Research On Low Temperature And Low Stress Electroless Copper Plating Solution Of 2,6-diaminopyridine As Accelerator

In recent years, new technology is required to have finer and thinner PCB substrates for the high integrated electronic devices. With the further finer and thinner of the PCB substrates and copper interconnection, a high surface roughness of the substrate leads to a dramatic attenuation for the high-frequency signal, while low adhesive strength is obained when surface roughness decreases. 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. At present, commercial low stress electroless copper plating solution is produced only by some companies abroad at present, such as the Dow chemistry, Atotech, Ebara and Uyemura. Low stress electroless copper plating solution has not been developed by domestic companies, so the cost of production is increased dramatically and PCB industry in our country is under restrictions.To obtain a low stress electroless copper plating sulotion, the low temperature electroless plating bath using the Rochelle salts as the complexing agent and formaldehyde as the reductant was investigated. First, some additives were studied and2,6-diaminopyridine as accelerator,2,2’-dipyridyl as stabilizer, sodium dodecyl sulfate (SDS) as surfactant and NiSO4·6H2O as stress-relief agent were chosed for the further research. Then, the effects of the additives on the deposition rate, deposited copper microstructure and surface morphology were investigated by deposition rate measurement and surface SEM observation. Though the deposition rate of electroless copper solution was accelerated by2,6-diaminopyridine addition (7.16μm·h-1) and was deceased by2,2’-dipyridyl addition (1.95μm·h-1), it reached3.45μm·h-1(the deposition rate of basic electroless copper solution was3.65μm·h-1) at33℃with the composite addition of the four additives, and the deposited copper film also became uniform and smooth. The analysis of copper film performance indicated that it had the tendency of crystal nuclear growth to the Cu (111) crystal plane by the addition of the additives and the copper film performance was improved. 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 and the role of the additives in the electrolyte was explained by the electrochemical analysis. The stresses of copper films were measured by XRD analysis method using the Cu (220) and Cu (311) peaks. It was found that, the internal stresses of deposited copper film were decreased with an addition of2,2’-dipyridyl, and the internal stresses was low when adding1.5-2.0mg·L-12,2’-dipyridyl to the electrolyte. The internal stresses of deposited copper film were increased with an addition of2,6-diaminopyridine, which were caused by the high deposition rate and introduction of impurities. The deposited copper film with low stress and high deposition rate could be obtained when2,6-diaminopyridine concentration was1.0mg·L-1. The internal stresses of deposited copper film on the Cu (220) and Cu (311) planes were decreased from-164.49and-122.34MPa to-16.50and-31.00MPa with an addition of15.0mg·L-1NiSO4·6H2O, respectively, which indicated that NiSO4·6H2O could relieve the stresses. In the copper deposition process, the addition of NiSO4·6H2O increased the number of the nodules and restrained coalescence of the islands within nodules by disrupting the transport of the Cu atoms and prevent lattice imperfection of Cu films. Consequently, the energy of stress was released significantly by the addition of NiSO4·6H2O with the formation of nodule boundaries and a lower stress was obtained. SDS as surfactant was used to decrease the surface tension of the electroless copper solution and eliminate hydrogen bubbles formed on the surface of electroless Cu film by the reducing agent dehydrogenation reaction. The stresses of copper films were decreased slightly with the addition of SDS.Finally, the low temperature and low stress electroless copper plating bath was obtained and the composition consisted of10g·L-1CuSO4·5H2O,28.25g·L-1C4H2O6KNa,5mL·L-1HCHO (37%),1.0mg·L-12,6-diaminopyridine,1.5mg·L-12,2’-dipyridyl,15.0mg·L-1NiSO4·6H2O, and4.0mg·L-1SDS at pH12.5and33℃. The adhesion strength between the substrate and electroless copper film reached1.12kN·m-1.