PCB007 Magazine

PCB-Feb2016

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76 The PCB Magazine • February 2016 face of the cathode; this results in increasing the effective thickness of the diffusion layer. The re- sult is better organization. This gives rise to a de- posit with a tighter grain structure. The carrier- modified diffusion layer also improves plating distribution without burning the deposit. The brightener is a grain refiner. Its ran- dom adsorption produces a film that will sup- press crystallographic differences. The bright- ener produces a fine-grained, non-directional (equiaxed) grain structure. It is the additive that directly affects the tensile strength and elonga- tion properties of the deposit. Levelers are small molecules that carry a par- tial charge that are attracted preferentially to the higher current density areas on the plating surface. Levelers, or leveling agents, are selec- tive inhibitors present at low concentrations in the electrolyte, as compared to the depositing metal. Vigorous solution movement is required to replenish the leveler at the surface of the pan- el. Figure 2 shows an example of a mis-located air sparging pipe resulting in vigorous solution movement on the filled side with hardly any movement on the voided side. Maintaining the organic additives within the recommended operating window is criti- cal to the success of the process. This is accom- plished by establishing a dosing system that is triggered by an ampere-hour accumulator. The additives are analyzed using cyclic voltametric stripping (CVS) analytical techniques. Filling Mechanism There are multiple concurrent mechanisms that take place for the via-filling process to oc- cur. The brightener and the carrier are evenly distributed throughout the via and their com- bined effect is to promote plating. The leveler component is only active at the knee of the hole (high current density area). It inhibits the plating at the via entrance which helps to keep the via open, allowing the brightener/car- rier combination to preferentially plate up the bottom of the via. Refer to the progress of plat- ing in Figure 3. Replenishment of the leveling component at the copper surface is a function of solution movement. Vigorous laminar flow across the surface must be designed in the origi- nal setup. In the absence of the leveler effect, the knee of the via would plate at an accelerated rate, closing the via before the filling is com- plete (Figure 4). This same mechanism can fill a small diam- eter, low aspect ratio through-hole like a 6.0-mil diameter hole in a 10-mil thick laminate. The Plating Cell Setup The plating cell design is not that different from standard acid copper plating. Attention must be made to ensure adequate anode cath- ode spacing (7–12 inches) in the initial design. The plating cell must be equipped with tem- perature control capabilities, mostly cooling. An overflow weir is recommended to maintain solution level and to facilitate filtration of sus- pended particles. Filtration thru 5–10 micron filter cartridges should be continuous and de- signed at a flow rate to ensure a minimum of two solution turnovers per hour (>2 STO). The filtration system draws from the bottom of the weir and returns below the cathode. The number of anodes (usually titanium baskets filled with copper balls) should be opti- mized for the platable area of the cathode; max- imum for panel plate and minimum for dot pat- tern (pads only). Anode placement should be such that the active length of the anode is 3–4 Figure 2: Misplaced air sparger; solution movement only on one side (filled vias). how to set up a suCCessful blind via hole fill dC plating proCess

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