Combined Pulse Laser Drilling Of Alumina Ceramic

Alumina ceramic has stable physical,stable chemical and excellent electrical properties,and it has become the main material of circuit substrate for electronic device in high-power LED lighting,5G communication,high-power semiconductor device,solar panel,automobile and aerospace.With the maturity and popularization of industrial laser technology,the laser drilling technology has become the primary choice for the micro-hole drilling of alumina ceramic circuit substrate.However,alumina ceramic has a low absorptivity for the laser in a large range of wavelength,and the drilling of alumina ceramic shows obvious thermal effects which makes the micro-hole accompanied by micro defect.Therefore,the assistance of the laser absorber and high-pressure gas jet has become an indispensable technical method for the conventional millisecond laser drilling,which limits the laser drilling speed of alumina ceramic.In this thesis,the novel combined pulse laser(CPL)drilling method is applied to drill alumina ceramic,the theory and technology of the CPL drilling of alumina ceramic are studied,and the laser drilling of alumina ceramic without the assistance of the laser absorber and high-pressure gas jet is realized.The following research results have been obtained:(1)The theoretical model of the CPL drilling of alumina ceramic is established,in which the CPL is composed of a nanosecond laser(ns)and a millisecond laser(ms).Firstly.the influence mechanism of the absorptivity of alumina ceramic is analyzed.The simulation results show that,when the ns-ms CPL is adopted for the drilling,using the characteristic that the absorptivity of a 1 mm thick alumina ceramic for 1064 nm laser is less than 0.05,the multi-reflection of the inner wall of the keyhole that ablated by the nanosecond laser can enhance the absorption of the millisecond laser by about 4 times,so no conventional laser absorber is required for the drilling.On the other hand,the theoretical analysis of the material removal rate(MRR)of alumina ceramic drilling is carried out.The theoretical calculations show that,when the ns/ms CPL is adopted for the drilling,the heating effect,recoil pressure and plasma shock wave generated by the ns laser with a peak power density of 2.68×10~9 W/cm~2 can not only enhance the absorption of alumina ceramic,but also introduce a ejecting speed higher than 200 mm/ms,so that the drilling of alumina ceramic can be achieved without the assistance of the laser absorber and high-pressure gas jet.(2)An experimental system of the CPL drilling is built.The CPL is spatially composed of a ns laser and a ms laser,and there is no laser absorber and high-pressure gas jet during the CPL drilling.The ns laser with a peak power up to 235 k W is generated by the end-pumped nanosecond solid-state laser,and the ms laser with a pulse energy up to 4 J is gated by a continuous wave(CW)fiber laser.The results of the ns-ms CPL drilling show that the ns pulse sequence with a pulse number of 150 and a peak power density of2.68×10~9 W/cm~2 can ablate a keyhole with a depth of 21.5?m,which makes the absorption of the ms laser reach saturation.Also,the laser fluence required for the drilling is reduced to 57 J/cm~2,which is two orders of magnitude lower than the conventional ms laser.On the other hand,the results of the ns/ms CPL drilling show that when a 1.5 ms,0.4 J ms laser is combined with a 17 ns,4 m J ns laser for drilling alumina ceramic,a through hole with a upper diameter,lower diameter and depth of 222?m,184?m and 1 mm can be drilled by only using a laser energy of 1.692 J.The MRR of the drilling is as high as 1.9×10~7?m~3/J,to the best of our knowledge,it is higher than all the reported MRR of alumina ceramic drilling.(3)The parameters of the CPL drilling are optimized,and the spatial matching relationship of the CPL drilling is analyzed.Firstly,the microscopic features of the hole are systematically analyzed from the aspects of the hole diameter,cracks,spatter deposition,recast layer and energy spectrum composition.The results show that,due to the improved laser absorption and melt ejection,the ns/ms CPL drilling results shows a bigger diameter,less recast layer and less massive oxygen vacancies.But the high temperature gradient introduced by the ns laser causes large cracks.In this context,the matching relationship between the two laser spots is further optimized,and a better drilling performance is obtained by the ns/ms CPL in which the ns laser and the ms laser have the compound radius of about 50?m.On the other hand,it is found that the off-axis incident ns laser has an elliptical focused beam,but the CPL drilling can still obtain holes with high circularity.Then,through experimental and theoretical analysis,it is confirmed that the quality of ns/ms CPL drilling can be independent of the spot shape of nanosecond laser.The self-homogenization effect of the laser optimizes the drilling quality.(4)The high-speed CPL drilling of alumina ceramic is explored.Firstly,the ns/ms CPL drilling experiment system based on the optimized parameters is used to perform high-speed drilling.Without the assistance of the laser absorber and high-pressure gas jet,a0.5 ms,0.15 J ms laser combined with a 17 ns,4 m J ns laser can be used to achieve a drilling speed of 10 holes/second.The low equipment cost,simple drilling process,and high drilling speed reflect the high performance price ratio of the ns/ms CPL drilling system,so it is more suitable for the industry application.Then,the high-speed drilling of alumina ceramic by the CPL based on a Nd:YAG amplifier structure is also studied.The results show that the single pulse energy of the ns laser can be increased to 50 m J.The CPL with a total energy of 1.62 J can drill a hole with a diameter of about 160?m in 1 ms,and the corresponding drilling speed was as high as 70 holes per second.This high-speed CPL drilling further confirms the effectiveness of the CPL drilling and also reflects the great potential of the CPL in alumina ceramic drilling applications.In summary,this thesis proposes novel CPL drillings method for alumina ceramic,solves the dependence on the laser absorber and high-pressure gas jet,analyzes the drilling mechanism and technology research of the CPL drilling,and explored the high-speed drilling of alumina ceramic.It can provide a reference for the high-speed drilling of alumina ceramic,which shows great potential in the industrial applications.