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64 The PCB Magazine • March 2016 was stripped of copper to expose the PI surface. A typical test coupon is shown in Figure 9. Each test condition was monitored for two primary responses: lateral copper growth (LCG) and plated through-hole (PTH) coverage. LCG was developed as an internal test parameter which can provide an indication of the copper growth properties without needing to evaluate PTH. While the PTH coverage indicates func- tional attributes. Figure 10 shows the experi- mental sequence for LCG and PTH coverage de- termination. Results from the screening experiment guid- ed us to a stable formulation where additional DOEs were performed to optimize the formula- tion, process parameters and establish a robust process window. An example of the optimiza- tion DOE matrix is shown in Table 4. Follow- ing the optimization DOE, smaller experiments were carried out to refine the operating window of the significant parameters. As before, all DOEs were blocked over different types of PI substrate. LCG Evaluation The test coupon as shown in Figure 9 is used to measure lateral copper growth. Following the DMS-E process the coupon was plated in an acid copper bath (Enthone Cuprostar ST2000 or LP1) for five minutes at 2 ASD. The coupon is then rinsed and dried before evaluation. As the edge of the test coupon usually has a higher copper growth than the center, an average value is used for LCG analysis. The difference in cop- per growth between the edge and center is due to the roughness of the substrate at the edges which allows more MnO 2 to be adsorbed. This results in a higher conductivity and hence high- er lateral copper growth. Copper growth takes place not only from the top, but also from the edges to the center of the coupons. The average growth is determined by measuring six points on the surface, as shown in Figure 11 and get- ting an average of the six points. direCt metallization system for flexible printed CirCuits Figure 9: lCG test coupon. Table 4: Example of DoE matrix for process optimization. Figure 11: Measurement location for lCG growth. Figure 10: Process flow.