Proper hand soldering is an art which you can excel at only after much practice. By knowing what mistakes to avoid, though, you may make the learning/practice phases of soldering much easier. Some very common mistakes, for example, include using improper tip sizes, using a higher temperature than recommended in order to finish sooner, and not using proper ESD precautions.
Other glaring mistakes can include:
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Insufficient wetting (of pad): This usually involves not properly bonding with the pad after soldering, possibly prompted by dirty circuit boards. This can be avoided by applying heat evenly both to the pad and to the pin.
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Insufficient wetting (of pin): This happens when not enough (if any) heat is applied to the pin, thus not allowing the solder to flow properly, creating an uneven wetting of the pad but not the pin.
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Inadequate wetting for surface-mounted components: The mistake involves not providing enough solder to flow unto the pad, but only unto the pin. Both the pad and pin need to be heated evenly to avoid this; the pad needs to be heated first.
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Disturbed joints (not same as “cold joints”): This may come about if the work is not put in a vise (when necessary)—i.e., failing to stabilize the board or immobilize the joint(s).
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Cold joints: This happens when the solder does not melt completely, creating a lumpy/rough surface and the use of too much solder. This can be repaired with a hot iron—i.e., by removing excess solder.
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Overheating of the joints: This comes about by applying the hot iron before the solder is ready to flow properly, leading to burned flux (usually causing nasty messes).
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Solder-starved joints: This usually involves not applying enough solder to joints, thus forming weak joints that may in time give in to stress cracks and, ultimately, malfunction.
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Excessive soldering: This usually leads to rounded globs over the tip and pad, often leading to lack of proper connection/adhesion. These mistakes don’t have the concave look of a properly soldered joint.
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Untrimmed leads: Leaving excessively long “tails” may bring about short circuits if the leads ever touch nearby traces. These leads should be trimmed at the solder joint tops.
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Solder bridges: This mistake involves putting so much excessive solder as to allow 2 joints to melt together (creating a “bridge). Obviously, these connections need to be fixed.
Conclusion
Hand soldering isn’t easy but you can identify mistakes that you can easily avoid simply by knowing what they are and what precautions you need to take.
