PCB + ASSEMBLY
PCB + ASSEMBLY Condenso’s vacuum system allows both the vacuum time and the desired final pressure to be adapted to the requirements of the assembly to be soldered. Source: Rehm Thermal Systems Combining vacuum and temperature profiling into one system, reduces failure rates and increases reliability. Source: Rehm Thermal Systems quantity of liquid can therefore be so precisely regulated that reproducible soldering results are achieved. “The liquid quantity influences how quickly the temperature rises, and in turn the soldering profile”, says Alf Schadel, responsible for Technical Sales East. A major advantage of this solution is that the solder material can be transported horizontally through the system, which saves time since a production line operates horizontally. “Another striking argument in favor of Rehm is that the solder material is kept safely stationed in the process chamber during both the pre-vacuum and the soldering process and there is no slippage of components on the assemblies. With other solutions on the market on the other hand, Galden or another medium is continuously heated, creating a vapor blanket into which the module is raised and lowered on a product carrier. These solutions would not have given us the reproducible process we require”, says Jahn. Diehl was also convinced by the simple implementation of the vacuum process, where the corresponding vacuum technology is adapted to the already existing, hermetically tight process chamber. This helps to avoid the oxidation of the components or the drying out of the solder paste or adhesive. It also stops voids from occurring by outgassing volatile substances from the solder paste. As this eliminates the need for additional handling through transport to another vacuum chamber, it saves a great deal of time between the start of the pre-vacuum process and the start of the soldering process. Furthermore, Condenso’s vacuum system allows both the vacuum time and the desired final pressure to be adapted to the individual requirements of the assembly to be soldered. “However, the purchase of a condensation soldering system alone is not enough”, says Alf Schadel. “We know that switching from convection to condensation soldering is a challenge for the user at the beginning. You have to gain experience, test and think.” Therefore, soldering tests were carried out with smoke detectors at Rehm. “Finally, we were able to say: Now, we have cracked it!” Jahn adds: “They were always there when we had questions and helped us quickly and without any fuss. From the very beginning, we were able to carry out soldering test, we were shown what solder profiles were possible, we were able to prepare assemblies and we could see what adjustment options we had to optimize the process. And even now, when the CondensoXP is in daily use, Rehm is at our side when needed”. In this way, Diehl Aviation can continue to supply aircraft manufacturers and mobility service providers with fail-safe technology and ensure that air and rail traffic remains safe. www.rehm-group.com Zusammenfassung Das Zusammenspiel qualifizierter Zulieferer ermöglicht, dass absolut zuverlässige Komponenten den Weg zu den großen Flugzeugbauern der Welt finden. Dafür ist eine Kondensationslötanlage im Einsatz, die materialschonende Fertigungsprozesse garantiert. Résumé L‘interaction de fournisseurs qualifiés permet à des composants absolument fiables d‘accéder aux principaux constructeurs aéronautiques mondiaux. Pour cela, une machine à souder par condensation est utilisée, ce qui garantit des processus de fabrication économes en matériel. Zusammenfassung russisch Взаимодействие квалифицированных поставщиков позволяет абсолютно надежным компонентам найти свой путь к крупным самолетостроителям. Для этого используется установка конденсационной пайки, которая гарантирует процессы изготовления, обеспечивающие бережное отношение к материалу. 36 EPP EUROPE November 2018
PCB + ASSEMBLY Reflow profile vs. solder structure at the joint In today’s SMT circuit assembly, lead-free tin-silver-copper (SAC) alloy composition solder pastes, such as Sn-3.0Ag-0.5Cu (SAC305), are commonly used. The solder structure of a lead-free SAC alloy composition solder paste consists of a Sn phase and a Sn-Ag eutectic phase, with an intermetallic compound (IMC) of Cu 6 Sn 5 dispersed within and in between each phase. The composition ratio of silver or copper doesn’t make much difference and results in a similar solder structure, except for the tin-silver eutectic phase which decreases in low silver alloy composition. Due to the difference in the heat capacity ratio between the board and components, the reflow conditions at the solder joint will vary in melting temperature, time, and cooling rate. This means the joint structure may not be identical, partly due to the metal diffusion from the board and component. Reflow soldering process is prominent for mobile electronics, especially for miniaturizing components, such as in 2.5D and 3D assembly. It is also important for automotive electronics due to the use of increasingly dense assemblies, such as a safety package for automated driving. This article shows the inevitable influence of the varying reflow conditions on the solder joint structure, and presents the results of verification performed with SAC305. Preparation of the test board The test board was made with the use of a SAC305 alloy composition solder paste, 3216 metric size chip resistors, and substrate. The thickness of the stencil was 120 μm. SK-5000, a high temperature observation equipment by Sanyo Seiko, was used for reflow, in a N 2 environment. Preparation of test sample The test board was cut in the perimeter of 3216 metric size chip resistor. The cutout was then sealed with heat-curing encapsulation resin, and grinded to the center of the encapsulated chip component, after it was cured to make the test sample for observation. Source: Koki Test board design (OSP). Applied standard reflow profile. Source: Koki Source: Koki Surface finishes of the test board. Reflow profiles for evaluation. Source: Koki EPP EUROPE November 2018 37
