This application is an application under 35 U.S.C. 111(a) and claims priority under 35 U.S.C. 119 from Taiwanese Patent Application No. 098142003 filed Dec. 9, 2009, the disclosure of which is incorporated herein by reference.
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1. Field of the Invention
The present invention relates to an inspection apparatus and a method for a light emitting diode (LED) package interface.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
A light emitting diode (LED) package process comprises steps of die bonding, wire bonding, encapsulation, and inspection, wherein the die bonding step uses a die bonding material, such as silver paste, eutectic alloy, or thermal conductive adhesive, for adhering LED dies to a package carrier or a substrate. If the thickness of the die bonding material is non-uniform, if voids occur in the die bonding material, or if the material ages during the die bonding step, such conditions will result in variances in characteristics of the die bonding material. With current process technology, the inspection step for sorting unqualified die bonding devices cannot be performed rapidly enough by the general apparatuses of rapid optical and electrical properties inspection. The unqualified die bonding material results in greater thermal resistance and poor heat conduction so that the LED devices may have problems with overheating, early light degradation, or burnout during consumer use.
The current process to evaluate the thermal conduction of LED devices utilizes a thermal resistance measurement apparatus for measuring thermal resistance based on industry standards including JEDEC-51, MIL-STD-883, and CNS 15248. However, the thermal resistance measurement step is too complex and time-consuming to be adopted for real-time quality inspection of LED package devices.
The present invention provides a rapid inspection apparatus and a method for an LED package interface which reduces time spent measuring the thermal conduction of LED devices. Differences in measurable qualities of the package interface, such as die bonding quality, between LED devices can be determined within seconds. The inspection apparatus and method of the present invention can be combined with the general apparatuses of rapid optical and electrical properties inspection for sorting unqualified die bonding devices before the LED devices are shipped from the factory.
According to one exemplary embodiment, the LED package interface inspection apparatus for an LED device includes a current source, a voltage measuring unit, and a testing control unit. The testing control unit provides at least one control signal to command the current source to output at least one current to the LED device, and provides at least two signals to command the voltage measuring unit to measure a first forward voltage of the LED device at a first time and a second forward voltage of the LED device at a second time. The testing control unit calculates a voltage difference between the first forward voltage and the second forward voltage, and determines that the LED device is defective if the voltage difference is larger than a predetermined threshold value.
According to another exemplary embodiment, the LED package interface inspection method for an LED device includes providing at least one current to the LED device, measuring a first forward voltage of the LED device at a first time and measuring a second forward voltage of the LED device at a second time with the at least one current, calculating a voltage difference between the first forward voltage and the second forward voltage, and determining whether the LED device is defective based on whether the voltage difference is larger than a predetermined threshold value.
According to yet another exemplary embodiment, the LED package interface inspection method for a plurality of LED devices includes providing at least one testing current, measuring a first forward voltage of each LED device at a first time and measuring a second forward voltage of each LED device at a second time with the testing current, calculating a voltage difference between the first forward voltage and the second forward voltage of each LED device, and classifying the plurality of LED devices according to the voltage difference of each LED device.
Another exemplary embodiment includes a computer program product for inspecting a package interface of an LED device. The computer program product comprising a computer readable storage medium having computer-readable program instructions embodied in the medium, the computer-readable program instructions comprising first instructions for providing at least one current to the LED device, second instructions for measuring a first forward voltage of the LED device at a first time and measuring a second forward voltage of the LED device at a second time with the at least one current, third instructions for calculating a voltage difference between the first forward voltage and the second forward voltage, and fourth instructions for determining whether the LED device is defective based on whether the voltage difference is larger than a predetermined threshold value.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present invention and, together with the description, serve to explain the principles of the invention.
Exemplary embodiments will now be described more fully with reference to the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this present invention will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art.
