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46 I-CONNECT007 MAGAZINE I MAY 2026 Minimal Surface Copper Formation Surface copper thickness is a critical factor, as excessive surface buildup limits achievable line and space resolution. To demonstrate the perfor- mance of the new flash plating system, a PCB with substantial copper overhang was used, as shown in Figure 4 (left). BMVs with this shape are very difficult to plate. With the newly developed flash plating system, excellent results could be achieved as illustrated in Figure 5. These results clearly demonstrate the system's excellent flash copper plating capability. Drilling de- fects such as wedge voids, copper overhangs, and glass protrusions can be effectively masked while keeping surface copper buildup to a minimum. Flash Plating of Small BMVs We also evaluated plating on smaller BMVs. Even at reduced diameters, the system achieved very good BMV reinforcement with minimal surface copper thickness. Copper Crystal Structure and Epitaxy Analysis The copper crystal structure was analyzed using Scanning Electron Microscopy (SEM) at two via diameters: 40 µm and 50 µm. The evaluation showed a well formed epitaxial interface between the electroless copper and the new flash copper layer at the BMV capture pad, in- dicating excellent metallurgical continuity for strong mechanical and electrical reliability. Distinct copper twin boundaries were also observed, confirming a robust copper to copper bond and high integrity grain growth. This improved epitaxial alignment is expected to support superior NCL test performance, further enhancing long term via reliability. Normalized Tentative Crack Length (NCL) Method Assessing copper epitaxy is often subjective, relying heavily on expert interpretation of SEM images. To provide a more objective and quantifiable evalua- tion, the Normalized Tentative Crack Length (NCL) Method 1 can be used. This approach enables direct comparison between the current POR and the new flash plating process through standardized image analysis and scoring criteria, ensuring consistent and reproducible assessment of interfacial integrity. Figure 8 and Table 1 illustrate the evaluation framework used to determine NCL values. Figure 4: As received condition of BMV with Ø75 µm, 45 µm depth with a) big copper overhang; b) big glass protrusions; and c) wedge formation. Figure 5: BMV after flash plating with only 1 µm of plated copper on the surface. Figure 6: BMVs with 40 and 50 µm opening plated with the new system. Figure 7: Crystal structure of 50 µm BMV corner (left) and center (right).