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Unsoldered joints during reflow are normally either caused by poor solder paste application or coplanarity of the leads. In both cases it requires checks on other parts of the process. Confirmation that a consistent volume of paste is being applied during printing.

Measuring the paste deposit will show any variation in the process. There should be a minimum of 0.005" paste height for 0.025" pitch parts and 0.007" for 0.050" pitch provided the component lead quality is sound.

Lead coplanarity should be better than the existing JEDEC standard of +/-0.002" or 0.004" variation from one lead to another.

Figure 1: Open joints are usually caused by either poor solder paste application or poor coplanarity of the leads.

SOT89 transistors are often found to reflow and lift on one side leaving one termination unsoldered. The most common cause is excessive solder paste on the centre termination. As the centre pad is also level with the base printing a full aperture of paste causes parts to seesaw back and forth. Always reduce the paste print aperture in this region to avoid the rock and roll.

Figure 2. This SOT89 transister lifted on one side, leaving one termination undersoldered.

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