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64 DESIGN007 MAGAZINE I JULY 2022 a better capability to meet the increased pro- tective demands of new applications, e.g., automotive industry thermal shock cycling or aerospace increased condensation resistance. UV-curable materials cure extremely rapidly (in seconds) when exposed to UV radiation of a suitable wavelength and intensity, making them an extremely interesting technology for increasing factory throughput and reducing production footprint. UV-curable materials also provide good protective properties and improved chemical resistance compared with other cure types. Why Do We Need a Secondary Cure? Unfortunately, light only travels in straight lines and curing is line of sight only. Due to the 3D nature of a PCB, it is likely that some areas of the board will not attain full exposure to the light, particularly material on the backside of component leads and underneath components. For this reason, UV-curable materials need to contain a secondary cure mechanism, which can be a moisture, chemical, or heat-based mechanism. Moisture-curing has traditionally been favored because it requires no additional processes. However, although the material is generally well cured in light exposed areas, the diffusion of moisture into the coating and the emission of the leaving groups can be difficult. e better the coating barrier, the longer the secondary-curing process; many days, weeks, months, and even years have been reported. What Are the Issues With Heat Activated Curing? Heat-activated secondary curing processes require additional processes and time, which eliminates the benefit of the rapid primary cure. Speaking of which, the initial rapid cure can generate significant levels of stress, and seldom leads to more than 70–80% of the theoretical conversion to polymer, meaning that the materials can contain reactive groups that remain dormant. Once exposed to high temperatures (100°C+) additional polymeri- sation can take place, resulting in the materi- als continuing to harden, change properties, and be more prone to cracking during thermal shock transitions. Chemical Secondary Cure Time and Thermal Aging Materials containing a chemical secondary process will cure completely within six to eight hours at room temperature aer exposure to suitable long wavelength UV light. Due to the unique formulation of these materials, resid- ual stress is minimised and the cure proceeds to a very high level of conversion, resulting in minimal changes in properties during thermal aging. Aer thermal age testing, the results showed that the conventional materials tended to be very stiff and inelastic at sub-ambient temperatures, whereas the secondary chemi- cal cure system remained elastic until -20°C, but still retained a degree of elongation even at -40°C. UV Chemical and UV Moisture Cure Test Results While the lack of changes in physical prop- erties during thermal aging are an important parameter in material selection, the key to per- formance in an end-user application is whether a material can survive the required thermal shock profile without cracking or impact- ing further stress on solder joints. To investi- gate this, we selectively coated 12 automotive engine control units at a normal thickness with five different coatings in a full thermal shock experiment test from -40°C to +130°C, 0°C to 130°C, and -40°C to + 60°C. Aer 250 cycles of each test cycle, the 12 boards were visually inspected for evidence of cracks in the coating, and the most striking observation from the tests was that the materi- als which had the most stable properties dur- ing the thermal aging process were the chemi- cal cure and UV/chemical cure materials. e