Aluminum-contact nickel activation for electroless Ni–P plating on copper spheres: process and protective performance
ZHOU Shengming;WEI Wei;LIAO Wanda;HAN Zhenbin;XING Guobiao;ZHANG Xingkai;[Objective] To address the issues of substrate etching and high cost associated with conventional palladium activation, an aluminum-contact nickel activation followed by electroless Ni–P plating process was proposed, aiming to prepare protective Ni–P alloy coatings with excellent performance on copper spheres of different sizes(submicron, micron, and millimeter). [Method] A Ni activation layer was firstly deposited on the surfaces of submicron-, micron-, and millimeter-sized copper spheres via aluminum-contact nickel activation, followed by the deposition of continuous and compact Ni–P alloy coatings through electroless plating. The surface and cross-sectional morphologies, chemical composition, and phase structure of the coatings were studied using scanning electron microscopy(SEM), energydispersive spectroscopy(EDS), X-ray diffraction(XRD), and focused ion beam scanning electron microscopy(FIB–SEM). The oxidation resistance and corrosion resistance of the copper spheres before and after Ni–P alloy deposition were studied through thermogravimetric analysis(TG) and inductively coupled plasma optical emission spectroscopy(ICP–OES). [Result] Complete, compact, and well-bonded amorphous Ni–P alloy coatings were obtained on copper spheres of all scales. The coatings exhibited a typical nodular structure, with uniform distribution of Ni and P elements, forming a distinct core–shell structure. [Conclusion] The aluminum-contact nickel activation effectively avoids corrosion of copper substrate while significantly reducing activation costs. The prepared Ni–P alloy coatings demonstrate excellent oxidation and corrosion resistance. This study provides a new process route for surface protection of copper spheres across multiple scales.
Copper electroplating of high-aspect-ratio through-silicon vias: suppressor selection and process window
LIU Cui;LIU Leyan;CHEN Shanyi;HONG Jiekai;ZHAO Xiang;JIANG Yuxin;LUO Jiye;[Objective] To address the issue of void defects in the electroplating filling of through-silicon vias(TSVs) with high-aspect-ratio, efficient suppressors suitable for copper interconnect processes were screened. [Method] The inhibition capabilities of four suppressors, namely polyethylene glycol 20000(PEG 20000), ethylene oxide/propylene oxide block copolymers(PE6400 and 17R4), and polyoxyethylene–polyoxypropylene monobutyl ether(50 HB-260), on copper electrodeposition were compared by cyclic voltammetry and verified through TSV filling experiments. The effects of current density and convection intensity on TSV filling effectiveness were studied to determine a suitable process window. [Result] PEG 20000 achieved void-free and seam-free TSV filling within a concentration range of 100 to 200 mg/L. It exhibited good adaptability to convection intensity, enabling defect-free filling under stirring rates no less than 1 L/min. When electroplating was performed using a multi-step current density sequence of “0.1 A/dm2 → 0.2 A/dm2 → 0.3 A/dm2 → 0.4 A/dm2”, the adsorption distribution of the suppressor in TSV was effectively controlled, achieving bottom-up superfilling. [Conclusion] PEG 20000 is an efficient and stable suppressor with a wide process window for TSV electroplating.
Effect of palladium activation on Cu/Ni interfacial voids and solderability of organic packaging substrate
LU Ran;WANG Guohui;DENG Zhicheng;ZHONG Weijie;KE Haibo;[Objective] To study the formation mechanism of Cu/Ni interfacial voids during electroless nickel/palladium/gold plating(ENEPIG) process and the effect of palladium activation on solderability of thin-nickel ENEPIG boards. [Method] The effects of two activation systems, i.e. palladium chloride and palladium sulfate, on Cu/Ni interfacial voids were compared under different palladium ion concentrations and activation time. Transmission electron microscopy(TEM) was employed to analyze the microstructure and elemental distribution of Cu/Ni interfacial voids. The crosssectional morphology and elemental composition of soldered samples were characterized using three-dimensional X-ray microscopy, scanning electron microscopy(SEM), and energy-dispersive spectroscopy(EDS). The relationship between interfacial voids and soldering voids was explored in combination with the mechanism of gas evolution during soldering. [Result] For both activation systems, the number and size of interfacial voids increased with the increasing palladium ion concentration and activation time. However, the palladium chloride system produced more and larger Cu/Ni interfacial voids than the palladium sulfate system, with void lengths exceeding 1 μm. TEM analysis revealed that voids generated by the palladium chloride system has a localized electroless nickel layer approximately 20 nm thick, leading to void sealing and residual moisture. After soldering, obvious soldering voids were observed inside the solder joints of boards treated with the palladium chloride system, primarily located above the intermetallic compound(IMC) layer, while a large number of Cu/Ni interfacial voids existed beneath the IMC layer. No significant soldering voids were found in boards treated with the palladium sulfate system. [Conclusion] Owing to the high activity of palladium and the permeability of chloride ions, the palladium chloride system is prone to forming severe Cu/Ni interfacial voids during the activation stage. Residual moisture trapped in these voids expands rapidly during soldering, which is a key factor leading to soldering voids in thin-nickel ENEPIG boards. The use of a palladium sulfate activation system can effectively reduce the extent of interfacial voids and improve soldering reliability. This study provides a theoretical basis for the selection and process optimization of activation systems in thin-nickel ENEPIG manufacturing for organic packaging substrates.
