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» PCB & ASSEMBLY White paper: ‘flying’ SMD components Investigating the ‘flying’ SMD components phenomenon Advancing miniaturization of SMD components not only results in advantages such as a more compact product design and low weight, it also poses a number of challenges for process technology. This white paper, which will be published in two parts, investigates the issue of so-called ‘flying’ components in which some components are being blown away during convection soldering processes. Dr. Paul Wild, Carsten Giersberg, Rehm Thermal Systems GmbH In recent years, much has been said about adapting solder paste printing, placement and inspection processes to the new challenges posed by the advancing miniaturization of SMD components. During this time, there has also been an increase in the frequency at which components are blown away during convection soldering of electronic PCBs. In particular, the SOD323 design (small outline diode) is frequently affected by this problem. The mechanisms that lead to this defect, their interactions and possible remedies are explained in this article on the basis of CFD simulations (computational fluid dynamics), analytical models and experiments. Fundamentals of heat transfer In convection soldering systems, heat transfer is based on the principle of forced convection. Ambient air or nitrogen is used as the heat transfer medium. After the medium has been heated up it is directed, About Rehm Rehm Thermal Systems is a specialist in the area of thermal system solutions for the electronics and photovoltaic industries and is among the leaders in technology and innovation in the modern, cost-effective production of electronic assemblies. As a global manufacturer of reflow soldering systems with convection, condensation or a vacuum, drying and coating systems, functional testing systems, equipment for metallisation of solar cells and numerous customised special systems, we have a presence in all the relevant growth markets and, as a partner with over 30 years of industry experience, we arrive at innovative production solutions that set new standards. Flow field of a jet stream striking a surface for example, through a nozzle array onto a PCB and heat transfer takes place. Heat transfer depends on the temperatures of the PCB and the medium, and on the heat transfer coefficient. Jet streams from a nozzle array which strike a product beneath it are very often used in thermal systems for cooling, heating or drying. The image above shows the flow field of a jet stream striking a surface along with its typical open jet, stagnation flow and wall jet zones. The heat transfer coefficient depends on the geometry of the nozzle, its distance from the impact surface, temperature difference and flow velocity. See the image overleaf for an example of simulated velocity distribution for an arrangement consisting of nozzle array and PCB with electrolytic capacitors. The flow zones described above are visualized here by means of flow lines. Maximum heat transfer occurs at the end of the open jet at the point of flow stagnation [Schabel & Martin, ‘Wärmeübertragung bei erzwungener Konvektion: Prallströmungen’, 2019]. Next to this is an example of simulated distribution of the heat transfer coefficient for an arrangement of several nozzles and a PCB with electrolytic capacitors. This visualization represents only a mo- Source: Rehm Thermal Systems 32 EPP Europe » 04 | 2023
Simulated velocity distribution for an arrangement of multiple nozzles and a PCB with electrolytic capacitors Source: Rehm Thermal Systems Simulated distribution of the heat transfer coefficient for an arrangement of multiple nozzles and a PCB with electrolytic capacitors Source: Rehm Thermal Systems mentary snapshot from a transient calculation. In the actual process, the PCB moves so that it’s alternately subjected to lower and higher heat transfer coefficients in the direction of transport. When designing the nozzle arrays, particular attention is focused on longitudinal and transverse profile requirements and the difference in temperature between small and large thermal masses. Optimum heat transfer and lowest possible wind forces are required as well, which interact with each other in a contrary manner. Nozzle arrays are static system components and they have to perform well with a broad range of PCBs and components. Borderline cases also exist, especially at high frequencies or fan power levels, as is the case for SOD323 components. Experimental investigations: flow velocity measurements During the first step of the investigation, wind forces to which the respective component is subjected relative to fan power need to be determined Zusammenfassung Die fortschreitende Miniaturisierung von SMT-Bauteilen stellt eine Reihe von Herausforderungen an die Prozesstechnik. In diesem zweiteiligen White Paper wird das Problem der so genannten „fliegenden“ Bauteile untersucht, bei denen einige Bauteile inzwischen so klein sind, dass sie während des Konvektionslötprozesses weggeblasen werden. Résumé La miniaturisation croissante des composants CMS pose un certain nombre de défis à la technologie des processus. Ce livre blanc en deux parties examine le problème des composants dits « volants », dont certains sont désormais si petits qu’ils sont soufflés pendant le processus de brasage par convection. Results of temperature and velocity measurement in transport direction on the surface of a PCB Source: Rehm Thermal Systems Резюме Продолжающаяся миниатюризация компонентов поверхностного монтажа предъявляет ряд требований к технологическому оборудованию. В этом аналитическом докладе из двух частей рассматривается проблема так называемых „летающих“ компонентов, т.е. когда некоторые компоненты настолько малы, что их буквально сдувает во время процессов конвекционной пайки. EPP Europe » 04 | 2023 33
