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62 SMT Magazine • November 2016 of the weight loss calculation. This resulted in a moisture absorption rate of 0.20%, matching the rate of standard FR-4 laminate. Copper Peel Strength (Used as a Measure of Interlaminate Adhesion) Another critical characteristic affecting delamination is the interlaminate adhesion strength. In the past, we have tested standard FR-4 materials to be in the 10–12 lbs./inch range, as displayed in the Table 1.. Due to the difference in material compo- sition, phenolic materials tend to have much lower peel strengths, typically in the 3–4 lb. range (Table 2). The data suggests that the higher temper- ature, phenolic-cured lead-free capable lami- nates have only about one-third (1/3) the peel strength of standard FR-4 materials. And Now... The Perfect Storm Isola's studies suggest that vapor pressure of moisture in a PCB is 225psi at standard tin- lead (SnPb) reflow temperatures (approx. 200°C to 217°C). However, at lead-free reflow temper- atures (approx. 250°C), the vapor pressure in- creases by about 2.5x to 550psi. So, you now have a material that absorbs twice the moisture, has one-third the adhesion strength, and has to go up against 2.5x the va- por pressure. Nice. This combination of factors is the primary reason we as fabricators have incorporated ad- ditional storage precautions and baking param- eters in our processes. Unfortunately, the work doesn't stop here as there is responsibility upon the PCB user to pre-bake any re-absorbed mois- ture from the board just prior to assembly. Iso- la suggests baking per the following parameters (Table 3): TO BAKE OR NOT TO BAKE Figure 1: Original phenolic laminate data sheet showing moisture absorption at 0.45%. Table 1. Table 2. Figure 2: Current phenolic laminate data sheet showing moisture absorption at 0.20%.

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