The defects in welding two specific materials can relate to different types of defects that exist after reflowing solder paste. An example of welding defects, from my personal world, include my car's 4th gear (pictured). Its welds broke due to a lack of fusion; more weld penetration was needed.
Solder paste defects include: solder balling, poor wetting, open solder joint, and tombstoning.
Solder Balling- occurs when the solder particles do not melt consistently because the oxide layer is present on them has not been completely removed or the surface is not wettable. Some of the reasoning why this happens is because too much paste is deposited on the pad due to the wrong specification on the stencil. Also the initial ramp could be too steep on the reflow profile.
Poor Wetting- takes place when the oxide layer has not been removed completely form the surface to be soldered. This can be caused by the solder paste not being active enough or the solder pads and component leads are not solderable.
Open Solder Joint- The solder is only wetted to one surface thus having a failed connection. Oxidation and contamination can be the cause. Having a poor reflow profile can cause this as well as it being due to all of the surfaces not coming up to the correct reflow temperature. If the problem is still persisting, not enough paste may be present.
Tombstoning- (one end of the component being lifted from the surface of the PCB (Printed Circuit Board)) - this is when the surface tension of the solder that has properly wetted only on one side causes the other side of the component to lift up. The component could have been incorrectly placed or the solder paste deposit was not uniformly distributed. Having the incorrect reflow profile can cause this type of defect.