Explore test solutions for COB packaging

The optical color electrical parameter test of COB device brings some special requirements to its laboratory test and production line test because of its high power, high price and various types of shapes. The author exchanges some experiences here for communication.

About the COB laboratory test program

Due to the high power of the COB, the junction temperature changes rapidly after power-on during the test. Different test methods and conditions have a great influence on the test results. The length of the test, the heat dissipation of the test fixture, etc. directly affect the test results. Under the same test conditions, the change in junction temperature can change the LED light flux, color temperature, light efficiency and other important indicators by more than 8%. Therefore, the junction temperature problem is a problem that must be considered in the COB test process. Most laboratory tests now use a heat sink to achieve thermal equilibrium and start testing. The scheme is simple and convenient to use, and the disadvantage is that it is time consuming. When testing similar products, if the thermal resistance of the device has a large deviation, the test error will be caused. The optional test solution provided by Star Spectrum is to test the optical color parameters under the set junction temperature conditions. The advantage of this scheme is that no specific temperature control fixture is needed, the test speed is fast, the test is consistent with the batch sample, and the test repeatability is good. The disadvantage is the need for specialized equipment (such as Starpower Optoelectronics' SSP8810-S series).

The increase in the configuration of the test fixtures is also inconvenient for the test, and the different fixtures will bring the deviation of the light color parameter test. The solution is generally to calibrate each fixture.

About the COB production line test plan

The following questions are inconvenient for COB's production line testing. First, there are no uniform specifications and standards for various specifications such as the shape of COB products, and most of them are set according to their own needs. Therefore, there are many kinds of COBs on the market, and the output of individual varieties is difficult to be large. This is detrimental to the development of test equipment for fully automated COB. Second, the cost per unit of COB is relatively high. The process of production according to the order requires a high yield. Therefore, the step of replenishing the powder in the production process is called necessary, and because of the variety of COB specifications, the test link needs to be personalized. service. Third, the method of installing the COB on the heat sink and testing when the temperature reaches equilibrium is too long to meet the production line test production requirements.

At present, there are many kinds of COB production line testing methods, which are generally tailor-made fixture fixtures to meet the COB test loading requirements of different specifications. The test methods mostly use transient testing, fixed delay testing, etc. The results of the test methods sometimes differ greatly, and there may be cases where the requirements for direct comparison are not met. The production line test equipment not only requires accurate testing of COB related performance indicators but also meets certain process and timeliness requirements, and needs to be perfectly connected with the existing production process of the production enterprise.

The root cause of the deviation of the COB test results due to different test methods is that the junction temperature is different during the test. Some companies have also designed different test methods according to their own understanding. For example, Philips has commissioned COBCO's custom COB and its module production line test equipment to consider the junction temperature factor. The requirements for comprehensive testing of COB light, color, heat and electricity at a specific substrate temperature are proposed. The advantages of this test method are obvious. The junction temperature during the test is close to the junction temperature in actual use, which makes the test result and the actual use state closer.

For manufacturers of LED application products, they are very concerned about the junction temperature data of LED devices after lighting under normal ambient temperature conditions. Because it can provide an intuitive basis for the reliability and life assessment of the product, it can also provide a basis for the product to evaluate the rationality of the thermal structure design. Uniformly recognized methods and standards for accelerated aging testing are still in the research stage, and the junction temperature of the product under normal operating conditions can reflect whether the product is safe to work, which is one of the main bases for life assessment. In terms of COB junction temperature test, the voltage method commonly used in the world has been used for COB junction temperature measurement. COB is regarded as a PN junction, and its integrated junction temperature is tested by voltage method, and a good effect is obtained.