Fabriaction Of LED Headlight Source Modules Based On Chip-Scale Package (CSP)

CSP LEDs have many advantages,such as small size,high injected current,good heat dispersion and high luminous intensity.With all these qualities,CSP LEDs have a wide market in the field of automotive lighting.Especially for Chinese vehicle industry.LED headlamps have good energy consumption.With the small package size,CSP LED can be regarded as point source,which is friendly to optical design in headlamps.Studying CSP LED headlamps is significant to reduce the cost and improve the quality.In this paper,we first introduce the structure and the manufacturing processes of CSP LEDs.Then we manage to employ coating process to manufacture CSP LEDs with 5 lightemitting surfaces,whose correlated color temperature(CCT)is about 6000 K and the color rendering index is greater than 75.After the manufacturing processes are done,some tests of their performance have been run.The max luminous flux is 393 lm at 1500 mA.The luminous efficacy is 153 lm/W at 100 mA.Next,considering during the tests of CSP LEDs,the test results of CCT are inaccurate due to the interference of the neighboring LEDs,which will deteriorate the accuracy of sorting later.That is because during the test,the light emitted from the LED being tested enters the fluorescent glue of neighboring LEDs,which leads to more blue light get absorbed and turn into light with longer wavelength,causing the tested CCT lower than the actual value.There are two solution for this problem.One is increasing the gap between the neighboring CSP LEDs,so that the light may not enter the fluorescent glue of neighboring LEDs.Another is employing random forests algorithm to correct the test results,and this method is what we use in this paper.The accuracy rate of the correction is 95.2% when the error-tolerant rate is ±5%,and the accuracy rate is 88.6% when error-tolerant rate decreases to ±4%.At last,we package the light source for vehicle illumination.During the package process,the CSP LED and PCB substrate are often badly soldered,and the reason for this is the thermal expansivity of fluorescent glue is greater than that of LED chips.To solve this problem,the bonding pad on the substrate should be designed as big as the size of blue CSP LED chips.In addition,the dosage of solder paste should be carefully controlled to avoid it heap up under the fluorescent glue.Comparing to traditional chip-on-board(COB)LEDs,CSP LEDs have better performance on heat dispersion and higher luminous intensity.At 1200 mA,the luminous efficacy of CSP LEDs is 5% more than COB and the surface temperature of CSP LEDs is 16% less than COB.So,CSP LEDs are better light sources for vehicle headlight.