» PCB & ASSEMBLY
» PCB & ASSEMBLY Zusammenfassung Der Lotpastendruck in Vertiefungen von LED-Leadframes konnte in partnerschaftlicher Kooperation erfolgreich entwickelt und erprobt werden. Elektronikhersteller sind so in der Lage, den Dispensprozess durch Schablonendruck zu ersetzen, für erhöhte Durchsatzraten und verbesserte Reproduzierbarkeit der Prozessschritt. Résumé L‘impression dans les creux des grilles de connexion des LED (leadframes) avec de la pâte à braser a été développée et testée avec succès en partenariat, dans le cadre d‘une coopération. Les fabricants d‘électronique peuvent ainsi remplacer le processus de dépose par l‘impression au pochoir, ce qui permet d‘augmenter les débits de transfert et d‘améliorer la reproductibilité de l‘étape du processus. Резюме Печать паяльной пастой в углублениях выводных рамок светодиодов может быть успешно разработана и протестирована в рамках партнерского сотрудничества. Таким образом, производители электроники могут заменить процесс дозирования трафаретной печатью, что повышает производительность и улучшает воспроизводимость технологического процесса. that are not completely filled. For the study described here, the special SAC305 Type 6 solder powder, manufactured using the patented Welco technique, was used. In the production procedure, the solder alloy is first heated in a temperaturestable dispersion medium above its melting range. A special rotor-stator process is employed to produce molten, homogeneous solder particles. This ensures that, after cooling, the required fine solder powder fraction has a very narrow particle size distribution (Type 6: 5–15 μm). It is characterized by very good sphericity (aspect ratio of ~1) and a smooth surface of the spherical shape. These solder powder properties help to ensure both very good initial printability as well as the required stability in long-term printing. These criteria will be of particular importance when it comes to ramping-up 3D-stepped stencil technology in manufacturing. In order to secure, not only excellent printability, but also the necessary good solderability with this ultra-fine solder paste, Heraeus has developed the LED131 NC flux series which has been tailored for use in printing in cavities, and with which the high wetting requirements can be met. Due to the smaller grain size, the surface area of the solder powder spheres increases considerably (high oxidation susceptibility), so for the necessary activation, a robust formulation had to be developed first. The aim was to achieve good wettability of all the joining surfaces involved (especially those of components and substrates) and a minimal void rate, while at the same time maintaining the necessary high surface isolation resistance (SIR) of the remaining flux residues. In addition to these considerations, as well as other standard requirements for solder pastes such as low slump and a low tendency to form solder balls, it was necessary to guarantee that the solder pastes had the appropriate tackiness. This ensures that the solder paste can be easily removed from the stencil and that its adhesion to the leadframe surface is ensured. Printing stencil technology One of the biggest challenges in the design of the stencil is the peculiar shape of the leadframe. The cavities on the leadframe are not angular, but circula meaning the use of a classic slit squeegee is not possible. Leadframes for use with LEDs, displaying the positions of the solder deposits, are shown in the illustration (see page 26, bottom right). 3D step stencils are used for printing in cavities. View of the print side of the stencil Source: Christian Koenen Microscope image of the printed leadframe Source: Christian Koenen 28 EPP Europe » 11 | 2021
Measurement example on the Meister S These are made using the Koenen company’s patented process. In this procedure, the base material of the stencil is removed layer by layer on both sides with the highest precision. As shown in the diagram (see page 27, top right), the stepped stencil with the substrate is displayed in cross-section. Following this production step, the apertures are lasered into the cavity and afterwards chamfered in the cavity using a very specific process. The printing area of the 3D step stencils including the apertures is shown in the image (page 28, bottom right). The slit squeegee used, just like the stencil, was designed to be equipped with special markings. These allow for the precise alignment of the squeegee to the stencil. If, for example, the squeegee slots are not aligned with the cavities in the printing direction, the squeegee cannot slide into the cavities and the dots will not be printed. Printer featuring advanced print head We printed the paste in the test series using a printing system from the scalable SERIO platform. This modern printer range can be retrofitted with all the relevant functions required for the evaluation conducted. Stencil printing reaches its full potential when applying paste in cavities. Very high throughput rates can be achieved through an industrialized and easily controllable manufacturing process with high yields. The characteristic height differences of stepped stencils require specific procedures in the printing process, and, for efficient printing, the equipment also needs to display some special technical features. If the paste transfer is realized via slotted squeegee blades, the positioning and alignment of the blade sections relative to the layout and squeegee stroke are crucial. This ensures the uniformity of the paste application across the substrate. Source: Christian Koenen Excellent adhesive performance is a function of collaboration. Identifying the key technical requirements and understanding the process for applying the right adhesive are critical elements in creating a reliable adhesion bond. Alpha offers a wide range of adhesives which provide increased bond strength for applications using a range of different plastics and metallized substrate materials. Process Expertise Reliability Needs Substrate Materials Adhesion Requirements Let’s get started. Alpha’s commitment to the development of innovative adhesion bonding technology has resulted in critical advancements in complex assemblies. Let’s work together to optimise the adhesion strength of your assembly. macdermidalpha.com Innovative Products Low Temperature Adhesives UV Adhesives UV-Thermal Adhesives SMD Adhesives Visit us at productronica Europe Booth #A4 466 EPP Europe » 11 | 2021 29 ALPHA HiTech PA 92x270mm EN 30Sep21.indd 1 05/10/2021 10:08:29
