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Soldering Processes used in the Electronics Industry

What is soldering?. The joining of metal parts using a metal alloy (the solder) with the application of heat.The melting temperature of the solder is below the melting points of the metals to be joined.When the right temperature is reached the molten solder flows between both metal parts.When the

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Soldering Processes used in the Electronics Industry

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    2. What is soldering? The joining of metal parts using a metal alloy (the solder) with the application of heat. The melting temperature of the solder is below the melting points of the metals to be joined. When the right temperature is reached the molten solder flows between both metal parts. When the conditions are right the connection between the solder and the metals is. firm sealed resistant to corrosion electrically and thermally conductive

    3. Overview of processes

    4. Conditions in the past With leaded components and track widths >200 µm on the PCB the wave soldering process was not critical with regard to temperature and air humidity provided activated flux was used. The inserted and soldered assemblies were mostly washed (with CFCs!). If there were soldering problems, subsequent tinplating would improve the connections.

    5. Conditions today When working with SMDs and progressive miniaturization wave soldering can only be used conditionally. Component manufacturers lay down conditions for compatibility which in practice cannot be adhered to, e.g. ceramic multilayer capacitors. Today PCB track widths are already <50 µm, dimensions which until a few years ago were reserved for semiconductors. Cleanrooms, controlled air humidity and temperature are requirements for a controlled soldering process.

    6. Influence of housing developments on the soldering process Very large chips in very thin housings require a careful soldering process which must be exactly controlled. Co planarity of the leads <40 µm. Wet layer thickness of the solder paste < 100 µm due to channelling. Ceramic capacitors require a very controlled temperature profile to avoid damage. Plastic housings must not exceed certain max. Temperatures in order to remain below glassivation temperature of the plastic.

    7. Terms

    8. Wave soldering

    9. Temp. Profile for dual wave soldering

    10. Remarks Components sit on the upper side, the connections go down through holes in the PCB. As the lower side of the PCB is pulled through the liquid mix of tin and lead the connections are soldered to the tracks on the PCB. A quality characteristic of the soldering point is the solder cup formed on the upper side. From time to time discrete SMDs were soldered to the underside resulting in high thermal load for components and higher temperature gradient when dipping in the wave! The significance of wave soldering has gone down with the ascendancy of the reflow process.

    11. Reflow soldering process In a reflow soldering process the solder mixed with the necessary flux is pressed onto the PCB before the soldering takes place with practically only a vapour phase and infrared with various modifications. The components are inserted into this paste and hold to the PCB with the adhesion of the soldering paste. Through heating the solder melts again, the flux vaporizes and the component connections are joined to the terminal pads on the PCB. In the various processes the components are subject to varying thermal loads.

    12. Vapour phase soldering

    13. Temperature profile for vapour phase soldering

    14. Infrared soldering system

    15. Picture of a reflow soldering system

    16. Soldering requirements Critical points

    17. Soldering with protective gas The soldering process lasts approx. 5 to 6 minutes at high temperatures. In this time the solder oxidizes quickly in a way which is comparable to 1 to 2 years of storage! Oxides of solder hinder a good soldering joint and compromise the reliability. Therefore it is essential that the whole oven is filled with nitrogen as a protective gas to keep the air oxygen away from the soldering joints.

    18. Temperature profile for infrared soldering

    19. Lead-free soldering In 2003 the EU issued the 2002/96/EC directive, better known as WEEE with the subsequent issue of directive 2002/95/EC, known as RoHS. WEEE = Waste of Electrical and Electronic Equipment RoHS = Restriction on Hazardous Substances Directive Due to environmental pollution the use of lead, cadmium, mercury and hexavalent chrome is prohibited as from 1st July, 2006. This will have an effect on the solder temperatures because the replacement solders will have higher melting temperature or melting zones. (Please see separate presentation on this theme). There are numerous exceptions, e.g. the military, aviation and aerospace

    20. Melting temperatures of common solder

    21. Comparison of thermal load for solder with and without lead

    22. Conclusion From 1st July, 2006 lead-free soldering will effectively become a legal requirement in the EU. The higher soldering temperatures represent considerable stress for the used components. It will be necessary to store components which are not RoHS-compliant separately from components suitable for lead-free soldering. Many customers have not yet made the changes necessary to comply with the new requirements and will face big problems.

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