Excessive Soldering It is completely conceivable that this glob of solder does not moisten either the pin or the pad and so does not provide a secure electrical connection. A lovely concave surface, like on the joint on the far left, is the finest proof of appropriate wetting (and excellent electrical contact).
If you keep adding more and more solder to a joint and it still won't melt, then yes, you can have too much solder. This is called a cold joint and it means that there is no physical contact between the pins on your PCB and the pads they are supposed to connect with. The only way to fix this is with heat, which will also help the joint look nicer!
As well as being undesirable, having cold joints on a board is dangerous because it means that those pins/joints are not connected to anything when there is power flowing through the board. If they were actually touching the metal case of your device, then it would be less of an issue but since they're not, they can float free in space and be damaged by ESD from outside sources. This is why ESD precautions need to be taken when working with components.
ESD is the term used for unwanted electric charges that can cause damage to devices if they aren't handled properly. These charges can come from anywhere on your body and can be charged by atmospheric events such as lightning strikes or terrestrial events such as frayed cable insulation.
A good solder bond should be bright and fill the pad but not overrun it. Almost all faulty solder joins are caused by one of three factors: failing to allow the wire and pad to heat sufficiently, using too much solder, or melting the solder with the soldering iron rather than the wire lead. If the solder is not hot enough, then it will not melt into the joint - only the tin side of the solder ball will contact the metal pad on the board. This will cause the connection to fail when the board is subjected to temperature changes or mechanical stress.
The correct amount of solder is dependent on several factors, such as the size of the component and its placement accuracy, but generally a fine dusting of solder over the components and their leads is sufficient. Too much solder may flow between pins on a component leading to short circuits, while too little will leave the pins open and vulnerable to corrosion.
The third factor causing failure is using the iron instead of the leaded wire to melt the solder. This can happen if you use more solder than recommended or if you apply excessive pressure when making the connection. The iron should be kept at least 1/4" from the component lead and should be used in an even motion. If you keep pressing the iron down hard, you will eventually cut through the wire.
Shiny solder joints are not necessary for success, but they do help to identify problems before you put the board into use.
After you've finished a solder junction, examine it to ensure it's in good condition. If it is dark or has an odd-looking surface, it means that there are some problems with the board or the joint.
Solder needs to make contact with both the copper and the component lead/tin for good electrical connection. Therefore, a filled joint is necessary for making a reliable connection. A filled joint also helps prevent oxidation of the metal surfaces which would otherwise increase the resistance across the connection.
As well as being bright, the solder should also be free from oxides. This can be checked by rubbing some solder cream over the joint - a fresh solder joint will leave a mark while an old one won't. Also, use a magnifying glass to check joints for defects such as voids or bumps that might indicate a leaky connection.
Solder should be shiny enough to reflect light; if it's not, it may contain too much lead. The European Union has set strict limits on the amount of lead that can be used in solders. As well as being dark, if your solder isn't reflecting light then it's time for it to be replaced.