Research On Materials Of Ni-P Thin-film And Inkjet-printing Ink To Fabricate Embedded Resistor Inside PCB

The intelligence, miniaturization, multifunction, high-reliability and portability developments of electronic products, promoted the wide-application of system integrated technology in PCB. It is generally realized by the ideal of embedding electronic components into PCB. Accordingly, the embedding of components is of great benefits to the decreasing of PCB geometries and weights, and the enhancement of electrical performance reliabilities. Embedded resistor, one of the important embedded component technologies, was studied from its as-fabricated materials of Ni-P thin-film and inkjet-printing ink. It is summarized as follows:(1) The resistive properities of the Ni-P thin-film electrolessly deposited with manganese sulfate monohydrate(denoted as MSM). The addition of MSM in electroless Ni-P plating(ENP) quadratically increased the resistance of as-plated Ni-P thin-film, which was contributed by both the decreasing of Ni-P thin-film thickness and the increasing of Ni-P thin-film electrical resistivity. Accordingly, the effects of MSM on the plating rate of Ni-P thin-film and the as-plated Ni-P thin-film properties, including crystallinity, surface morphology, elemental electronic state and phosphorus content, were investigated to clearify the electrical resistivity increasing. Experimental results demonstrated that thickness decreasing was dedicated by the decreasing of Ni-P plating rate and electrical resistivity increasing was resulted from the interface between the particles with diameters of 20-50 nm inside the Ni-P nodules. The Ni-P density of the interface, as the dominated factor of Ni-P thin-film resistance, was decreased at the addition of MSM. The more addition of MSM resulted in the lower density interface and then higher electrical resistivity for Ni-P thin-film.(2) The stabilization mechanism of MSM on ENP. The effect of MSM on the ENP and its mechanism were studied from the investigations of the decomposition time of ENP solution, plating rate, elemental composition of as-plated Ni-P thin-films on the nickel substrate. The investigation results confirmed that the MSM acted as a stabilizer in the ENP. In addition, the ENP bath p H effects on the ENP plating rate was also studied. It can be found that an increasing in p H decreased the plating rate in the presence of MSM, which is completely converse to the conventional regular that higher p H results in higher deposition rate at without or with other stabilizers. In addition to the results of MSM effects on bath p H in deionized water and the obtained hydrolysis constant, the component to realize stabilizing action was preliminary confirmed to be Mn OH+. The residence of Mn OH+ on nickel surface decreased the number of vacant catalytic sites. On the other hand, the interaction of H in Mn OH+ and O in hypophosphite is preferred, and therefore decreased the possibility of the absorption of H in hypophosphite on the Ni catalytic site. We therefore concluded that such absorption of Mn OH+ caused the difficult absorption of the hypophosphite on Ni catalytic surface, i. e., the increasing of adsorption energy on Ni surface. It then leads to the increase of the reaction barrier for nickel reduction and phosphorous deposition and oxidation. In addition, the addition of MSM resulted in the pores in the Ni-P thin-film at lower concentration, and the nodule structure at higher content, indicating that the decreased Ni-P density of interface between Ni-P particles was found to be formed by as-absorbed Mn OH+ retard the escape of as-produced hydrogen gas in the ENP.(3) The application of Ni-P thin-film deposited with MSM in PCB embedded resistor fabrication. Prior to the practical application, the electrical resistance properties of Ni-P thin-film due to the existence of particle interface must be evaluated through electrical and thermal stability test by means of I-V, TCR and thermal shock tests, respectively. Experiment results showed that the as-fabricated resistor behaved very stable adhesion with PCB substrate without cracks after 288 ℃ thermal shock, stable in resistance with the electrical current loadig of 0 to 100 m A, and stable TCR lower than ±100 ppm/℃. It is demonstrated the Ni-P thin-film deposited with MSM is qualified for the practical application of embedded resistors. For embedded resistor, its resistance tolerance is belived to be contributed by the combination of thin-film resistance tolerance and the geometry variations of resistors. The geometry effects would dominate the resistance tolerance of embedded resistor if the geometry of resistor is lower than 4 mm ×4 mm.(4) The research of carbon-epoxy inkjet-printing ink for PCB embedded resistor. The inkjet-printing ink for embedded resistor was prepared with carbon black filler, epoxy polymer binder and solvent. The dispersion of carbon in the ink was efficiently improved by the chemical grafting of epoxy polymer on the surface of carbon black. Therefore, the grafting shows the promising capability in the application of advanced printable resistor ink. The curing condition of this ink was investigated by DSC and TGA, which showed that the curing condition of 180 ℃, 4 hour is desirable. For its application of embedded resistor, the thermal reliability of as-cured ink was verified by Tg, TCR and CTE investigations. It was indicated the Tg, TCR, and CTE are 180 ℃,600 ppm/℃ and 47 ppm/℃, respectively. The resistances are ranged from 1.2 k?/sq to 100 M?/sq. at the carbon black loading decreased from 17 wt% to 5 wt%. It indicated that this embedded resistor ink would be one of the promising materials to fabricate embedded resistors.