|
The main objective of developing soldering methods for electronic devices in recent years was to ensure homogenous distribution of the temperature over the entire board. However, it is no longer sufficient to satisfy the requirements of merely distributing heat homogeneously nowadays and for future applications. New demands are additionally being made on reflow machinery and processes by the transition to lead free manufacturing processes. This situation particularly applies to issues such as the parallelism of conveyor railsas well as process gas cleaning. New packaging technologies are also making higher demands on components and also jointing techniques. The application of polymer electronics as well as the integration of optical components into the PCB results in a maximum admissible soldering temperature of 150°C on the one hand, while the introduction of new lead free solders raises the soldering temperature up to 260°C on the other hand. Therefore, current demands made on polymer electronics, electro-optical assemblies and high-temperature electronics require a new technology for heating soldered joints, which allows the solder paste deposit to be heated stronger and faster than temperature-sensitive components and substrates. Leveraging convection soldering
The introduction of convection soldering showed great advantages compared with the infrared soldering processes which were being used previously. Whereas infra-red radiation that impinged on components with large thermal capacities only caused them to heat up very slowly, small components were overheated far beyond the requisite soldering temperature. ‘Forced convection’ had therefore become the dominant reflow soldering process during the 1980s. However, this process had reached its limits as in the case of particularly demanding components such as large BGAs for example, where the soldering connections are located underneath the components. The surfaces of printed circuit boards and components often reach temperatures that are more than 15°C to 30°C above the temperature of solder balls which have to be soldered. Multi-layered printed circuit boards with at least 20 layers require very long soldering times - until the entire assembly has been heated up and the soldering deposit has reached the soldering temperature. Today, it is possible to reduce the differences in temperature further by using the vapor-phase soldering process. Vapor-phase soldering meets the demands of special components and assemblies which can only withstand slight variations in temperature. Negligible differences in temperatures can be achieved by means of the condensing vapor’s high coefficient of thermal transmission, even for demanding assemblies. The aforementioned demands made by polymeric electronics, electro-optical assemblies and high-temperature electronics necessitated further development of this soldering technique, so that only the actual soldered connections will be heated up in the process. While this can be achieved nowadays with selective soldering processes like stirrup soldering or laser soldering, however, sequentially working, selective soldering processes considerably limit the number of components for treatment to one assembly each time. A new technology, which is particularly useful for the production of RFID tags or ‘Smart’ labels, combines a simultaneous soldering process (jointing of all components at the same time) with selective heating (the soldered joints can be heated up more than the substrate and components). Such a process combines convectional reflow soldering and microwave heating. This joint project called ‘MICROFLOW’, which is being funded by BMBF (the German Federal Ministry of Education and Research), is intended to develop a combined reflow soldering machine. The following conceptual solution has been derived from developments in the manufacture of assemblies in a study which was commissioned by SEHO. Using electromagnetic fields
Electromagnetic fields – which are well-known in the area of household microwave ovens – can be used to achieve selective heating in a simultaneous process during which energy penetrates the work piece heated and stimulates its material properties in specific areas. This process of inductive heating in the low, medium and high frequency ranges requires, in principle, the inductor to be geometrically adapted as close as possible to the soldered assembly’s shape. While this method offers essentially more flexibility to heating solder joints of various sizes in large volume and with high throughput, specific prerequisites must be fulfilled in order to be able to transfer this to the manufacture of electronic assemblies. • The system’s reliability must be ensured, even in an open throughput system. • No damage is allowed to occur in the assemblies and components because of thermal or electrical stress. • The volume (solder and soldered connections) that has to be heated effectively must be able to be warmed up noticeably quicker and more intensively than all of the remaining components. Modern micowave systems can ensure reliability, even in an open throughput operation under manufacturing conditions. Appropriate systems can be made nowadays – which are ready for serial production and can typically be used for drying wood and other materials – by means of suitable screening measures in the internal and external areas, even for large quantities with high throughput. However, it is more difficult to answer the question on such systems’ adverse effects on electronic components and assemblies. Examples from adhesive technology have already shown that it is possible to apply microwaves reliably in microelectronics. Care has to be taken while doing so – as with every other form of energy – that specific limiting values are not exceeded. The question remains about the selectivity of heating with regards to the utilization of soldering materials. Initial preliminary tests have shown that solder paste can only be warmed up very slowly and with a high power density. Examples taken from recent literature also specify that solders only melt in microwaves after very long processing times and normally indirectly via the heated surrounding assembly. This is why another approach for using microwaves is being pursued by the research project that was derived from the study. It was found that the applied power density can be increased substantially by adding a defined quantity of an extremely well coupled material, which can be coupled to the solder paste, i.e., a so-called susceptor. Such susceptors must have appropriate polar or dielectric properties and they can be added to the solder paste in solid or liquid form, as shown in Figure 1. The solder can be warmed up considerably more quickly than the other assemblies in this way, whereby the requisite energy will be further reduced too. Basic tests with microwaves
