02

Sunken Joints
Sunken solder joints are seen on the topside of the board and on the bottom side and may be caused by a number of things. The most common reason is the hole to lead ratio. If the hole is large in comparison to the lead diameter the solder literally drops in or out of the hole. They can be caused by incorrect pre heat or poor fluxing when seen on the top side of the board in cases where the solder has not reflowed directly on to the top side of the board. This would not be the case in this example as the tin/lead has either reflowed on to the top side or caused the existing solder coating to reflow. Sunken joints on the base of the board may be caused by outgassing. If the soldering process is functioning correctly when the hole outgasses the solder tends to shrink back into the hole to fill the void.