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Page 68 of 103

OCTOBER 2020 I PCB007 MAGAZINE 69 Anisotropic Etching for SAP and mSAP The final fabrication step for circuit forma- tion is the final etch that removes the 1 µm of SAP electroless copper in the SAP process or 2–3 µm of ultra-thin foil in mSAP or ATP pro- cessing. High-performance sulfuric-peroxide etchant systems are utilized to ensure maxi- mum circuit density, trace geometry, and ex- cellent adhesion. A precision etching process that does not significantly reduce trace width is an enabler for circuit density because the line and space tracks can be placed closer to- gether. The anisotropic etching aspect of the process means that it etches, at a greater rate, the cop- per between the traces without reducing the surface height or impacting the square shape of the traces, maintaining electrical performance. Furthermore, these processes must also not undercut the interface between the substrate and the trace or adhesion issues could appear during the device assembly and end-use. Summary and Conclusion The brief overview given on metallization technologies for advanced packaging only scratches the surface of what cur- rently is possible for these increas- ingly used interposer solutions. The adoption of panel-level substrate de- signs will only further the leveraging of these metallization technologies to meet the needs for cost reduction and quick design solutions in the coming wave of advanced designs utilizing heterogeneous integration to enable the future of post-Moore computing. PCB007 References 1. Dharmarathna, S., Bowerman, W., Wat- kowski, J., & Feng, K., "Embedded Trace and RDL Copper Plating Process for Panel Level Packaging Applications," iMAPS Device Packaging, 2020. 2. Gugliotti, C., Bellemare, R., Oh, A., Blake, R., & Desalvo, D., "Single-Step Metallization Process for the Filling of Through-Holes With Copper," SMTAI, September 2020. Leslie Kim is IC substrates manager, MacDermid Alpha Electronics Solutions. Jordan Kologe is technical marketing specialist, MacDermid Alpha Electronics Solutions. Bill Bowerman is director of primary metallization technologies, MacDermid Alpha Electronics Solutions. Kesheng Feng is director of research, metallization, MacDermid Alpha Electronics Solutions. Rich Bellemare is director of electrolytic plating, MacDermid Alpha Electronics Solutions. Figure 16: It is critically important that the final etch does not undercut the trace and retains a rectangular cross-section after etching, without removing significant amounts of surface copper.

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