The junction temperature of an LED device with unqualified die bonding interface is higher than that of an LED device with qualified die bonding interface when the same rated current flows through both LED devices. Based on the above characteristic, the present invention provides a method for inspecting LED package interfaces instantaneously so as to resolve complex and time-consuming issues associated with sorting the LED devices by measuring thermal resistance of LED devices in a conventional way.
The measurement principle of the present invention is based on the phenomenon of an increase of the LED junction temperature resulting in a decrease in LED forward voltage. When a current flows through an LED device, a pn junction of the LED device luminesces and produces heat. Therefore, the junction temperature of the LED device increases, and the forward voltage of the LED device decreases. As the forward voltage of the LED device decreases, a voltage difference dv (a negative value), i.e., a second forward voltage V2 minus a first forward voltage V1, increases as shown in
During the package process of the LED device, an unqualified die-bonding interface can be sorted out by the above measurement of the forward voltage difference. Moreover, when the LED device is assembled to a circuit board or a heat dissipation metal plate, a high thermal resistance interface formed due to an unqualified assembly interface can be sorted out according to the above measurement of the forward voltage difference.
In order to explain the LED package interface inspection method of the present invention more clearly, an inspection apparatus that performs the method of the present invention is described as follows.
Hereinafter, referring to
In a first embodiment of the present invention as shown in
The testing control unit 24 reads and records the two forward voltages V1 and V2 of the LED device 25 measured by the voltage measuring unit 23, calculates the voltage difference of the voltages V1 and V2, and then determines that the LED device 25 is defective if the voltage difference is larger than a predetermined threshold value.
In a second embodiment of the present invention, the testing control unit 24 provides at least one control signal S1 to command the current source 22 to generate at least one current for the LED device 25, and provides at least two signals S2 and S3 to command the voltage measuring unit 23 to continuously measure forward voltages of the LED device 25 so as to obtain a plurality of forward voltages including voltages V1 and V2 at different times as shown in
Moreover, the testing control unit 24 reads and records the plurality of forward voltages of the LED device 25 measured by the voltage measuring unit 23, and calculates the voltage difference between two forward voltages V1 and V2 of the LED device 25 measured at two predetermined times t1 and t2, wherein the plurality of forward voltages decreases continuously over time, and the value of the forward voltage V1 is larger than that of the forward voltage V2. Subsequently, the testing control unit 24 determines that the LED device 25 is defective if the voltage difference between the forward voltages V1 and V2 is larger than a predetermined threshold value.
The predetermined threshold value is set according to the package structure of the LED device and the interval between two measuring times. Poorer heat conduction of the package structure of the LED device corresponds to larger voltage difference. Also, for the same LED device, a longer interval between two measuring times corresponds to a larger voltage difference.
In the first and second embodiments, the testing current provided by the current source 22 can be a pulse current or a direct current, and the value of the testing current can be set according to the structure of the LED device 25. A rated current value of the LED device 25 with 1 mm2 LED die area, for example, about 250 mA to 350 mA, is generally adopted. Moreover, a rated current value of the LED device 25 with 0.1 mm2 LED die area, for example, about 10 mA to 20 mA, is generally adopted. In addition, a time interval td of the voltage measurement can be set according to the value of the testing current and the types and the structures of the LED device 25, and the general time interval td is between 100 μsec and 1 sec. However, the present invention should not be limited to the first and second embodiments.
In a third embodiment of the present invention as shown in
Subsequently, the testing control unit 24 reads and records the forward voltages V1 and V2 of the LED device 25 measured by the voltage measuring unit 23, and calculates the voltage difference between them. Subsequently, the testing control unit 24 determines that the LED device 25 is defective if the voltage difference is larger than a predetermined threshold value.
