PCB007 Magazine

PCB007-July2018

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JULY 2018 I PCB007 MAGAZINE 61 storage followed. The intermediate or subse- quent testing procedures discussed in this pa- per consisted of: • Breakdown voltage testing • Tracking resistance (CTI) • Adhesion using different pre-cleaning modes and were carried out with a screen printing LPiSM with a 20 µm dry thickness. The thermal performance of high-tempera- ture-resistant LPiSM cannot be viewed as an isolated entity. It must always be looked at in combination with the substrate used and its treatment (i.e., cleaning, degreasing, deoxida - tion). Without a matching (Tg/CTE) laminate, even the best possible pre-treatment prior to solder mask coating will not lead to the de- sired results in terms of adhesion and inevi- tably lead to fissures and cracks as shown in Figure 1. Figure 2 shows the colour change of an LPiSM before and after a thermal load of 2000 hours at 175°C and 200°C, which is acceptable to most as long as the insulation properties are within tolerances. The discolouration is caused by a certain degradation/oxidation of the pig- mentation and binders. Depending on the ac- tual coating tested, electrical properties can of course decline (e.g., insulation resistance) and must be looked at individually. Figures 3, 4 and 5 very clearly show the impact of pumice brushing, brushing and a (multi-level) chemical pre-treatment at 150°C, 175°C and 200°C for up to 2000 hours dwell time in combination with an aqueous-alkaline Figure 1: Corner crack of an LPiSM on laminate/colour change–relevant or not? Figure 2: Visual appearance of LPiSM before and after 2000 h storage at 175°C and 200°C–influence of pre-treatment on solder mask adhesion. Figure 3: Adhesion of LPiSM after temperature storage at 150°C.

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