrated terminal, and a base portion. The base portion includes a base mold and a base terminal. The base terminal includes pads suitable for a surface mount reflow process.
Thermal cut-off (TCO) devices suitable for surface mount reflow processes are disclosed. The TCO devices are modeled after existing lead attached TCO device structures, but are improved with compact and miniaturized structures suitable for surface mount reflow operations. The arm and base terminals include pads for surface mount reflow. The base molds are designed for receiving the PTC device, bimetal device, and arm terminal feature and may operate using either a single base terminal or a multi-part base terminal. Multiple cover designs are disclosed to lower the heat capacity of the upper plate of the TCO device relative to the base terminal. A TCO device featuring an integrated arm and bimetal device terminal is also disclosed, with an updated base portion to support the integrated terminal.
The base portion 102 consists of the base mold 104 and the base terminal 106. In an exemplary embodiment, the base terminal 106 and the base mold 104 are molded together as one component, the base portion 102. The bimetal disc 108 is placed in the base portion, and the arm terminal 110 is then molded to the base mold 104. The cover 112 is placed over the assembly and then the second mold 114 encases the components, resulting in the TCO device 100 as shown. In an exemplary embodiment, both the base terminal 106 and the arm terminal 110 are made of copper. In exemplary embodiments, the cover 112, which is not part of the conduction path, is made of stainless steel or copper without isolated material covering, or made of stainless steel or copper with isolated material covering like epoxy coating or plastic molding material.
During normal operation, the TCO device 100 is connected in series to a device to be protected, such as a battery, or other circuitry. Current flows from the base terminal 106, which could be thought of as a first terminal, through the PTC device, if present, to the arm terminal 110, which could be thought of as a second terminal, and to the device being protected, and vice-versa. When an abnormal condition, such as an overtemperature condition, occurs, the bimetal disc 108, which is disposed between the base terminal 106 and the arm terminal 110 (or between the PTC device and the arm terminal) bends or deforms, such as in a convex position. This has the effect of causing the bimetal disc 108 to “lift” itself, such that the connection between the base terminal 106 and the arm terminal 110 is removed, disrupting flow of current to and thus protecting the device or circuitry to which the TCO device 100 is connected.
Existing TCO devices, such as MHP TCO devices of Littelfuse®, are designed for lead attached applications to provide overcurrent and overheating protection for a circuit, circuit components, or a device. By contrast, the exemplary TCO device 100 disclosed herein is particularly designed for surface mount reflow processes. In exemplary embodiments, the TCO device 100 takes the robust design concepts of the existing MHP TCO platforms and update them for compact and miniaturized applications, as well as surface mount reflow.
In contrast to the traditional soldering techniques used to attach components to printed circuit boards (PCBs), reflow soldering is a recent technique that enables mass production and speed by separating the placement of the components from the soldering process. In traditional soldering, the component is positioned on the PCB and soldered. This is followed by a second component being placed and soldered, and so on, until all the components of the PCB are soldered in place. With reflow soldering, the pads of the PCB, which are usually copper, are covered with soldering paste. With miniaturization, the pads may be very small and close together, relative to legacy PCBs. Therefore, a stencil is often used to ensure accurate application of the soldering paste to the pads. The components are then placed, one by one, onto the sticky soldering paste, where the stickiness of the soldering paste ensures that the components stay in place. Finally, the PCB is heated to a temperature that equals the melting temperature of the soldering paste, such as 265° C. Though there are different techniques for heating/melting the soldering paste, one common technique is to place the PCB in a specialized oven for this purpose. The melted soldering paste then “reflows” onto the terminals of the components that have been placed thereon, after which the PCB is allowed to cool. Thus, by using a surface mount reflow process, all components on the PCB are simultaneously assembled.
In
Further, the base portion 102 of
Further, the base portion 102 of
An edge portion 302 of the base terminal 106 is also visible in both the top view 100A and the bottom view 100B of the TCO device. In an exemplary embodiment, the edge portion 302 is inserted through a rectangular opening of one side of the TCO device, such that the edge portion is sticking out and slightly visible. The contact portion 118 of the arm terminal 110 is also shown, for making contact with the connecting portion 208 of the base terminal 106 (
In a legacy TCO device, the terminals of the device establish a connection to external components of the circuit or device to be protected. In contrast to the legacy TCO devices, the TCO device 100 additionally includes pads on each terminal for connections to external components. The pads 304, 306, 308, and 310 thus enable the TCO device 100 to be suitable for surface mount reflow process applications.
