» PCB & ASSEMBLY
» PCB & ASSEMBLY Representation and dimensions of an SOD323 [Infineon Technologies AG, Silicon PIN Diodes BAR63..., München, 2009] and the model derived therefrom Source: Rehm Thermal Systems experimentally. Whereas temperature measurement in an oven belongs to the state of the art and a variety of profilers etc. are available for this purpose, the measurement of flow velocities in the vicinity of the PCB poses a challenge. In particular, this is due to very high process temperatures and complicated flow conditions. As part of an internal research project, a measuring system was developed at Rehm Thermal Systems on the basis of a vane anemometer, which is capable of recording flow in all 3 spatial directions, as well as directional changes, at typical soldering temperatures. The graph on the previous page shows the results Process settings for blow-off tests Process settings 1. Without solder paste 2. With solder paste and 23°C 3. With solder paste and 180°C Fan power (%) 45 100 91 Effect Blown away Not blown away Blown away of a series of measurements performed with this system. The flow velocity curve shows a number of direction reversals in proximity to the nozzle arrays. This is due to the paths of the individual nozzle jet streams: if the vane wheel is moving towards a jet stream, it rotates in one direction. After passing the stagnation point, flow is reversed. This explanation also correlates with the temperature curve, which shows small temperature fluctuations when passing individual nozzles (red marking). Velocities lie within a range of ± 0.75 m/s. Due to the size of the vane wheel (approximately 15 mm), measurement results tend to have a coarse resolution and can’t be extrapolated to significantly smaller components. Experimental investigations: Blow-off test In order to investigate the influence of flow and temperature on SOD323 components being blown away, testing was conducted with the settings listed Calculation space of the model (left) and 6 gas jets with impact on 36 components Source: Rehm Thermal Systems 34 EPP Europe » 04 | 2023
in the table. In each case, fan power at which the components were blown away was determined. Tests conducted at 23°C and 180°C served the purpose of determining the influence of temperature on the properties of the solder paste. Displacement or rotation of components near the pads was most frequently observed without solder paste (1) and with solder paste at 180°C (3). This is due to the geometry of the component: the connections can be no more than 0.05 mm longer than the height of the component, so that the component nearly rests on the PCB. For this reason, it’s assumed that the printed solder paste is stressed to the point of shearing when subjected to wind. Many components outside the pad area had been knocked over. This is due to unevenness (conductor paths etc.) on the circuit board, against which the connections come into contact, causing the component to tip over. The components blown off of the PCB were located exclusively in zone 2. No components were blown off with standard temperature and the fan set at 73% power. Numerical investigations: Model description Simulations for determining flow velocities were carried out concurrent to the experimental investigations. The Solidworks Flow Simulation tool was used for this purpose. The diagram on the left shows the dimensions of an SOD323 taken from a data sheet, as well as the geometry derived therefrom. In the simulation model, the components are firmly connected to the PCB. In order to take the influence of orientation into account, the components are aligned in the direction of transport, as well as transversely. Loads to which the PCB is subjected are determined in the stationary state, i.e. without moving the PCB through the oven. The left-hand image at the bottom of the previous page shows the calculation space with periodic boundary condition. Ambient pressure was defined as an external boundary condition. The blowing force of a total of 6 gas jets is simulated, which act on 36 components with orientation parallel and at a right angle to the direction of transport (see the image on the right). This static arrangement of components and boundary conditions is intended to approximate the forces acting on a moving PCB. Spatial directions are defined as follows: X – in transport direction, negative, Y – vertically upward, positive, Z – at a right angle to the direction of transport, positive, to the left. Flow velocities and resulting forces were simulated for experimentally determined fan powers of 100%, 91%, and 45%. Simulation results As the second part of this paper will detail, the results of the simulation demonstrated 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 considered the greatest force. www.rehm-group.com To be continued... The second part of this white paper, set to be published in our October 2023 issue, will evaluate the results of the simulations for determining flow velocities, discuss the role of solder paste, and summarise the results of the investigation overall. DISCOVER 1 STOP SMART SOLUTION GET UNIQUE BENEFITS FOR EFFICIENT AND HIGH-QUALITY PRODUCTION Check Cutting-edge capability for every process in your line Check M2M communication with our YSUP intelligent factory software Check Easy to use with our intuitive graphical interface Check Backed up by world-class support OUR PORTFOLIO: PRINTER DISPENSER MOUNTER HYBRID PLACEMENT 3D SPI 3D/2D AOI YSUP SOFTWARE INTELLIGENT STORAGE EPP Europe » 04 | 2023 35 smt.yamaha-motor-robotics.eu
