EPP Europe P2.2023

Yield points of various solder
pastes as a function of temperature
[A. Sharma, S. Mallik,
N. Ekere and J.-P. Jung,
‘Printing Morphology and
Rheological Characteristics of
Lead-Free Sn-3Ag-0.5Cu
(SAC) Solder Pastes’, 2014]
Source: Rehm Thermal Systems
In the Y direction
Buoyancy force in the Y direction at 100% fan
power is 34.5 µN and is thus smaller than the weight
force of the component which amounts to 44 µN.
Taking the additional wet tackiness of the solder
paste into account, the component cannot be displaced
in this direction by the wind forces.
In the Z direction
As described in ‘Experimental Investigations’,
blown off components (at 91% fan power and 180°
C) in proximity to the pads exhibit failure due to
shear stress. The diagram overleaf offers a schematic
representation of shear stress on the solder paste
layer as a result of lateral wind force.
The fact that the shear stress on the solder paste
does not occur abruptly, but gradually, must also be
taken into account. The component moves into the
flow of air at 16.7 mm/s, and blowing force increases
as it gets closer to a nozzle. When this force exceeds
the solder’s yield force, the component moves.
Yield force F F
with cross-sectional surface area A
amounting to 1.35 x 10–7 m², which is equivalent to
the contact surface of a lead, and a yield point σ F
of
273 N/m² at 23° C can be calculated with the
formula: F F
=2Aσ F
and amounts to 73.7 µN. Blowing force in this direction
is 71.3 µm and is smaller than the calculated
yield force. It should also be noted that the solder
paste is squeezed out around the lead after placement,
increasing the effective surface area of the
paste being sheared. If this arises, an area larger than
just the connection area must be anticipated.
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EPP Europe » 11 | 2023 41