Study of the Thermomechanical Performance of FR-4 Laminates During the Reflow Process

The behavior of FR4 laminates for printed circuit boards (PCBs) during a double reflow process and the main factors associated with warpage are discussed. The microstructure, glass transition temperature (T_g), decomposition temperature (T_d), coefficient of thermal expansion (CTE), time to delamination and % water absorption were characterized, and the temperature profiles in the board during its pass through the reflow oven were recorded. Bow and twist measurements were taken on a set of 30 PCBs. Both T_d and T_g were found to be below their nominal values. The CTE is highly anisotropic and vary at different locations, and it is more than three times higher in the through thickness middle section with respect to the bottom and top locations. Temporary temperature gradients of more than 50 degrees between different locations were detected at the start of the reflow step as a result, the bow values were found to be strongly anisotropic, particularly after the first reflow, in which different sides can present bow values up to more than six times higher than others, with maxima values of 0.47 mm. The anisotropy of the CTE appears to affect only slightly the twist values. Compared with the first reflow, the bow and twist values are lower, exhibit less dispersion and are more homogeneous for the second reflow. The histograms of both bow and twist do not seem to follow a normal distribution, possibly as a result of the combined effect of spatial variations and anisotropy of properties as well as the temperature gradients during reflow.