Selective Soldering with Alternative Lead-Free Alloys
We are still searching for alternative lead-free alloys. Selective soldering requires relatively high solder temperatures to get proper hole filling.
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Authored By:
Gerjan Diepstraten
ITWEAE
Oosterhout, Netherlands
Summary
The electronics industry is still searching for alternative lead-free alloys. Selective soldering requires relatively high solder temperatures to get proper hole filling. Alternative alloys with lower melting temperatures may wet faster and open the process window. These low temperature solders (LTS) are of interest in SMT soldering because of reduced warpage and voiding risks. The question is how these alloys respond in liquid soldering applications. Lower operating temperatures reduce the risk of damaging plastic parts or secondary reflow on mixed technology boards. The preheat temperatures are close to the melting point of the alloy which will make the solder flow easily to the solder destination side of the board. Fluidity of these alloys may change at higher temperatures and it will be interesting to see the impact of the melting range on solidification of the solder.
Other alloys are designed to be compatible with extreme thermal exposure in operating environments such as under-hood automotive, avionics/aerospace and other severe operating environments. Their wetting properties are important to get hole fill at reasonable solder temperatures. This study investigates new alloys and their properties in lead-free selective soldering applications. Does their fluidity influence the wave stability on wettable and non-wettable nozzles? The impact of the solder temperature on hole fill and bridging is tested. A test-board with fine pitch components from 1.00 mm to 2.54 mm pitch is used to define the bridging risk.
There are new technologies on the market that make it possible to design very odd-shaped nozzles. De-bridging can be much more efficient with this second generation of non-wettable nozzles in combination with advanced nitrogen-gas nozzles to eliminate bridges. This patented nozzle technology will be compared to the conventional nozzles.
Conclusions
Selective soldering through-hole components with low melting solder is interesting for multiple reasons including less warpage, risk for fillet/pad lifting, better hole filling, lower material cost and dross reduction. From a reliability point of view, through-hole connectors are up to 20x stronger than SMD components. The solder joints made with this lower melting alloy are strong enough for a wide range of products but are not reliable for products in harsh environments.
Selective soldering with the tested alloy is feasible at temperatures >240°C.
Initially Published in the SMTA Proceedings
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