Pulse Plating In The Printed Circuit Board

Recently,high-density interconnection(HDI) has been one of the major trends in printed circuit board(PCB).The HDI substrate uses micro-vias and through-holes to connect adjacent conductive layers and plating through-hole(PTH) is naturally a major process in PCB.The trend in the manufacture of PCBs is toward more layers with smaller through-hole.As the through holes get smaller and smaller,the aspect ratio of PTH gets higher and higher.In high aspect ratio PTH,it is great difficulty in achieving uniform copper deposition.The technique of pulse reverse plating(PRP) has been used to solve this problem.In this paper,the uniformity of copper deposit in through-hole, which has a diameter of 250μm or 300μm,has been studied.And an acid copper plating bath has developed to apply for plating through-hole with PRP.Polyethylene glycol(PEG),bis(3-sulfopropyl) disulfide(SPS) and chloride ions(Cl^-) are used to the acid copper plating bath to improve the deposit morphology and throwing power.The effect of PEG and the concentration of SPS upon the uniformity of pulse electroplating through-hole was studied in acid copper bath.The results showed that the uniformity of PTH will increase with increasing of the molecular weight of PEG,and the uniformity of PTH can be up to more than 90% when the molecular weight of PEG increase from 8000 to 12000 g/mol.Keeping agitation ratio of 700rpm,the uniform copper conducting layer can be got as the concentration of SPS changed from 4 mg/L to 6 mg/L.Further study showed that this process is especially applicable for plating through-holes with the diameter of 250μm and the proportion of depth to diameter of 8.8:1.The acid copper plating bath in the presence of additives(PEG-SPS-Cl^--JGB) were optimized by orthogonal experimental design(OED) for plating through-hole.Analysis of variance show that the concentration of PEG8000 and SPS have much influence on the uniformity of copper deposit in through-hole,and the additive of Cl^- is of certain influence to test results,but the concentration range of JGB have no remarkable effect on the uniformity of PTH.The effective concentration range of each additive has been found by OED.Finally,the effectiveness of the approach is verified by the experiments,and the best composition of acid copper plating bath is CuSO₄ 0.28 mol/L,H₂SO₄ 2.04 mol/L,Cl^- 100 mg/L,SPS 5 mg/L,PEG 400 mg/L。In traditional direct current(DC) plating,only the current or potential can be varied.However,in pulse plating many variables such as pulse duty cycle pulse current and pulse duration exist. Nowadays,PRP are widely used in the PTH process,but the reasonable and suitable pulse parameters are very important to the uniformity of copper deposit in through-hole.Until now,few reports in the literature have discussed the effect of pulse plating parameters on PTH.In this paper, The effect of pulse parameters in electro deposition of copper has also been studied by OED.The result show that the change of forward pulse duty cycle(γ_c) have much influence on the uniformity of PTH,and the suitable pulse parameter to PTH is thatγ_c of 80%with reverse pulse duty cycles(^'a) of 5%.The forvard and reverse pulse current are 6 A/dm² and 0.31 A/dm²,while the forvard and reverse pulse duration are 100 ms and 10 ms.In the process of pulse reverse plating through-hole,only if setting suitable reverse pulse current and higherγ_c can achieve uniform copper deposition in high aspect ratio PTH.Furthermore,the uniformity and deposition rate of PTH is not only greatly influenced by forward pulse duty cycle, but also affected by the interaction amongγ_c,forward pulse peak current(J_p.c) and reverse pulse peak current(J_p.a).