There are many various difficulties in each of manual assembly and surface mount technology within soldering.
For manual assembly a variety of these problems include disturbed joints which is when the solder moves as it’s solidifying. Another problem that commonly occurs is a ‘cold joint’ where the solder has been removed from the board too early and has not been allowed to melt properly. Another frequent problem is an overheated joint, which is where the solder has not yet flowed and the residue of the burnt flux makes this joint hard to fix. Insufficient wetting is when the solder has wet the leads but then has not went on to form a good bond with the pad.
There are also many problems with surface mount technology these include, solder balls which are mainly caused by poor process conditions which could include someone moving the board while the solder is settling. Another is solder beads which can occur when the reflow ramp up is either too fast or too slow. A further problem is bridging which can occur both due to hot slumping and cold slumping.
As well as this another common problem is tombstoning which is most commonly caused by unequal components placement on the board before reflow results in unbalanced solder forces. Circuit boards are made up of extremely sensitive components, and moving them about in a careless manner will almost certainly cause the circuit board to break and no longer usable, especially putting tension on the small components, static electricity damages boards completely so the best way to prevent damage is to keep any sort of static electricity near the board to a minimum and discharge any static charge that has been built up in the circuit over time and hold the circuit board with care. While transporting boards you should wear rubber soled shoes in order to ground yourself before handling a board. When you are in control of circuit boards you should make sure the humidity within your workspace is higher than normal as static electricity builds up much more and much faster in low humidity and in dry air.
Also you are recommended to put on gloves to handle a circuit board.