Effects of curing conditions on structural evolution and mechanical properties of UV-curable polyurethane acrylate coatings

Tin whisker growth phenomenon, which can lead to short circuits in the electronic components and boards, becomes more critical in lead (Pb)-free electronics. One potential solution to this problem is to use a conformal coating to mitigate the tin whisker growth, for which its mechanical properties will play an important role. In this study, a polyurethane acrylate film under the dual curing system (UV and moisture) was investigated in terms of chemical structure evolution and mechanical properties with various curing conditions. To establish the relationship between curing conditions and mechanical behavior of the film, both the UV curing portion (10 min, 20 min, 30 min and 40 min) and moisture curing portion (2 h, 3 h, 4 h and 5 h) were evaluated. Infrared spectroscopy, Raman spectroscopy, differential scanning calorimetry and thermogravimetric analysis were used to examine the curing status and microphase separation of film under various curing conditions, which in turn influenced its mechanical behaviors. Mechanical properties of the film were not only evaluated by a uniaxial tensile stress-strain test but also by stress relaxation and creep tests to quantify the long-term, time-dependent behavior. In addition, the influence of strain rate in tensile test on the mechanical behavior of the film was studied. The tensile test results show that partial cured film (UV20 min and moisture 4 h) has optimum mechanical performance from the strength and ductility point of view. The fully cured film (UV40 min and moisture 4 h), however, possesses more strength to resist plastic deformation during a long-term loading.