PCB Arts GmbH
Instruction Manual Vapor Phase One
Page
25
of
27
5.
Operation tips
5.1
Soldering defects
5.1.1
Tombstone effect
The tombstone effect (or "straightening effect") means that small two-pole components
(SMD capacitors and resistors) straighten up under certain circumstances when the solder
paste melts unevenly and the surface tension of the solder now acts on one side.
The main reasons for building elements to stand up are:
•
The layout of the PCB is not or only poorly adapted to the component geometry.
•
A poor or unsuitable solder paste is used
•
The solder paste print is uneven and/or poorly positioned
•
The solder paste thickness is too low. (optimal approx. 0.15 mm thickness)
•
The stencil size is not reduced (optimal 10 to 20% reduction)
•
The placement offset is too large
•
The metallization of the components and or the connection pads is insufficient
It can also happen that the solvent in the solder paste evaporates abruptly due to
insufficient drying or incorrect storage. Accordingly, components can be thrown up.
5.1.2
Wicking effect
The wicking effect describes the rising of the molten solder on the component legs. This
does not result in a proper solder connection with the underlying connection pad. The
likelihood of this effect occurring can be minimized by the amount of solder paste, the
contact force when mounting the components and by adjusting the temperature profile.
5.1.3
Solder beads
Solder beads are small balls of tin that can move loose on the assembly, causing short
circuits on the assembly. There can be several causes for this:
•
The temperature gradient is too high, so that the solvent in the paste evaporates
abruptly and rips solder balls out of the paste
•
The paste pressure is too high and is on the solder resist - good results can be
expected with approx. 10 - 15% reduced paste pressure
•
The press-in pressure of the components into the paste is too high
•
The solder paste stencil is not clean, so that paste residues get onto the assembly
•
The solder paste is old or of poor quality
•
Due to the poor wettability of the pad or component, complete wetting does not
occur