High-phosphorus electroless nickel plating for aluminum alloy microwave enclosure
PAN Liang;DAI Yong;XIA Hongliang;GUI Zhenbing;[Objective] To address the issues of poor adhesion, insufficient corrosion resistance, and low bath stability of electroless nickel plating on aluminum alloy microwave enclosure, it is urgent to develop a high-phosphorus electroless nickel plating process suitable for mass production. [Method] Using 5A06 aluminum alloy as the substrate, a process was proposed involving zinc immersion followed by alkaline electroless nickel plating, then high-phosphorus electroless nickel plating, and finally sealing treatment with the home-developed ZY-30 sealing agent. The main process flow and key control points were described, and improvements were made to both the electroless plating and sealing processes. [Result] The obtained nickel coating had a phosphorus mass fraction of approximately 12%, exhibited a semi-bright silver-gray appearance, and demonstrated good adhesion. After sealing with ZY-30, the coating passed a 96-hour neutral salt spray(NSS) test without affecting the adhesion of subsequently applied paint films. The electroless nickel plating bath remained stable and did not decompose after five consecutive production cycles, with a product qualification rate exceeding 98%. [Conclusion] This process is stable and reliable, offering both excellent coating performance and bath stability, and has been successfully applied in actual production.
Pretreatment process for electroless bright nickel plating on 6061 aluminum alloy
LI Junjie;WANG Shichun;QIN Weiheng;HU Guanghui;LUO Jiye;HAO Zhifeng;REN Changyou;[Objective] To develop a secondary zinc dipping pretreatment formulation for electroless bright nickel plating on 6061 aluminum alloy, optimize the process parameters of secondary zinc dipping and zinc stripping, and obtain a uniform, thin, and compact zinc displacement layer as a favorable substrate for subsequent electroless bright nickel plating. [Method] The effects of FeCl3 concentration and surfactant type and concentration in zinc dipping solution, first zinc dipping time, zinc stripping time, and secondary zinc dipping time on the morphology, elemental composition, and surface roughness of the zinc immersion layer were studied using scanning electron microscopy(SEM), energy-dispersive spectroscopy(EDS), and laser confocal scanning microscopy(LCSM). The microstructure, glossiness, and adhesion of the electrolessly plated Ni–P alloy coating were characterized. [Result] The zinc dipping formulation was optimized as follows: zinc oxide 6 g/L, sodium hydroxide 60 g/L, potassium sodium tartrate 15 g/L, ferric chloride 1 g/L, and polyethylene glycol 6000(PEG 6000) 500 mg/L. The optimized pretreatment process parameters were as follows: first zinc dipping for 40 s, zinc stripping for 3 s, and second zinc dipping for 20 s. With this process, a uniform and compact zinc dipping layer was obtained on the 6061 aluminum alloy surface. The subsequent Ni–P alloy coating obtained after 1 hour of electroless plating exhibited a mirror-bright appearance(glossiness of 502 GU), uniform thickness(average value of 13.5 μm, coefficient of variation of 1.22%), adhesion rated as class 0, and a compact and defect-free microstructure. [Conclusion] Appropriate pretreatment of aluminum alloy facilitates the subsequent electroless plating to obtain Ni–P alloy coatings with excellent performance. This study provides a reliable pretreatment scheme for electroless nickel plating on aluminum alloys.
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