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.
- 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.
- Mis-aligned placement of a component may mean that leads are not in the correct position for joint formation.
- 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.
- 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.
- 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.