In this embodiment, the testing current provided by the current source 22 can be a pulse current with a pulse width about 20 μsec to 100 μsec, and the value of the testing current can be set according to the structure of the LED device 25. A small current value of the LED device 25, for example, between 0.1 mA and 5 mA, is generally adopted. The heating current provided by the current source 22 can be a pulse current or a direct current, and the value of the heating current can be set according to the structure of the LED device 25. A rated current value of the LED device 25 is generally adopted. In addition, the heating interval th can be set according to the value of the testing current and the types and the structures of the LED device 25, and the heating interval th is generally between 100 μsec and 1 sec. Compared to the first embodiment, the testing current in the third embodiment is smaller, and thus errors caused by extra heat, due to large current heat generation, can be avoided.
Similarly, for obtaining a plurality of forward voltages including voltages V1 and V2 at different times in accordance with a fourth embodiment as shown in
The testing control unit 24 reads and records the plurality of forward voltages of the LED device 25 measured by the voltage measuring unit 23, and calculates the voltage difference between two of the forward voltages V1 and V2 of the LED device 25 measured at two predetermined times t1 and t2, wherein the plurality of forward voltages decreases continuously over time, and the value of the forward voltage V1 is larger than that of the forward voltage V2. Subsequently, the testing control unit 24 determines that the LED device 25 is defective if the voltage difference between the forward voltages V1 and V2 is larger than a predetermined threshold value.
In a fifth embodiment of the present invention as shown in
The testing control unit 24 reads and records the forward voltages V1 and V2 of the LED device 25 measured by the voltage measuring unit 23, and calculates the voltage difference between them. Subsequently, the testing control unit 24 determines that the LED device 25 is defective if the voltage difference is larger than a predetermined threshold value.
Similarly, for obtaining a plurality of forward voltages at different times in accordance with a sixth embodiment as shown in
The testing control unit 24 reads and records the plurality of forward voltages of the LED device 25 measured by the voltage measuring unit 23, and calculates the voltage difference between two forward voltages V1 and V2 of the LED device 25 measured at two predetermined times t1 and t2, wherein the plurality of forward voltages decreases continuously over time, and the value of the forward voltage V1 is larger than that of the forward voltage V2. Subsequently, the testing control unit 24 determines that the LED device 25 is defective if the voltage difference between the forward voltages V1 and V2 is larger than a predetermined threshold value.
In addition, a decrease of the LED junction temperature will result in an increase in LED forward voltage. After a heating current flows through an LED device for a period of time, the heating current is stopped, and a testing current, i.e., a pulse current with a short pulse width, is used to continuously measure a plurality of forward voltages of the LED device. In this condition, the forward voltage of the LED device increases rapidly so that a voltage difference dv (a positive value) increases continuously over time. In particular, the LED device with unqualified package interface will exhibit a larger positive voltage difference than the LED device with a qualified package interface.
In a seventh embodiment of the present invention as shown in
The testing control unit 24 reads and records the forward voltages V1 and V2 of the LED device 25 measured by the voltage measuring unit 23, and calculates the voltage difference between them. Subsequently, the testing control unit 24 determines that the LED device 25 is defective if the voltage difference is larger than a predetermined threshold value. Because the LED device 25 is heated by the heating current for the interval th and is measured by the testing current with the short pulse width, the junction temperature of the LED device 25 will decrease after the heating interval th, and the measured voltage difference will be a positive value.
Similarly, for obtaining a plurality of forward voltages at different times in accordance with a eighth embodiment as shown in
The testing control unit 24 reads and records the plurality of forward voltages of the LED device 25 measured by the voltage measuring unit 23, and calculates the voltage difference between two forward voltages V1 and V2 of the LED device 25 measured at two predetermined times t1 and t2, wherein the plurality of forward voltages increase continuously over time, and the value of the forward voltage V1 is smaller than that of the forward voltage V2. Subsequently, the testing control unit 24 determines that the LED device 25 is defective if the voltage difference between the forward voltages V1 and V2 is larger than a predetermined threshold value.
