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66 The PCB Magazine • March 2016 PTH Coverage Evaluation For the though-hole evaluation a test cou- pon with sufficient holes is required. These cou- pons are treated in the same way as the LCG test samples except that the plating time in the acid copper bath was increased to 30 minutes. The coverage is evaluated from a cross section as through-hole efficiency (THE). Equation 1, along with Figure 12, describes the through- hole efficiency calculation method. (1) For substrates where it was not possible to prepare separate coupons for through-hole eval- uation, samples were taken from the LCG test coupon with through hole and a different evalu- ation method was employed. Due to the shorter plating time for copper, a quantitative evalua- tion of through-hole efficiency was difficult. For these samples a qualitative method was used to estimate the through-hole coverage (THC). This is shown in equation 2 and Figure 13. (2) Results and Discussion Common PI substrates, as provided by dif- ferent customers have been used to develop an optimized conditioner system for DMS-E pro- cess. Presenting results from all substrates used in this experimental work is beyond the scope of this paper. Hence the discussion will be fo- cused on a customer-supplied, hard to plate adhesive base PI substrate labeled as "A." Dur- ing the experiment, only the conditioner was varied; all subsequent steps of the DMS-E and copper plating processes followed the standard process. Screening Experiments Results from the screening experiments are shown below through a series of graphs show- ing the effect of individual factors on the two key response variables. Figures 14–16 show the effect of pH, reducer and sweller on both LCG and THC/THE for substrate "A". It is evident from these figures that the through-hole perfor- mance is not significantly affected by the three chosen factors. However the LCG is affected by all three factors. The graphical analysis shows alkaline (higher) pH, reducer 2 and sweller 1 provided higher LCG. As LCG is a good indi- cator for plating speed, this finding was con- sidered to be significant. This was also true for most of the substrates tested in this study. Based on this analysis, the follow up DOE was con- ducted with high pH and reducer 2. Both of the swellers were kept in the optimization DOE to further understand its effect on different stan- dard flexible and rigid substrates. Optimization Experiments The LCG result from the optimized DOE (Table 4) was analyzed using MiniTab. The main effect plot and the ANOVA table are shown in direCt metallization system for flexible printed CirCuits Figure 12: Measurement locations for Th efficiency. Figure 13: Measurement location for Th Coverage.