Electronics Assembly Knowledge, Vision & Wisdom
Selective Soldering for Interconnection Technology
Selective Soldering for Interconnection Technology
Paper evaluates three processes for selective surface mount soldering. A Xenon Lamp-based system, a hot air system, and a reflow oven system.
Production Floor

Authored By:
Mark Woolley, Wesley Brown, Dr. Jae Choi
Avaya Inc.
Westminster, CO USA
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Transcript
Selective soldering techniques vary widely in their ability to produce high quality, reliable electrical and mechanical connections. Much of the variation occurs because the selective soldering techniques heat the pads and the materials surrounding them unevenly.

In many cases, the heat source is from one side of the connection only which can result in a severe thermal gradient through the solder connection and within the components.

Lead-free solders must be heated to higher temperatures than traditional tin-lead solders. The amount of heat energy necessary to raise the temperature and melt Tin-Lead solder can be significantly different from that of the commercial RoHS-Compliant solders.

This paper evaluates three processes for selective surface mount soldering. A Xenon Lamp-based heating system, a hot air heating system, and a reflow oven heating system are compared for use with both RoHS and non-RoHS compliant solders.
Summary
At times, wave soldering or reflow involving the entire assembly are not applicable for soldering of components. Selective soldering techniques vary widely in their ability to produce high quality, reliable electrical and mechanical connections. Much of the variation occurs because the selective soldering techniques heat the pads and the materials surrounding them unevenly. It is necessary to heat the pads, contacts and solder to a temperature sufficient to melt the solder and form a good intermetallic layer at both interfaces with the solder, while at the same time not over heating and damaging the PWB and nearby components. In many cases, the heat source is from one side of the connection only which can result in a severe thermal gradient through the solder connection and within the components. The construction of the PWB itself plays a part in the heating of the structures to be soldered.

Newer, lead-free solders must be heated to higher temperatures than traditional tin-lead solders. Due to the differences in specific heat of the materials, the amount of heat energy necessary to raise the temperature and melt Tin-Lead solder can be significantly different from that of the commercial RoHS-Compliant solders used in today's assemblies. This is not a problem with reflow ovens where the heat energy is replaced as fast as it is absorbed and can be considered infinite. The heating of the assembly is driven by a temperature difference between the oven air and the components. However, in other selective soldering techniques, such as hot air or heating via a focused light beam, the amount of energy is limited and must be taken into account during the soldering process.

This paper evaluates three processes for selective surface mount soldering. A Xenon Lamp-based heating system, a hot air heating system, and a reflow oven heating system are compared for use with both RoHS and non-RoHS compliant solders. Pull strengths of the solder connections, and intermetallic thicknesses of the connections are used to evaluate the solder connections.
Conclusions
All of these selective soldering methods have the potential to perform well and produce quality solder connections. Each method has its drawbacks which must be balanced with the benefits. The method used to selectively solder connections to a PWB must be determined based on a number of variables. These variables include the geometry and mass of the components, and the composition of the materials surrounding the area to be selectively soldered. The expertise of the factory personnel also plays a part in the selection of the soldering methods.
Initially Published in the IPC Proceedings
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