Assembly Defect
Thermal process defects occurring during the soldering of surface mount components characterize the reliability risks of the assembly cycle. Categorization of reflow failure modes includes the specific categories of physical and electrical faults that arise when the solder paste is melted in a convection oven. These include common issues such as tombstoning and solder balling.
Each mode points to a specific deviation in the temperature profile or the volume of the solder deposit. Identifying these modes is the primary task of post reflow inspection.
Process Root
Root causes for these failures range from incorrect stencil thickness to improper ramp rates in the oven. For instance, if the preheat phase is too short, the flux may not activate correctly, leading to poor wetting. When the cooling rate is too slow, the resulting grain structure of the solder becomes coarse and brittle.
Mechanical stresses during the cooling phase can also lead to microcracks that are not visible to the naked eye. Non uniform heating across a large board can cause warping that lifts pins away from the pads.
Inspection Goal
Automated optical inspection and X-ray systems are the tools used to detect these failures. A programmed algorithm compares the solder fillet shape against a golden board to find anomalies. Verification through cross sectioning provides a detailed view of the intermetallic layer.
