Issue link: https://iconnect007.uberflip.com/i/1172746
78 SMT007 MAGAZINE I OCTOBER 2019 This can be explained in part by thickness and coverage—although the particular chemistry of the formulation also plays a significant role, as explained by the significant difference in per - formance between UR4 and UR3—even though the applied thicknesses (150 µm) are similar, as shown in the cross-section in Figure 3. Thermal Shock Resistance Automobile electronics are usually required to work between -40°C and 125°C with rapid transitions between the temperature extremes. The Electrolube SIR test coupon shown in Fig- ure 4 was designed with a number of compo- nents laid out in a difficult configuration to bet- ter simulate a production assembly. Coupons were selectively coated with polyurethanes UR3 and UR4 at a target thickness of 250 µm and subjected to 1000 air-to-air thermal shock cycles at the temperature extremes previously indicated with a rate of temperature change in excess of 40°C/min. These coupons were visually inspected at 20X magnification for evidence of cracking, delamination, and solder joint or component damage. After 1,000 cycles, UR3 showed some signs of surface cracking and discoloration but did not expose any metal surfaces and did not propagate to the surface of the board, whereas UR4 showed almost no change in appearance. Salt-mist Resistance To assess the protection provided under salt-mist conditions (simulating win- ter driving conditions), the test coupons shown in Fig- ure 4 that were previously exposed to the 1,000 ther- mal shock cycles were sub- jected to a 196-hour salt-mist test (5% NaCl solution). The coupons were also contin- uously powered at 50 V for the duration of the test, and the insulation resistance was measured at periodic inter- vals throughout the test. Figure 3: Cross-section of 3-oz. coated coupons, showing the importance of applied thickness and coverage in condensation resistance. Figure 4: Electrolube test coupon for the assessment of conformal coating performance.