Nanosecond multi-pulse laser milling for certain area removal of metal coating on plastics surface
Introduction
In the fields of aerospace, energy, biomedical and MEMS, the precision parts that are obtained by manufacturing of metal coating with functional pattern on engineering plastics surface have gained increasing attention recently; the typical precision parts include microstrip antenna [1], [2], micro-power sources [3], miniature sensor [4], etc. With ever-escalating expectation of the performance for engineering applications, this kind of precision part needs higher geometric accuracy and edge quality of the functional patterns. Moreover, the engineering plastics substrate presents three-dimensional (3D) structure, so as to meet the performance requirements or the limitation of installation space. Through various studies by many scholars, this kind of precision parts can be fabricated through adding or removing certain area of metal coating on engineering plastics surface.
When using the method that adds metal patterns onto the engineering plastics substrate, such as 3D printing or laser Direct-Write [5], [6], the manufacturing of parts with a 3D structure can be realized. However, after printing or deposition, the whole parts need to be annealed at 150–550 °C to make the metal layer cured, as annealed temperature is higher than the thermal deformation temperature for most kinds of engineering plastics—the applications of this method are limited. Another method makes the surface of plastic substrate coated with a metal coating first, and then removes a certain area of metal coating and leaving functional pattern on the substrate surface. The common machining method for certain area removal of metal coating on the engineering plastics surface is photolithography [7], [8]. Through this method, the functional pattern of metal coating on planar substrate can be manufactured precisely. However, due to a series of steps like mask alignment, exposure and development, it is difficult for photolithography to complete the manufacturing of metal coating with functional pattern on the 3D substrate.
Because of the above processing method, it is not suitable to fabricate metal coating with a functional pattern on the engineering plastic surface when the substrate has a low thermal deformation temperature or a 3D structure—an advanced manufacturing technology is urgently needed. At present, the laser-processing technology shows desirable superiority compared with the conventional machining method, such as high flexibility, speed, resolution and precision—especially capable of 3D parts processing and manufacturing [9], [10]. Therefore, precision fabrication of metal coating with functional pattern on the surface of the 3D substrate could be promisingly achieved by the laser-processing technology.
In this study, the machining method for certain area removal of metal coating on the engineering plastics surface is studied based on the nanosecond multi-pulse laser milling. The manufacturing steps of the metal coating with functional pattern are as follows: First, thin metal coating is uniformly plated on the surface of the engineering plastics substrate; then the metal coating outside the functional pattern is completely removed by the laser-processing technology, and the damage of the engineering plastics substrate should be avoided as far as possible; finally, the functional pattern of metal coating with a high accuracy could be obtained and there are no burrs and warps of the pattern edge.
In order to remove certain area of metal coating on an engineering plastics surface via laser machining technology, the choice of an appropriate laser source and the corresponding machining parameters are directly relevant to the machining quality of functional pattern, machining cost and operation efficiency. Nowadays, the nanosecond multi-pulse laser technology is more widely used in industrial application with its sophisticated development—for instance, fabrication of the fiber grating to realize optical switching [11], [12], hard disk texturing, silicon wafer marking, IC repair, cutting of thin Al-alloy sheet and lubricant groove processing [13], [14], [15], [16], [17]. For the existence of narrow strip-line in the functional pattern and the short pulse laser, this technology exhibits the advantage of a high accuracy in micro-machining. The nanosecond multi-pulse laser can be chosen for laser machining of metal coating with functional pattern based on the optimal processing parameters.
To meet the requirements of industrial production, nanosecond multi-pulse laser processing technology for various materials and the optimization of processing parameters are also studied by numerous academics and research institutions [18], [19], [20]. Most studies have been done mainly on homogenous target materials such as metals, ceramics and polymers [21], [22], [23], [24]. However, there are few reports about the study of nanosecond multi-pulse laser technology aiming at thin metal coating on engineering plastics with low melting point like metal cladding engineering plastics. Meanwhile, many researches about laser processing technology focus on the effect of single pulse laser [25], [26] or laser drilling and punching that laser spot does not move continuously [27], [28]. Although these researches are very significant, they cannot be used for the functional pattern of metal coating machining directly by nanosecond multi-pulse laser milling.
