Research On Low Temperature And Low Stress Malic Acid Copper Plating Solution As Accelerator

At present, along with the further finer and thinner of the copper interconnection on printed circuit board, it is very difficult to achieve the high adhesion strength between electroless copper film and substrate by surface etching process to enhance the surface roughness of the substrate in the past. A high surface roughness of the substrate leads to a dramatic attenuation for the high-frequency signal which decreases mechanical performance of PCB. In order to assure the adhesive strength at the same time reduce the attenuation for the high-frequency signal, a low stress electroless copper plating bath became very essential.In this paper, To meet the requirement of PCB copper interconnection, the effects additives on the the electroless copper plating were investigated, a low temperature and low stress electroless copper plating bath was achieved using Rochelle Salt as the complexing agent and formaldehyde as a reducing agent, The formation mechanism of various additions in the electroless copper were investigated by electrochemistry method. Finally, a low temperature, high speed and stabilizing electroless copper plating solution was obtained.The content of this thesis is as follows:The deposition rate of electroless copper was influenced by a great many factors. First of all, the basic composition of electroless copper plating solution was follow: copper sulfate (CuSO4·5H2O,10g·L-1), sodium potassium tartrate tetrahydrate (C4O6H4KNa-4H2O,28.25g-L-1), formaldehyde (HCHO,5mL·L-1). The operating temperature was33℃and the pH of bath was12.5.The electroless copper bath is usual easy to be decomposed without a stabilizer addition, but the addition of the stabilizer should lead to a significant decrease of the deposition rate. In order to improve the stability of electroless copper solution at the same time maintain a high deposition rate at low bath temperature, a accelerator should not only accelerate electroless copper deposition, but also have a better synergistic effect with stabilizer to inhibit a significant decrease of the deposition rate. In this work, the deposition reaction in the electrolyte was inhibited by an addition of2,2’-dipyridyl, but was accelerated by an addition of L-malic acid. The deposition rate of electroless copper solution was increased from3.55to5.96μm·h-1with alone addition2mg·L-1 L-malic acid. To decrease deposited copper film stress, NiSO4·6H2O as stress-relieving agent was added to the electroless copper solution. When the L-malic acid (2mg-L-1) added in electroless copper plating solution, the deposition rate achieve Maximum (7.16μm·h-1) with NiSO4·6H2O concentration from5to20mg-L"1. But, when L-malic acid concentration was2mg-L"1and NiSO4-6H2O was5mg-L-1, the deposition rate of electroless copper solution decreased from7.16to3.78μm·h-1with an addition of2.0mg-L-12,2’-dipyridyl, which indicated a better synergistic effect between L-malic acid, NiSO4·6H2O and2,2’-dipyridyl on the deposition rate. At last, the quality of Cu film were improved better by an addition of SDBS. And, the concentration of the additives in the Rochelle salt bath was obtained.The effects of additives on the polarization behaviors of the electroless copper plating bath were investigated by the linear sweep voltammetry method and mixed potential theory. The microstructure and surface morphology of samples were characterized by scanning electron microscope (SEM). The internal stresses of copper films were measured by XRD analysis method using the (311) peaks. The internal stresses of deposited copper film on the (311) crystal planes was significantly decreased from-76.90to-13.73MPa with an increased of NiSO4-6H2O from5to20mg-L-1, and the adhesion strength between the ABS substrate and electroless copper film was also increased from0.94to1.22kN-m-1. Finally, a low temperature and low stress electroless copper plating bath was obtained.In this dissertation, a newly-designed etching treatment system was used to etching FR-4substrate before the electroless copper. When the smooth FR-4epoxy resin laminates were treated by an alkaline potassium permanganate solution at70℃for5min, the highest adhesion strength were obtained, the adhesion strength between the FR-4substrate and electroless copper film was also increased from0.3to0.72kN-m-1. Simultaneously, it was found that the stress of deposited copper film on the FR-4substrat was reduced with adding nickel sulfate.