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The most obvious cause of a non-reflow of paste is that the temperature of the reflow process has been incorrectly specified. Solder paste needs to be raised above its liquid temperature to reflow and then form a joint by wetting both surfaces to be joined. If the temperature is not reached or not held for sufficient time the paste will not completely flow, as in Figure 1.

Figure 1: There was either not enough time or not enough temperature for the paste here to flow completely.

This may be the first board of the shift before the oven has reached reflow temperature, or it could be that the incorrect paste is being used. It may also be that the oven is used for dual functions, like curing adhesive, and operators have not changed the process settings for reflow.

Figure 2: More non-reflow.

Faiulre to reflow can occur if a partly assembled board is left exposed to the environment over a couple of days before reflow. The temperature for reflow may have been reached, but there is no activity left in the flux to allow reflow and wetting to occur. The defect in Figure 3 was caused by a long delay between assembly and reflow, two days waiting for late components.

Figure 2: Non-reflow here was due to degredation of the flux..

Chip Leaching	Component Cracking	Cracked Joint	Crystallized Joints	Failure to Reflow
Joint Failure	Joint Voiding	Lifted Component	Open Joints	PCB Measelling
Plastic Coplanarity	Plastic Softening	Poor Pad Wetting	Poor Solderability & Wetting	Solder Beading
Solder Short	Solder Wicking