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62 PCB007 MAGAZINE I AUGUST 2019 Finally, when the copper plating inside the via approaches coplanarity with the surface, the plating rates inside of the via and on the surface become equal, and the bottom-up fill- ing stops. However, depending on how strong the additive adsorbs and desorbs, the bright- ener may not diffuse as expected, and the high concentration of the brightener will keep ac- celerating the plating, resulting in over-plate referred to as a "momentum pump." Fine-line Profile Measurement Figure 4 shows the calculation of the profile % and the R-value. The profile % is defined as the ratio between the height difference of the lowest and highest points and expressed as a percentage, while the R-value is the height difference between the pad area and fine lines. Minimum values for both numbers are desirable. Process I is designed to fill vias with flat tops and to plate fine lines with better trace profile %; therefore, the plating conditions are opti- mized as shown in Table 1. To achieve desired via fill capability, higher CuSO4 concentration (200 g/L) was used in combination with low sulfuric acid (50 g/L). Typical performance of Process I is shown in Figure 5 in which vias 60 x 35 µm in size were filled while the total surface Cu thickness was 15 µm. Due to the ability of Process I to fill the vias with minimal dimple, no additional planarizing steps are necessary. Profile % was generally in the range of 10–15%; however, there were a few instances where it was observed be- tween 15–20%. Plated Cu thickness for the lines was 15–16 µm. The R-value was between 1–2. Pad shape was closer to square and had a flat top while lines showed a slight dome. Further evaluation of the via filling capability of the formulation was done using vias of different sizes. Four different via sizes were test- ed: 90 x 25 µm, 80 x 35 µm, 90 x 60 µm, and 100 x 80 Figure 5: Typical plating performance of Process I. Figure 4: Profile % and R-value calculation.