Initial basic tests were carried out with BiSn solder pastes having low melting points and possible susceptors at a melting temperature of 139°C, in order to check the proposed concept’s feasibility. It has been clearly proven already in the test results, which are shown in Figure 2, that the requisite temperatures can be transferred to the solder with suitable susceptors, even if the non-molten solder paste’s appearance still does not correspond to the soldering technique’s requirements. Further tests and selection of susceptors identified some versions with which noticeably higher temperatures were able to be achieved and conventional soft solders could be melted around. The most effective possibilityfor thermal transmission in the microwaves resulted from using liquid susceptors (Figure 3). An even quicker rise intemperatures resulted from better contact of the liquid witha solid surface: the requisite soldering temperature of 245°Cfor lead-free solders was safely reached in this case too. Afurther advantage of utilizing liquid susceptors is a smallerquantity of residues which remain around the solderingpoints after the soldering process has been completed.The process materials (e.g., fluxing agents) can thus becompletely vaporized at an optimally chosen boiling point and soldering temperature. Nevertheless, it was found that it is insufficient to bring the solder paste up to melting point solely by means of microwaves: the effect of microwaves on the components as well as the changing effect of the assembly’s individual components among themselves being decisively significant factors. Appropriate tests on the effect of incompletely vaporized liquid residues on the printed circuit board led to the finding that the susceptors did not damage the base material or metallization. In addition, it has been ascertained by adjusting the frequency and the highest homogenously possible distribution of the field, that it is possible to maintain production of soldered connections without damaging the components. A reliable processing time without any damaging effect on the printed circuit board to be processed was able to be defined (Figure 4). Combined microwaves – reflow
The use of hybrid warming-up is a further important aspect of the concept which is being pursued by the research. Basic heat can be generated with a pre-heating process by means of convection because it is required to keep the differences of temperature on the assembly as low as possible to minimize the thermal stresses on the assemblies. The soldered connections must only be heated up by the microwaves to an extra 30°C at a pre-warming temperature of typically 200°C in the lead free solder process in order to enable reliable melting around them. The combination of selective input via the susceptors and hybrid warming-up ensures that the reflow soldering process is conserving and effective. Figure 5 shows the arrangement for integrating a microwave module into a convectional soldering system, in order to ensure that the hybrid warming-up principle functions. A further advantage of such a hybrid conceptual system is that conventional solder pastes without susceptors can continue to be processed as well and the microwave energy will be ‘added in doses’ as needed to suit the assembly which has to be processed. Results
Laboratory samples have adequately proved that the idea behind this project can be implemented successfully. Figure 6 shows a printed circuit board where the solder deposits have been completely melted around the components, as well as some soldered components on the printed circuit board without further detectable faults. The following photographs in Figure 7 document the well-known appearance of the lead free soldering technique, not only in the macroscopic condition but also by means of metallographic analysis. Summary
The results which were gained from the current ‘MICROFLOW’ combined research project confirm the hypothesis that it is possible to induce selective heating in a simultaneous process by coupling hybrid warming-up and thermal input into the solder paste using susceptors. Adaptation of the materials has meanwhile led to the development of solder pastes on the basis of lead free SnAgCu solders with a melting point of 217°C. The object of the final work to be done before the project finishes consists of optimizing the hybrid systems’ engineering and defining the processing time for sufficient reliability.
Field distribution - a household microwave perspective A conventional household microwave oven has a very inhomogeneous distribution of the field – not only chronologically but also locally – because of the source’s simple phase, which will only be compensated for by rotating the article to be warmed up. However, such compensation is much too sluggish and would inevitably lead to the article’s destruction: that is why it is vital to ensure that the field’s distribution is as extensive and homogeneous as possible, without local and chronological peaks occurring. This problem can be overcome with the present state of technological development in the construction of modern microwave systems.
Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Outlook and acknowledgements
A session will be held about the project’s results within the framework of the ‘Electronic assemblies and printed circuit boards: EBL 2008’ conference, which will be jointly organized by the FED. All of the described results from this contribution are based on the joint work by the project’s consortium, which comprises the firms of SEHO Systems, Fricke and Mallah, Heraeus, Daimler-Chrysler, Peters Research and Inboard, as well as Fraunhofer IZM. The ‘MICROFLOW’ combined project is being sponsored by the Federal Ministry of Research. We would like to take this opportunity to thank all the participating partners as well as the project’s organizer – Karlsruhe Research Centre, which is in charge of this project – for their good cooperation and support. Bibliography
[1] Wittke, K.; Scheel, W.; Nowottnick, M.: Künftige Entwicklungen von Lötverfahren für die Fertigung von Elektronikbaugruppen, Berichtsnummer: IF 2001.329/2002-250, Studie im Auftrag der Firma Seho; Berlin, Juli 2002 [2] Projektträger Forschungszentrum Karlsruhe: Verbundprojekt Niedrig-temperaturmontage hochintegrierter elektronischer Baugruppen durch selektive Mikrowellenerwärmung (MICROFLOW), Förderkennzeichen: 02PW2163; Karlsruhe, Juli 2003 [3] Haseneder, R.: Mit Mikrowellen schneller Härten; EPP April/Mai 2002, S. 52/53 [4] Kyoung-sik Moon, Yi Li, Jianwen Xu and C.P. Wong: Lead-Free Solder Interconnect by Variable Frequency Microwave (VFM); ECTC 2004, Las Vegas 1-4 June 2004 [5] Reid, P.P.: Variable Frequency Microwave Reflow of Lead-Free Solder Paste, Dissertation, Georgia Institute of Technology; May 2004 [6] Nowottnick, M.: Liquid Solders Joints for High Temperature Electronics, MicroCar 05, Leipzig 2005 [7] Pape, U.: Bleifreie Lötverbindungen für die Hochtemperaturelektronik, 4. Löttechnisches Forum (DVS, Fachgesellschaft Löten, Berlin 2005 [8] Scheel, W.: Die Leiterplatte als multifunktionale Systemplattform, FED-Konferenz, Fulda, 23. September 2005 [9] Diehm, R.L.: Lötanlagen für miniaturisierte elektronische Baugruppen, Karlsruher Arbeitsgespräche Produktionsforschung 2006, Karlsruhe, März 2006 | 
