EPP Europe P2.2023

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» 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
Forces in X direction (transport direction) on all SOD323s, parts 1 through 20 at right angle to and parts 21 through 36 parallel to transport direction Source: Rehm Thermal Systems Forces in Y direction (positive, upwards) on all SOD323s, parts 1 through 20 at right angle to and parts 21 through 36 parallel to transport direction Source: Rehm Thermal Systems static frictional force is 32.7 µN assuming a static friction coefficient µ of 0.74 for tin on tin [P. J. Blau, Friction Science and Technology: From Concepts to Applications, 2008]. Static frictional force is smaller than the determined lateral wind force of 33.7 µN, and thus the component is blown away. 2. With solder paste at 100% fan power and 23°C Solder paste rheology Solder paste has to meet a broad range of requirements for the production of electronic PCBs. The solder paste’s flux has to demonstrate specific properties for each process step. Solder pastes can be EPP Europe » 11 | 2023 39
Page 1 and 2: Issue 11 | 2023 www.epp-europe.eu E
Page 3 and 4: » EDITORIAL Dear readers, Waste no
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EPP Europe P2.2023

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