Surface mountable over-current protecting apparatus

Details Surface mountable over-current protecting apparatus Surface mountable over-current protecting apparatus

2002-03-19

2003-12-16

Sterrett, Jeffrey (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

Circuit interruption by thermal sensing

Reexamination Certificate

active

06665164

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an over-current protecting apparatus, and particularly to an over-current protecting apparatus which can enhance solderability.
2. Description of Related Art
Due to extensive application of portable electronic products such as cellular phones, notebook, hand-carried camera and PDA, it is more and more important for these electronic products to have an over-current or over-temperature protecting functions.
A Positive Temperature Coefficient (PTC) over-current protecting apparatus is often seen in electronic products. Since the PTC over-current protecting apparatus has advantages of reusableness, sensitivity to temperature and stable reliability, it has been extensively used to protect batteries, especially to secondary batteries, such as a nickel-hydrogen battery or a lithium battery.
The PTC over-current protecting apparatus utilizes a PTC material as a current-sensing element, and the resistance of the PTC material is sensitive to temperature variation. In normal operation, the resistance of the PTC material is at a low resistance value to ensure normal circuit operation. When an over-current and over-temperature situation occurs, the resistance of the PTC material will raise immediately over ten thousand times of the initial value to a high impedance state. Thus, the over current is suppressed and limited to a safe level. The purpose of protecting circuit elements and batteries is achieved.
FIG. 1
shows a prior art PTC sheet
10
, whose inner structure is disclosed in U.S. patent application. Ser. No. 09/542,283, now U.S. Pat. No. 6,377,467 entitled “Surface mountable over-current protecting device” and filed by the inventors of the present invention. In a cross-sectional view, the PTC sheet
10
has a PTC material
11
, an upper electrode
13
and a lower electrode
14
covering the PTC material
11
, a first metal termination
15
electrically connected to the upper electrode
13
, a second metal termination
16
electrically connected to the lower electrode
14
, a solder mask
18
between the first metal termination
15
and the second metal termination
16
, and insulating layers for isolating the upper electrode
13
and the second metal termination
16
and isolating the lower electrode
14
and the first metal termination
15
. As shown in a top view, the PTC sheet
10
has a plurality of electrically conductive holes
12
, and a conductive material is deposited on each electrically conductive hole. In the stage of manufacturing finished products, a dicing saw is used to cut the PTC sheet
10
along the center of the electrically conductive holes
12
(called half-circle manufacturing method). The components after cutting are separated and packaged as shown in FIG.
2
.
Due to the trend of minimizing electronic products, a typical dimension of a PTC over-current protecting component is gradually decreased according to the following order: 1812, 1210, 1206, 0805, 0603, 0402; where the first 2 digits and the last 2 digits respectively represent the length and the width of the component in unit of hundredth inch. For example, the form factor 1812 means that the component is 0.18 inches in length and 0.12 inches in width. As the component form factor gets smaller, so does the diameter of the hole. Under the dimension of 0603, the thickness of the dicing saw is getting close to the diameter of the electrically conductive holes
12
. Since the hole is so small, there is less room and less tolerance for dicing saw to cut through the hole. Just little off-center cutting could form obvious asymmetry geometry which results in one of the electrically conductive holes of PTC over-current protecting components will have too small surface area on the side wall. Consequently, poor solderability of the PTC over-current protecting component is observed when mounting the PTC over-current protecting component on a PCB. Poor solderability also means poor bonding of PTC component to the PCB. Therefore, any slight vibration or impact will separate the PTC over-current protecting component from the PCB, and causes a reliability problem.
SUMMARY OF THE INVENTION
A main object of the present invention is to propose an over-current protecting apparatus which can raise solderability. By improving solderability, the over-current protecting apparatus of the present invention can be stably adhered to a PCB and the reliability can also be improved.
Another object of the present invention is to propose an over-current protecting apparatus with a simple manufacturing flow. The present apparatus is suitable to a half-circle or full-circle surface mount manufacturing method with no need of increasing footprint on the PCB.
For achieving the above objects and avoiding prior art disadvantages, the present invention proposes an over-current protecting apparatus comprising at least one PTC over-current protecting component and a body. The PTC over-current protecting component has a PTC material, electrodes covering the PTC material and metal terminations electrically connected to the electrodes. The body has an insulating layer and a first conductive and second conductive regions covering the insulating layer. One end of the first conductive and second conductive regions is electrically connected to the metal termination, and another end of the first conductive and second conductive regions is mounted to a PCB. The metal material occupies over 20% area of the sidewall of the first conductive and second conductive regions for increasing solderability. Besides, another body can be adhered to the top of the PTC over-current protecting component to form a symmetrical components.
According to another embodiment of the present invention, the above body is replaced by a known leadframe to obtain the effect of increasing solderability.


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