Characterization of Through-Hole Filling by Copper Electroplating Using a Tetrazolium Salt Inhibitor

Abstract

Through-hole (TH) filling of an advanced printed circuit board (PCB) by copper electroplating is performed using a direct current (DC) plating method and a simple copper plating formula composed of a single organic additive, namely, nitrotetrazolium blue chloride monohydrate (NTBC). Copper is preferentially deposited at the hole center to form a butterfly-shaped copper cross-section in the filled TH during plating. Three key factors are identified for enhancing the filling performance via a center protrusion deposition in the TH and are characterized by practical filling plating results and cyclic voltammetry (CV) analyzes. The filling performance of the copper plating formula for the TH is sensitive to forced convection, chloride ion concentrations and H 2 SO 4 concentrations. An appropriate forced convection combined with weak accelerators [3-(Benzothiazolyl-2-mercapto)-propylsulfonic acid, sodium salt (ZPS) or 3-S-Isothiuronium propyl sulfonate (UPS)] pre-adsorbed on the copper seed layer of the TH can effectively shorten the fill plating time from 9 h to 5 h. The center-protrusion filling mechanism and the role of NTBC in the TH filling are discussed in this work based on the plating results and electrochemical analyzes.

Superconformal Filling of High Aspect Ratio through Glass Vias (TGV) for Interposer Applications Using TNBT and NTBC Additives

“…This suggests that at higher pHs some kind of 'weak boundary layer' was left on the Cu surface after plating. 6,[15][16][17]19,20,27,28 Parts plated in solutions below pH 2.75 did not exhibit this peeling. Interestingly the pH of the above 'acid-free' plating solution is expected to be approximately 3.1 due to hydrolysis of Cu(H 2 O) 6 2+ , i.e.…”

“…However, formazans are known to form stable Cu(II) complexes 38 and the diformazan reduction product of TNBT could form such a complex. 19 Umemoto 36 implies that the diformazan can form through disproportionation, after transfer of only 2 of the 4 electrons needed to complete the reduction process. Complexation of the diformazan by Cu(II) could conceivably shift the reduction potential of TNBT into our operating range, provided that this irreversible process could get to the point of some level of diformazan production.…”

“…Plugging is then followed by continued filling of the 'blind' holes formed on either side of the plugs. [15][16][17]19 The degree of convection, 15,20 the solution pH, and the specific acid 20 used were found to be important factors in achieving void-free Cu filling of through-holes. These parameters also affect the degree of incorporation of organic species into the Cu deposit and the morphology of the Cu deposit.…”

Superconformal Filling of Through Vias in Glass Interposers

“…Dow has reported successful super conformal filling (SCF) of through-vias (TVs) in printed circuit boards (PCBs) having aspect ratios (AR) of about 1.5 to 2.5 using DC plating and formulations containing the single organic additives tetranitroblue tetrazolium chloride (TNBT) or nitrotetratetrazolium blue chloride (NTBC) and Cl − ions. [8][9][10][11] These bulky cationic additives and their reduction products, e.g. diformazan derivatives, formed at typical Cu electroplating potentials, adsorb on Cl − covered Cu surfaces and inhibit further Cu deposition.…”

“…[8][9][10][11] Thickening plating eventually forms a blockage at the centers of the holes transforming them into two blind vias. [8][9][10][11] In cross section this blockage sometimes has the appearance of a "butterfly", from which the term butterfly plating originates. [8][9][10][11][12] Continued plating under certain conditions uniformly fills the two resulting blind vias without development of voids, while plating thickness on the surface, where additive inhibition is most effective, is kept relatively low.…”