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What are the principal causes of open joints on a reflowed surface-mount assembly?

   

Close inspection can often answer this question in a particular instance. Some of the commonly encountered reasons are: Non-planarity of component leads, especially for multileaded IC (integrated circuit) packages. If a lead is significantly displaced vertically (say more than 0.1mm) - perhaps because of mishandling - the chances of an open joint are increased.

  1. A blocked stencil, such that solder paste is not deposited on the board. If there is no paste (or insufficient paste), then at reflow, an appropriate joint will not form.
  2. Mis-aligned placement of a component may mean that leads are not in the correct position for joint formation.
  3. Poor wettability of a pad or component lead may mean that the solder alloy will not bond to the surface and thus not form a joint.
  4. Tombstoning of chip components. This may be helped by reducing the temperature slope on the reflow profile or by a redesign of the board pads.
  5. Solder drain into a connected via-hole may reduce the amount of alloy needed to bridge the lead and the pad. This may be difficult to identify depending on the position of the via.

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