Further, the receiving opening 502 may be shaped as a circular or other shaped opening, depending on the shape of the PTC device.
As with the base portion 102 (
In an exemplary embodiment, the base portion 402 further includes a connecting portion 508. This connecting portion 508 is part of the base terminal 406, and is in a raised or higher plane than the portion visible in the receiving opening 502 and is also in a higher plane than the receiving opening 504. Returning to
Returning to
Relative to copper, stainless steel is a poor conductor of electricity, while brass is about 28% as conductive as copper. Thus, in exemplary embodiments, these materials are selected for their strength, moldability, and resistance to rusting, not their conductive properties. In exemplary embodiments, relative to the cover mold 112 of the TCO device 100 (
Further, in an exemplary embodiment, the cover mold 412, and specifically, the cover plate 604, is chosen to have a lower heat capacity than that of the base terminal 406 of the base portion 402. Table 1 shows the heat capacity of four different exemplary cover plates 604, relative to that of the base terminal 406, which is made of copper. The material property, T, stands for thickness.
As Table 1 shows, the specific heat capacity parameter, A, for a given material does not change based on thickness. Thus, the SUS304 cover plate of 0.08 mm and the SUS304 cover plate of 0.01 mm both have a specific heat capacity of 460 J/(kg K) (Joules per kelvin per kilogram). Similarly, the density parameter, B, is unchanged with a change in thickness of materials. The volume parameter, C, is changed with a change in thickness. The heat capacity of the four versions of cover plate 412 given in Table 1 are all lower than the heat capacity of the base terminal 406. Thus, in an exemplary embodiment, for the TCO device 400:
Heat capacity of cover plate<heat capacity of base terminal (1)
In an exemplary embodiment, the cover plate 712 has a special structure that makes it stronger. The cover plate 712 does not include a separate plastic cover, as in the TCO device 400. Instead, in some embodiments, the cover plate 712 is covered with bumps or dimples that are raised slightly from the surface of the cover plate. In an exemplary embodiment, the special structure of the cover plate 712 increases its strength, allows a thinner material to be used, and adds strength to the TCO device 700 that facilitates the surface mount reflow process, described above. The high-temperature reflow process will enable a bigger bimetal disc to be made, relative to prior art TCO devices. The bigger bimetal disc will deform to lift the arm terminal 710 with a higher force, in some embodiments. Thus, having a strong cover plate that is able to withstand the lifting force without any deformation from the arm terminal is preferred. A poorly designed cover plate will be unable to bear the force from the arm terminal, which will deform the TCO device and may cause structural issues, such as cracking of the molding material and loosening of the arm terminal with high resistance. In one embodiment, the SUS304 stainless steel material is used to make the cover plate 712.
Table 2 shows the heat capacity of the cover plate 712, relative to that of the base terminal 706, which is made of copper, according to exemplary embodiments. As before, the heat capacity of the cover plate 712 given in Table 2 is lower than the heat capacity of the base terminal 706. Thus, in an exemplary embodiment, the TCO device 700, like the TCO device 400, satisfies equation (1), above.
In an exemplary embodiment, the base portion 802 is structured for receipt of the integrated terminal 808, which is described in more detail in
Further, similar to the TCO device 100, the base portion 802 of the TCO device 800 includes four pads 1008, 1010, 1012, and 1014. The first portion of the base terminal 806A includes the pads 1010 and 1008 while the second portion of the base terminal 806B includes the pads 1014 and 1012. In an exemplary embodiment, the pads 1008, 1010, 1012, and 1014 facilitate attaching the TCO device 800 to a substrate such as a PCB using the surface mount reflow process.
The TCO device 800 thus integrates the arm terminal and bimetal device into a single structure, simplifying the device. Further, the base terminal 806 is separated into two distinct parts, in contrast to the base terminals of the TCO devices 100, 400, and 700.
As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural elements or steps, unless such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features.
While the present disclosure makes reference to certain embodiments, numerous modifications, alterations and changes to the described embodiments are possible without departing from the sphere and scope of the present disclosure, as defined in the appended claim(s). Accordingly, it is intended that the present disclosure not be limited to the described embodiments, but that it has the full scope defined by the language of the following claims, and equivalents thereof.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CN2020/127258 | 11/6/2020 | WO |
