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Figure 8: SIR testing of conformal coated boards.
The impact of the double spray process is clear for acrylic-2, where there is clearly a thicker coating on the track. The results of a SIR test, 40°C/93%RH and 5V bias, for the coated boards are shown in Figure 8. These results clearly show that the uncoated board, along with the nano coated board and one of the polyurethane materials have the highest SIR, and one of the thick polyurethanes has the lowest SIR. It is well known that polyurethane coatings can have lower SIR, due to the less hydrophobic backbone, since the absorbed moisture provides an additional conduction path lowering the SIR.
The effect of using the condensing condition is now explored, using a platen temperature of 38.5°C and a 3-hour cycle time with the same environmental condition of 40°C/93%RH and 5V bias. The results of this test are shown in Figure 9 and Figure 10 for all the coatings, and include an uncoated test board. The nano-coating now performs very poorly with little difference between the results and the bare board. All three acrylic results follow the condensation cycle but have less than a two decade drop in SIR, with the values always above 108Ω. The polyurethanes are far less susceptible to the condensation cycle, particularly polyurethane-1. For all coating results the response from the four SIR patterns on each test coupon are very similar. Clearly, all the coatings, except the nano coating, are working to prevent a large drop in SIR as seen with the bare test board. This is contrary to the SIR results seen in Figure 8.
Figure 9: Condensing conditions with the 1oz copper tracks and the acrylic coatings.
Figure 10: Condensing conditions with the 1oz copper tracks with the nano and polyurethane coatings.
The coating coverage on the 3-oz copper test boards are shown in Figure 11. Again the polyurethanes are easily covering the copper tracks, and with the nano coating again there is no evidence of coating. However, with the acrylic, acrylic-2 was not used on the 3-oz copper SIR boards, it is difficult to determine from these images if the acrylic coating has covered the copper track and in particular, whether the corners are coated. The results from the condensation testing of these coated boards are presented in Figure 12.
Figure 11: Coating coverage on the 3oz copper tracks of the SIR patterns.
Figure 12: Condensing conditions with the 3oz copper tracks with the acrylic-1 and polyurethane coatings.
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