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30 SMT Magazine • April 2015 After reflow, each coupon was rated accord- ing to the following criteria: • Class 1 (C1): Solder spreads more than printed area with no evidence of de-wetting or non wetting • Class 2 (C2): Solder spreads equal to printed area with no evidence of de-wetting or non-wetting • Class 3 (C3): Solder spreads less than printed area or evidence of de-wetting or non-wetting The results are summarized in Table 3, with pictures. Wettability on Fr-4 Copper Substrates us- ing reflow Oven The purpose was first to quantify the wetting performance of solder pastes and secondly to evaluate their ability to avoid graping. Graping phenomenon is characterized by unreflowed solder powder on the surface of solder joints. Graping is due to the consumption of the flux activators during preheating. This phenomenon rEliaBiliTy aSSESSMENT OF NO-ClEaN aND WaTEr-SOluBlE SOlDEr paSTES, parT ii continues FeAture Table 3: Wetting on copper coupons summary.