This paper will present results from production trials that show that "lean" or "insufficient-solder" defects may be closely correlated with the locations of solder bridges between adjacent connections or "shorts". The implication of this coupling of defects is that, during the soldering process, solder has flowed to the bridge from the lean fillet and that the lean joint must therefore have been part of a temporary bridge, spanning several pads, which subsequently collapsed. This process has beenconfirmed through the observation of the formation and collapse of bridges which were deliberately created using a modified solder paste stencil.A computational modelling tool, The Surface Evolver, has also been used to examine the formation and stability of these solder bridges and has shown how the process of formation of bridges may also result in adjacent lean or open circuit joints, therefore indicating a possible mechanism for the observed phenomenon. Further development of these models should help to define more robust design rules for surface mount assemblies.Ideally all assemblies should be designed so that, for a sensible volume of solder, a bridge is impossible. However, it would appear that many existing designs fall into the region where bridges are possible but not inevitable. The incidence of solderbridges may be reduced by reducing the quantity of solder applied, but this also results in an increase in the number of open circuit defects. However, the understanding from the observations and models reported has helped in the development of a revisedsolder stencil design which has been shown, by controlling the flow of solder during the reflow process, to reduce the incidence of bridges without resulting in an increase in the occurrence of opens.
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