Feature: Soldering
fusing with the paste deposit, even though it is dipped back into the remelted paste deposit. Since the flux has been used up at this point, the oxide layer can no longer be reduced and no connection occurs, giving the ball the appearance of resting on a pillow, hence “head-in-pillow” or “head-on-pillow”. If oxidation during separation is prevented or vapour phase),
by an inert atmosphere (N2
the paste deposit and the ball can fuse. A small degree of residual activation by the flux must still be present, however. Here, too, an inert atmosphere protects the flux, since there are fewer oxides to be reduced.
Wetting defects Te good wetting conditions under nitrogen usually lead to the solder’s better spreading and prevent soldering defects due to wetting deficits of the soldering surfaces (following IPC 610 guidelines). While this is well known, the expected improved soldering results are not always achieved. Chip soldering is characterised by the
shape of the resulting meniscus, among other things. Te height of the raised solder is a quality characteristic. Te observed wetting height may be lower under nitrogen than air; i.e., the non-wetted area (gap) increases. Te reason for this is improved spreading of the solder under nitrogen. Since the solder has to work against gravity to wet the component connection, the PCB pad is wetted more easily. In addition, the sphere height is lower when there is greater spreading, resulting in less solder available to rise at the component connection.
Tombstones Tombstones are caused by differences in wetting times between the two sides of a two-pole solder; see Figure 3. If one of the solder joints melts before the other, the wetting forces and surface tension of the liquid solder liſt one end of the component. Te second solder deposit, which melts with
Figure 2: BGA ball head-in-pillow defect
a delay, is then no longer able to wet the other connection. Under a nitrogen atmosphere, an increased
number of tombstones are oſten observed aſter reflow soldering. Te reason for this is
once again the improved wetting, which usually increases the wetting time difference between the two component connections. However, interactions with other influencing factors can also be considerable in tombstoning.
Figure 1: Graping defect
Figure 3: Phases in the formation of a tombstone
www.electronicsworld.co.uk July/August 2022 37
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