Electronics Assembly Knowledge, Vision & Wisdom
Development and Testing of a Lead-Free Low Melting Point Alloy
Development and Testing of a Lead-Free Low Melting Point Alloy
This paper describes the development and testing procedure of a new lead-free low melting point alloy.
Materials Tech

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Adhesives, Chemicals, Cleaning Solutions, Coatings, Components, Design, Embedded Technology, Fasteners, Finishes, Flex Circuits, Flip Chip, Fluxes, PC Fab, Solders, Solder Masks, Solder Paste and more.
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Authored By:
Steven Teliszewski
Interflux Electronics NV
Gent, Belgium

Summary
This paper describes the development and testing procedure of a new lead-free low melting point alloy.

Current lead-free soldering alloys involve soldering process temperatures that can be harsh on temperature sensitive electronic components and PCB materials. These temperature sensitive parts can be found on virtually every electronic unit. As a result electronic units can come out of the soldering process in an already aged condition, sometimes even damaged or pre-damaged. An obvious solution would be to lower the temperatures in those soldering processes. This however can only be realized when using soldering alloys with lower melting points. Current low melting point alloys are mainly the eutectic Sn42Bi58 and Sn42Bi57Ag1. Unfortunately they have limited shock and vibration resistance and are not really suitable for selective and wave soldering. This substantially limits their field of use.

The focus of the development of a lead-free low melting point alloy should be in improving these weak points in such a way that the alloy is suitable for most of the electronic applications nowadays being soldered with lead-free soldering alloys. Therefore the low melting point alloy will need to have acceptable mechanical properties and will have to pass intense mechanical reliability testing. Furthermore the low melting point alloy should be suitable for the soldering processes that currently are being used in electronics assembly production environments.

In a pre-study, a first selection of metals is made based upon their ability to reduce the melting point, availability, cost, hazard classification and impact on solderability. The first selection procedure of different development alloys is done by the analysis of the microstructure before and after an annealing test. An elongation test is performed to determine the ultimate tensile strength and the elongation at break. Thermal cycling properties were tested on different QFN components soldered on a NiAu surface. Shear testing has been performed on chip components on the Cu OSP surface and NiAu. Vibration and shock testing has been performed on real functional electronic boards. The suitability of the alloy for the wave and selective soldering processes has been tested in cooperation with a leading manufacturer of soldering machines.

Test results indicate that the new low melting point alloy can be a viable alternative for current lead-free alloys.

Conclusions
Bi was chosen to be the most interesting alloying partner to reduce the melting point of Sn in order to create a new low melting point alloy that could be a viable alternative for soldering most electronic applications that are currently soldered with traditional lead-free alloys. Prototype alloys were developed and a first selection was made by performing an annealing test. Prototype alloys that gave too strong grain coarsening and showed cracks in the intermetallic layers were rejected. In an elongation test, prototype alloy Q showed very good values for tensile strength and elongation at break. Also in thermal cycling test on QFNs prototype alloy Q performed very well. In a shear force test on chip components, the component body broke before the solder joint did. Vibration and shock resistance was positively tested on a high accuracy electronic measuring device soldered with prototype alloy Q. Wave and selective soldering performance and the compatibility with machine parts was tested by a manufacturer of soldering machines with positive results.

Initially Published in the SMTA Proceedings

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