EPP Europe P2.2023

» PCB & ASSEMBLY
White paper: ‘flying’ SMD components - part two
How to combat the ‘flying’ SMD
components phenomenon
The first part of this white paper - published in our April 2023 issue - described how
the investigation to determine the influence of flow temperature on SOD323s being
blown away was conducted. This second part examines the test results, and reveals
the best methods to ensure these components are reliably processed.
» Dr. Paul Wild, Carsten Giersberg, Rehm Thermal Systems GmbH
The simulation results demonstrate that the
maximum forces act upon different components
and that there is no component for which a force resulting
from all three directions of action must be
taken into account as the greatest force.
As the graphs opposite show, the maximum force
in the X direction acts upon the smaller side surface
of component 8. This force amounts to 45 µN. The
maximum force in the Y direction (buoyancy force)
acts upon the upper side of component 14 and
amounts to 25 µN. The maximum force in the Z direction
(graph on page 40) acts upon the larger side
surface of component 25 and amounts to 52 µN. As
the forces in the X and Z directions act upon component
side surfaces of equal size, only the force in the
Z direction will be regarded as the blowing force in
the following, and the force in the Y direction will be
regarded as the buoyancy force. The table below
summarizes simulation results for the Y and Z directions.
If flow velocity can be measured or determined
numerically as described above, the force acting
upon an object as a result of such flow can be calculated
using the equation: F W
=c 1 W
/ 2
pv 2 A
The case in hand involves a rectangular body with
a flow circulating around it. With a body of this type,
resistance coefficient c W
comprises 100% of the
form resistance of the body and amounts to 1.1 [J.
Zeitler & G. Simon, Physik für Techniker, 2016].
Forces in the Z-direction can be calculated using
density of air p (1.225kg/m³), the largest side surface
area A of the component (1.53 x 10–6m²) and velocity
values shown in the table below.
Comparison of the forces reveals that, especially
for higher velocities, the results of the two methods
differ greatly. This can be attributed to finer flow
state resolutions achieved with the CFD method, including
detachment phenomena and so on.
Calculation & comparison of forces
The holding forces for the three cases are calculated
and compared with those numerically ascertained.
1. Without solder paste at 45% fan power
In the Y direction
In accordance with the formula: F G
=mg
with a component mass m of 0.0045g and gravitational
acceleration of 9.81m/s², force due to
weight amounts to 44 µN. This is greater than the
buoyancy force of 16.2 µN, so the component cannot
be raised.
In the Z direction
Without solder paste and according to the
equation: F H
=μmg
Simulated maximum flow velocities and blowing forces acting on SOD323
Direction
45
Fan Power [%]
91
100
Force [µN]
Velocity [m/s]
Force [µN]
Velocity [m/s]
Force [µN]
Velocity [m/s]
Y
16.2
31
34.5
Z
33.7
5.1
65
9.6
71.3
10.5
38 EPP Europe » 11 | 2023