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38 PCB007 MAGAZINE I JULY 2018 it is possible that full polymerization can never be achieved. This is usually the case if there is too much photoinitiator in the matrix. This ex- cess of photoinitiators has the potential to re- sult in outgassing volatiles during reflow. Relating Theory to Reality In practice, critical solder mask types can be potentially responsible for soldering issues. To demonstrate this, a test was performed at a PCB facility. Four solder mask types were used in the test and the post solder mask bake was varied in terms of time, and the UV energy for the solder bump was varied (refer to the test matrix in Table 3). The outputs from Table 3 are an evaluation of the solderability in the form of a solder gap test and a measure of the ionic contamination which can be correlated to the volatiles within the solder mask. Figure 10 is verification that the ionic con- tamination is a viable representation of sol- der mask polymerization. The evidence for the solder mask being the dominant cause of ion- ic contamination is proven by the result for the sample without solder mask, (the column marked as 'none'). As a point of interest, sol- der mask C also displays low values suggesting complete polymerization. To evaluate solderability, a test vehicle was selected that simulates wettability. The test ve- hicle is a tool generated by the chemical sup- plier responsible for the data generated for this paper. The coupon is shown in Figure 11. Table 3: Test matrix. Figure 10: The relationship between solder mask, UV energy, and ionic contamination.