In a ninth embodiment of the present invention as shown in
The testing control unit 24 reads and records the forward voltages V1 and V2 of the LED device 25 measured by the voltage measuring unit 23, and calculates the voltage difference between them. Subsequently, the testing control unit 24 determines that the LED device 25 is defective if the voltage difference is larger than a predetermined threshold value. Because the LED device 25 is heated by the heating current for the interval th and is measured by the testing current with a short pulse width, the junction temperature of the LED device 25 will decrease after the heating interval th, and the measured voltage difference will be a positive value. In this embodiment, the value of the heating current is equal to the rated current value of the LED device 25, and the value of the testing current is between 0.1 mA and 5 mA for reducing the heating effect of a large current. The pulse width of the testing current is between 20 μsec and 100 μsec. However, the present invention should not be limited to the embodiment.
Similarly, for obtaining a plurality of forward voltages at different times in accordance with a tenth embodiment as shown in
The testing control unit 24 reads and records the plurality of forward voltages of the LED device 25 measured by the voltage measuring unit 23 after the time interval th, and calculates the voltage difference between two forward voltages V1 and V2 of the LED device 25 measured at two predetermined times t1 and t2, wherein the plurality of forward voltages increase continuously over time, and the value of the forward voltage V1 is smaller than that of the forward voltage V2. Subsequently, the testing control unit 24 determines that the LED device 25 is defective if the voltage difference between the forward voltages V1 and V2 is larger than a predetermined threshold value.
In the aforementioned embodiments, when the testing control unit 24 provides a control signal to command the current source 22 to generate a testing current for the LED device 25, and provides a signal to command the voltage measuring unit 23 to measure a forward voltage of the LED device 25, a time delay can be set before voltage measuring to reduce voltage error. The time delay is between 5 μsec and 50 μsec. The time interval td of the voltage measurement and the heating interval th can be set according to the amount of the testing current and the types and the structures of the LED device 25, and the general time interval td and interval th are between 100 μsec and 1 sec. However, the present invention should not be limited to the embodiment. The voltage measuring unit 23 of the inspection apparatus 20 is a rapid voltage measuring apparatus with high resolution, and the degree of resolution should be less than 5 mV while the preferable degree of resolution is less than 1 mV. However, the present invention should not be limited to the embodiment. The sampling rate should be greater than 200,000 times per second, and the preferable sampling rate is one million times per second. However, the present invention should not be limited to the embodiment.
The present invention further provides a computer program product having computer readable program instructions for carrying out any of the aforementioned LED package interface inspection methods. The computer program product includes a computer readable storage medium having computer readable program instructions embodied therein, the computer readable program instructions comprising: program instructions configured to provide at least one current to the LED device, program instructions configured to measure a first forward voltage of the LED device at a first time and to measure a second forward voltage of the LED device at a second time with the at least one current, program instructions configured to calculate a voltage difference between the first forward voltage and the second forward voltage, and program instructions configured to determine whether the LED device is defective based on whether the voltage difference is larger than a predetermined threshold value.
When the package interfaces of a plurality of LED device are inspected, the testing control unit 24 can classify the plurality of the LED devices 25 according to a voltage difference of each LED device.
The present invention provides a rapid inspection apparatus and a method for a light emitting diode (LED) package interface. Based on the phenomenon whereby an increase in the LED junction temperature results in a decrease in LED forward voltage and a decrease in the LED junction temperature results in an increase in LED forward voltage, the voltage difference of an LED device can be measured according to differences in time interval of a testing current and a heating current, and according to differences in current value of the testing current and the heating current. Therefore, differences in measurable qualities of the package interface, such as die bonding quality, between LED devices can be determined within seconds. The inspection apparatus and method can be combined with the general apparatuses of rapid optical and electrical properties inspection for sorting unqualified die bonding devices before the LED
The above-described exemplary embodiments are intended to be illustrative of the invention principle only. Those skilled in the art may devise numerous alternative embodiments without departing from the scope of the following claims.
Number | Date | Country | Kind |
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098142003 | Dec 2009 | TW | national |