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82 The PCB Magazine • February 2016 The Cleaner Step The cleaner is composed of a solvent, an acid and a surfactant. The solvent removes or- ganic surface residues by dissolution. The acid removes oxidation and the surfactant wets the surface. The wetting is very important when plating small high-aspect ratio holes or blind vias; a well wetted surface will not entrap air. Intermittent vibration in this step is helpful to ensure that all air is dislodged from tight spac- es. Follow vendor's recommendations of con- centration, temperature, dwell time, and dump and remake schedule to ensure the cleaner functions as intended. Good two-step rinsing is important after the cleaner as its components in trace amounts, if dragged down the line may adversely affect the plating. The Micro-etch Step The micro-etch is made up with a strong oxidant like sodium persulfate or hydrogen per- oxide in a sulfuric acid medium. The micro-etch oxidizes the surface copper and literally etches it away, exposing a fresh copper surface for plat- ing. If the metallization used is electroless cop- per; the concentration and dwell time should be controlled to ensure that the electroless copper is not etched away. In addition, the micro-etch can undercut minute stubborn residues that the cleaner did not remove. Concentration, tem- perature, dwell time, and dump/remake sched- ule must be adhered to. The panels are double- rinsed after the micro-etch. The Pre-dip Step The pre-dip is made up with sulfuric acid at 5–10%. This step ensures that the surface is oxidation-free and is acidic to match the acidity of the electrolyte. There is no rinse between the pre-dip and the acid copper bath. Voids Ideally there should be no voiding in the copper-filled via. The latest version of IPC- 6012D spells out acceptability criteria as de- tailed in Figure 7. The primary cause of voids is poor wetting of the via by "entrapped air." All air must be removed from the via in the initial cleaner step. Cleaners with low surface tension are better suited for this task. Intermittent vibration in the cleaner ensures that all air is dislodged and that the surface of the via is fully wetted. Figure 8 demonstrates air entrapped voiding. Another form of voiding occurs when the plating at the knee of the via proceeds at a faster rate than at the bottom (Figures 2 and 4). Here the hole is closed at the entrance before the bot- tom plating has filled in. This is the result of imbalance in the leveling component, poor so- lution agitation or both. With a good understanding of the plating principles of the process, proper choice and control of the chemical process, periodic car- bon treatment, and paying attention to the de- tails of cell design, the "via fill" process can run trouble-free. PCB George Milad is national accounts manager for technology with Uy- emura International Corporation. Figure 8: Void due to air entrapment in the via. how to set up a suCCessful blind via hole fill dC plating proCess

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