Advanced packaging surface metallization research
Yang Yanzhang, Zhong Shangbiao, Chen Zhihua.
Guanghua Academy of Science and Technology (Guangdong)**
Summary. Advanced packaging is an important part of the future development of the semiconductor industry, and it is a key technology that surpasses Moore's Law. In this paper, it is found that the roughness and coating stress have significant effects on the adhesion of the coating by metallizing the surface of different packaging materials. Selecting the appropriate roughening method and low-stress electroplating copper plating solution can improve the adhesion of the coating (peel strength) without significantly increasing the surface roughness of the encapsulation material53 nm), which facilitates the production of fine lines (line width and spacing = 15 m 15 m).
0 Introduction. Advanced packaging, including PLP, SOC, SIP and other packages, is a new high-tech packaging technology that conforms to the development trend of the semiconductor industry towards smaller size and higher performance [1]-[4]. Advanced packaging surface metallization can realize the functions of electromagnetic shielding, heat dissipation, and electrical conductivity of the package, further reduce the size of the package device, and improve the performance of the package device [5]-[7]. At present, the surface metallization of advanced packaging has problems such as high roughness and low adhesion, and it is difficult to make fine circuits [8]-[10]. In order to solve this problem, this paper successfully realizes the coating with low roughness and high adhesion by optimizing the surface roughening technology of the packaging material and using the low-stress electroplating copper plating solution, and completes the fabrication of fine circuits.
1 Roughness.
Roughness is a parameter that characterizes the surface topography of a material (as shown in Figure 1), and its magnitude has a significant impact on the adhesion of the coating [11]. Generally speaking, the larger the roughness, the more conducive to the increase of the adhesion of the coating, so an important means to improve the adhesion of the coating is to increase the roughness of the contact surface. However, too much roughness is not conducive to the production of fine lines [12] .
2 Plating stress.
Stress exists in a wide range of materials and has an important impact on the mechanical and chemical properties of materials [13]. The coating stress of the electroplating layer can affect the hardness and cracking of the plating, for example, the higher the stress of the plating, the worse the mechanical properties of the plating. There are many factors that affect the stress of the plating layer, such as the plating solution formulation, plating parameters, etc. [14] .
3 Protocols.
3.1 Principle.
As shown in Figure 2, the surface of the encapsulation material is first roughened, then the surface is coated with a seed layer of metallic copper (<1 m) using electroless plating, and finally the thickness of the coating is increased by electroplating copper (>10 m).
As shown in Figure 3, the resin area on the surface of the encapsulation material EMC-1 is bitten by roughening method A to increase the surface roughness, then the surface is plated with a seed layer of copper metal, and finally the thickness of the coating is increased by electroless copper plating.
As shown in Figure 4, the coating area on the surface of the packaging material EMC-2 is bitten by roughening method B to increase the surface roughness, then the surface is plated with a seed layer of copper metal, and finally the thickness of the coating is increased by electroplating copper.
3.2. Test materials and testing equipment.
The packaging materials used in this article are epoxy molding compounds (EMC), which account for more than 90% of the entire electronic packaging materials. There are two types of EMC materials, and the difference is mainly reflected in the different sieving particle sizes of the fillers - EMC-1 and EMC-2 are 50 m and 20 m respectively. The test equipment includes a laser confocal microscope, an electron scanning microscope, a peel strength tester (as shown in Figure 5), and a stress tester (as shown in Figure 6).
4 Experimental results and analysis.
SEM** on the front and rear surfaces of EMC-1 and EMC-2 coarsening are shown in Figure 7. From the results in the figure, it can be seen that the surface morphology of the roughened EMC material becomes rougher than that before coarsening: (1) the size of the micro-pits in the resin area of the roughened EMC-1 surface is significantly increased(2) Clear occlusion cracks appeared in the roughened EMC-2 surface packing area.
To further analyze the EMC surface roughness before and after roughening, we used laser confocal microscopy to characterize the EMC surface roughness, and the results are listed in Table 1. As can be seen from Table 1, the surface roughness of EMC-1 coarsening increases significantly compared with that before coarsening, while the surface roughness of EMC-2 coarsening does not increase significantly compared with that before coarsening. This is consistent with the characterization results in Figure 7.
The interface structure of the EMC surface after copper plating is shown in Figure 8. As can be seen from the figure, the EMC-1 coating has a large undulation at the interface, which is due to the large roughness of the surface roughness of the coarsened EMC-1 (consistent with Figure 7D-F and Table 1). The EMC-2 coating interface is flatter than the EMC-1 coating interface without significant fluctuations (consistent with Figure 7J-L and Table 1), making it easier to fabricate fine traces.
We use peel strength to characterize the adhesion between the coating and EMC. As can be seen from Table 2, the peel strength of the surface coating of the EMC material after surface roughening treatment is significantly increased compared with the EMC material without surface roughening treatment, which indicates that the EMC surface roughness plays an important role in the adhesion of the coating. In addition, different electroplating solutions have different peel strengths: under the same pretreatment conditions, the adhesion of the plating solution 2 is better than that of the plating solution 1. This is due to the lower coating stress of solution 2 (as shown in Table 3), so the adhesion between the obtained coating and the substrate is higher.
Based on the results of the previous experiments, we use the SAP process to fabricate fine traces on the surface of EMC-2. As shown in Figure 9, the SAP process was used to successfully fabricate a fine line with a line width of 15 m and a line spacing of 15 m on the surface of EMC-2, and the line did not fall off, which indicates that the metallization process can meet the manufacturing requirements of the fine line.
5 Conclusion. Epoxy molding compound is a commonly used encapsulation material for advanced packaging. In this paper, the effects of pretreatment and electroplating on the surface coating adhesion of two EMC packaging materials with different filler particle sizes are studied, and it is found that increasing the surface roughness and decreasing the coating stress of the electroplating copper layer can effectively improve the adhesion of the coating: the maximum peel strength can reach 092 n/mm。Choosing an EMC material with a smaller filler size can achieve 0. at a low surface roughness (SZ<18 m).58 N mm coating adhesion, and the SAP process is used to produce fine lines with a line width = 15 m and 15 m. These experimental results provide a solution to meet the higher requirements of metallization of advanced packaging in the future, and also provide a technical reference for the dielectric-metal interconnection process.