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PCB-Jun2018

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44 PCB007 MAGAZINE I JUNE 2018 ization of designs. The use of blind and bur- ied vias, made possible in part by advances in plating chemistries, have enabled the use of sequential build up (SBU) technologies to accommodate more and finer pitch surface mount components [1] . These new designs with blind and buried vias directly impact multilay- er processing. Reliable via filling materials and methods are necessary and in high demand to fulfill the needs of PCB boards with deeper blind and buried vias. Acid Copper Via Fill Electrodeposited copper has become the fun- damental choice for via fill applications due to the exceptional conductivity and cost. Electro- plating is done in a typical electrolyte consist- ing of acid, copper and chloride ions. Sulfate- based electrolyte systems are preferred in the PCB industry owing to their low cost, conve- nient operation, safety and ease of waste treat- ment [2] . Compositions for copper via filling baths typically run with high concentrations of copper (up to 250 g/L copper sulfate) and lower concentrations of acid (approximately 100 g/L sulfuric acid). An additive-free elec- trolyte will give a deposit with poor physical properties and conformal plating. To get the desired consistent via fill while plating a mini- mum amount of copper on the surface, careful- ly designed additives are vital. These additives are key for filling vias of various diameters and depths. Typical systems will contain carriers, brighteners, and levelers. In theory, it is possi- ble to fill vias with only carrier and brighten- er [3] . However, from a practical standpoint, the dimple will be large, and fill will be conformal, also the brightener concentration will be very low making it difficult for the cyclic voltamet- ric stripping (CVS) analysis [4] . To raise the con- centration of the brightener and obtain a good fill, levelling agents must be used. Carrier and leveler in these formulations act as suppressors in different ways and can be classified as different types of suppressors [5] . Type I suppressors, like carriers, can be deac- tivated by the brightener (antagonistic, selec- tive adsorption to chloride only) whereas type II suppressors, like levelers, do not undergo de- activation (synergistic, non-selective adsorp- tion due to electrostatic interaction). Carriers or suppressors are typically high mo- lecular weight polyoxyalkyl type compounds. Typically, carriers are adsorbed at the surface of the cathode where they form a thin layer by interacting with chloride ions. Thus, carrier suppresses the plating rate by increasing the ef- fective thickness of the diffusion layer [6] . Con- sequently, the energy level over the cathode surface topography is being equalized (same number of electrons locally for plating at any cathode surface spot) so that the resultant de- posit becomes more uniform and a more even- ly distributed copper deposit thickness can be obtained. On the other hand, brighteners in- crease the plating rate by reducing the suppres- sion. They are typically low molecular weight sulfur-containing compounds, also called grain refiners. Levelers typically consist of nitrogen-bear- ing linear/branched polymers, heterocyclic or non-heterocyclic aromatic compounds be- ing typically quaternized (positively charged). These compounds will adsorb selectively on high current density sites such as edges and corners, local protrusions and prevent copper over-plating in high current density areas [7] . Via Fill Mechanism The difference in growth rate of copper in- side and outside any via is governed by the ad- ditives. Figure 1 shows a schematic representa- tion of via growth. Chemical adsorption is ex- aggerated here to show the different role played by each additive. However, both selective and non-selective adsorption occur during plating. Additive compositions must be controlled in a set range to get the desired "bottom-up filling." Analytical tools such as CVS analysis, the most common in the industry, may be utilized, al- though other techniques such as HPLC (high performance liquid chromatography) are avail- able to determine the concentrations of the ad- ditives. In Figure 1, wetter or suppressor is repre- sented with green, leveler with red and bright- ener with yellow. Wetter molecules predomi- nantly adsorbed on the surface suppressing

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