» PCB & ASSEMBLY
» PCB & ASSEMBLY Source: Seica S.p.A. Donut shape with variable diameter Source: Seica S.p.A. A convergent beam irradiates on the reophore, resulting in instability, B) A divergent beam does not irradiate on the reophore • The possibility to operate on boards having the bottom side facing upwards (which is not uncommon in some automated lines). • The possibility to modulate the amount of energy and to vary the area of distribution depending on the specific joint to be soldered. • The capability to implement a highly reliable and repeatable process. • The flexibility to adapt to changing production needs without any additional tools or equipment. These are only few of the reasons in favor of its application. Laser and selective soldering: a constantly evolving technology In the past, using a laser beam coming from a completely vertical angle with respect to the solder joint had already been excluded from the approach of laser selective soldering, due to a series of contraindications. In particular, it is impossible to eliminate the portion of the laser beam which, by force of circumstance, passes through the narrow space between the component lead and the walls of the hole and hits, and consequently damages, the component mounted on the opposite side. For this reason, the standard approach to this type of process was to use an angled conical laser beam, which considerably reduced the amount of radiation which could pass through the hole. The angled conical laser beam, combined with the possibility to modify the distance between the laser source and the target, also provided the capability to change the radiated surface, thus adjusting the process parameters. Despite the application of sophisticated control technologies, such as a laser distance sensor between the power source and the board to compensate for board warpage, a pyrometer to obtain feedback on the trend of the thermal profile, and a wire feeder equipped with a sensor to ensure the repeatability of the correct distribution of the filler material, the result of this approach was not exempt from basic problems. In particular, it was not possible to exclude damage to the component. Although minimized, there could be instances where the laser radiation could still pass through the hole, reaching the component mounted on the opposite side. A portion of the laser beam aimed at the lead of the component could be reflected and reach the pad edge, damaging both the PCB and the adjacent solder mask. The analysis of the results, combined with in-field experience, identified the issues of this standard approach, and pointed to two features of the solution: on one hand, the angle of the laser beam (causing reflection on the lead), and on the other, the shape of its cross section, its full-section oval footprint (main cause of energy “infiltration” through the hole). For this reason, the study was focused on the optical system, with the goal of obtaining a “hollow” beam (“donut-shaped”), which would eliminate the passage through the hole and enable a return to a vertical position of the laser beam over the target. In essence, the shape of the laser beam would be opti- 40 EPP Europe » 04|2021
mized, in order to be able to transmit energy exclusively to the copper pad and only from a vertical direction, thus reducing the possibility of unwanted reflections on the lead, while preventing any possible radiation going through the hole. Regarding the laser beam direction, it should be noted that its shape, defined by the optics and created to fit the standard pads and holes of printed circuit boards, is not convergent towards the pad, but divergent. By changing the distance between the laser source and the target, the characteristics of the laser beam allow the external and internal diameters of the two circumferences to be modified, in order to achieve optimum energy transmission to the targeted pad. From a practical point of view, instead of maintaining the architecture of the standard approach, which implemented each element of the soldering system separately, the new solution combines all of them in a single unit with a shared optical path, allowing for a better balance and calibration of the entire system. In fact, in Seica’s innovative solution, the laser source, camera and pyrometer are all positioned on the same path, minimizing variances and preventing misalignments. The FireFly Next A winning choice Today’s industry needs dedicated solutions, specific competences, optimisation of processes and investments. An automated, laser-based selective soldering system offers a solution for electronics manufacturing needs by virtue of the integral characteristics of the solution: • Laser technology allows for no-contact soldering especially in situations where there is limited access, for whatever reason (component density, pad size, product casing, etc.) • Complete automation ensures that soldering is constant and repetitive, components are soldered correctly without undesirable thermal stress, and the entire process has total traceability. • The laser soldering process is clean, which eliminates the need to clean, therefore subsequently handle, the processed boards, and power consumption is extremely low compared to other types of soldering technologies. • The process is totally waste-free and highly flexible for all applications. Selective soldering is a well-known and consolidated technology, used to solve many issues concerning the soldering of THT components. Among the existing solutions, the one which uses laser as the energy source has reached high levels of applicability and repeatability, resulting in accurate and reliable systems, perfectly capable of meeting the high qualitative demands of the electronics manufacturing industry. The scalability of systems, combined with the possibility of integration into custom automated lines, provide an additional level of flexibility to best meet factory requirements. www.seica.com Source: Seica S.p.A. Zusammenfassung Selektivlötsysteme, die den Laser als Energiequelle nutzen, erreichen ein bedeutendes Maß an Anwendbarkeit sowie Wiederholbarkeit und sind so perfekt in der Lage, die hohen qualitativen Anforderungen der Elektronikfertigungsindustrie zu erfüllen. Résumé Les systèmes de soudure sélective utilisant le laser comme source d‘énergie, atteignent un niveau élevé en matière d’applicabilité et de répétabilité et sont parfaitement en mesure de satisfaire les exigences qualitatives élevées de l’industrie de l’électronique. Резюме Селективные системы пайки с использованием лазера в качестве источника энергии применяются все чаще и обеспечивают все более высокий уровень воспроизводимости, в полной мере удовлетворяя высоким требованиям к качеству, предъявляемым в электронной промышленности. EPP Europe » 04|2021 41