As the metal coating with functional pattern on the engineering plastics surface can be applied under various extreme conditions, the edge quality of the functional pattern should be superior, the residual copper layer and damage of substrate should be avoided, so as to guarantee better electrical and mechanical performances of the parts. In this paper, the machining method for certain area removal of metal coating on the engineering plastics surface is studied based on the nanosecond multi-pulse laser milling. In order to improve the edge quality of the functional pattern, the generation mechanism and the corresponding avoidance strategy of the processing defects are studied. Additionally, a prediction model for the laser ablation depth is proposed, which can effectively avoid the existence of a residual metal layer and reduces the damage of the substrate. The machining parameters of nanosecond multi-pulse laser milling mainly include laser fluence F0, pulse repetition frequency f and feeding speed of laser spot v. Subsequently, the optimizing selection for machining parameters is carried out. As the laser spot is very small compared to the parts, the machining results on the 3D structure are basically as same as the planar one; therefore, a planar copper-clad polyimide (CCPI) is chosen as the raw material to carry out the experiments. At last, with the optimal machining parameters, a typical functional pattern sample on CCPI is machined based on the laser milling.
Section snippets
Mathematical modeling for the processing defects and laser ablation depth
Metal coatings with equiangular spiral pattern with two arms on the 3D structure are widely used for a signal receiver or a sensor. The manufacturing process of the parts is shown in Fig. 1. It can be seen from the picture that the curvatures of the pattern change continuously; therefore, the machining parameters should be chosen appropriately, so as to guarantee the machining accuracy meeting the requirement for different curvatures. To meet the requirements of industrial applications, the
Experimental material and equipment
The experimental material in this study is a copper plating layer with a thickness of 10 μm on the polyimide (PI) substrate, as shown in Fig. 4. The preparation process of the experimental material is that: First, the surface of the PI substrate is covered with the thin copper coating by magnetron sputtering,and the thickness is 1 μm, the operate temperature on the surface of substrate is below 50 °C, then the copper coating is thickening to 10 μm on the basis of electroplating technology.
To ensure
Optimization analysis of machining parameters
In order to meet the manufacturing requirements of the functional pattern, the machining parameters need to be analyzed to optimize the machining results. First, in order to guarantee the machining quality of the pattern edge, the appropriate displacement of two adjacent laser pulses should be determined, so the connection between the laser repetition frequency and the feeding speed of laser spot is defined. Then the laser fluence can be obtained based on the prediction model of ablation depth.
Machining results of equiangular spiral pattern on the CCPI
In order to verify the study, a metal coating with an equiangular spiral pattern is chosen to be machined. The model of the equiangular spiral pattern with two arms is shown in Fig. 19. The width of the spiral pattern increases constantly from the center. There are four edges for the equiangular spiral pattern, which correspond four equiangular spiral lines. The equations of the four equiangular spiral lines are as follows:
In which, r0 is the
Conclusion
The nanosecond multi-pulse laser milling technology is presented for making an attempt to fabricate metal coating with functional pattern on an engineering plastics surface with high quality for the edge of pattern and minimal damage for the substrate. Further research can explore the generation mechanism and avoidance strategy of the processing defects induced by geometry characteristic of the functional pattern. Moreover, a thermodynamics model is established to predict the ablation depth of
Acknowledgements
The project is supported by the National Basic Research Program of China (no. 2014CB046503), Science Fund for Creative Research Groups (no. 51321004) and National Natural Science Foundation of China (no. 51205041). The authors wish to thank the anonymous reviewers for their comments which led to improvements of this paper.
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