New Technology to Meet Challenging Reflow Requirements

- 07.01.08

New Technology to Meet Challenging Reflow Requirements

Uwe Pape - Fraunhofer IZM Berlin
Rolf L. Diehm - SEHO Systems GmbH
Prof. Dr. Ing. Mathias Nowottnick - University of Rostock

Abstract

New packaging technologies are 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. The introduction of new lead-free solders raises the soldering temperature up to 260°C on the other hand.
The main objective of developing the soldering methods for electronic devices in recent years was to ensure homogeneous distribution of the temperature over the entire board. The introduction of convection soldering therefore showed great advantages compared with the infrared soldering processes which were being used previously. Vapour-phase soldering meets the demands of special components and assemblies which can only withstand slight variations in temperature.
It is no longer sufficient to satisfy the requirements of merely distributing the 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 rails as well as process gas cleaning.
The current demands made on polymer electronics, electro-optical assemblies and high-temperature electronics require a new technology for making the soldered joints, which allows the solder paste deposit to be heated stronger and faster than the temperature-sensitive components and substrates. This new technology, which is particularly interesting for the production of RF-ID tags or "smart" labels, combines a simultaneous soldering process (jointing of all components at the same time) with selective heating (the solder joints can be heated up more than the substrate and components). Such a process combines convectional reflow soldering and microwave heating. A 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.

Initial Situation

The general objective of developing various soldering processes for assembly technology in the past was to ensure that the temperature could be distributed as homogeneously as possible over the soldered assemblies. Whereas infra-red radiation that impinged on components with large thermal capacities only caused them to warm 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. This process had also reached its limits occasionally 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 reach temperatures that are often 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 warmed up and the soldering deposit has reached the soldering temperature too. Nowadays, it is possible to reduce the differences of temperature further by using the vapour-phase soldering process. Differences of temperatures that are virtually nothing can be achieved by means of the condensing vapour's high coefficiency of thermal transmission, even for demanding assemblies.

The aforementioned demands made by polymeric electronics, electro-optical assemblies and high-temperature electronics necessitate further development of the soldering technique, so that only the actual soldered connections will be warmed up in the process if possible: this can only be achieved nowadays with selective soldering processes like stirrup soldering or laser soldering. However, the matter in this case always concerns sequentially working, selective soldering processes which considerably limit the number of components for treatment to one assembly. Nevertheless, a simultaneous soldering process that has a selective effect at the same time is required for a cheap industrial process which meets the aforementioned objectives, but such a process is certainly not available for electronic assemblies yet. The conceptual solution that is described below has been derived from generally developing the manufacture of assemblies in a study which was commissioned by the firm of SEHO [1].

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