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Ultrasonic Soldering: Green Technology for Electronics

By Shankar Srinivasan on Tuesday, May 25th, 2010

The European Union’s (EU) directives on Waste Electrical and Electronic Equipment (WEEE) and Restriction on Hazardous Substances (RoHS) that became effective in 2005 and 2006, respectively, restricted the use of hazardous materials like lead-based solders in consumer electronics applications. The automotive industry, for its part, is dealing with the EU’s 2002 End-of-Life Vehicle (ELV) directive that restricts the use of hazardous substances like lead in passenger and other vehicles. Beginning in 2011, the ELV directive bans the use of lead-based solders in all automotive electronics; those in electronic circuit boards and electrical applications, including those on glass. Unlike RoHS, there are no exemptions for high-lead and flip-chip components. In addition, the automotive industry has not yet developed appropriate lead-free solder compositions for many electronic applications. For example, in flip-chip soldering of a die to package, a high temperature solder is required to prevent the flip-chip solder joints from melting during subsequent soldering of the package to the board. Currently, only gold-tin (Au-Sn) solders are available in that temperature range, but they are not the most desirable due to obvious cost reasons.

EWI has made rapid advances in developing fluxless ultrasonic soldering technology, as well as in developing lead-free solders that could help the automotive industry meet their ELV requirements of “green” electronics. Conventional soldering, be it manual, reflow, or wave, requires the use of flux to prevent the formation of surface oxides to enable wetting of the solder to the base materials. In contrast, ultrasonic soldering is a fluxless joining process that consists of introducing high-frequency vibrations through a solder tool into the molten solder and inducing a cavitation action at the tool tip shown in Fig. 1. Upon near-contact of the tool tip with the base material, the cavitation forces disrupt and disperse surface oxides, thereby enabling solder wetting to the base metal. Thus, the oxide layer of the bonding surface is disrupted and removed by cavitation and not by the use of any fluxing agent.
Elimination of flux, which is the key advantage of the ultrasonic soldering process, leads to many associated benefits during soldering in electronics applications; e.g., electronic components soldered to printed circuit boards:

1. Elimination of hazardous exposure, clean-up issues, and reduction of material costs
2. Elimination of corrosion caused by flux entrapment
3. Elimination of post-solder cleaning operations and cleaning material costs
4. Maintaining dielectric constant of printed circuit boards
5. Improved wetting inside sharp corners and small crevices

Clearly, ultrasonic soldering is a Green Technology!
EWI has developed a patented lead-free solder, EWI SonicTMSolder that is a binary alloy of Sn and Al. This solder melts at 230.9°C, and in conjunction with ultrasonic soldering has been found to wet metals, ceramics, and plastics. The lack of Ag or other precious elements in its composition makes it cheaper than conventional Sn-Ag-Cu (SAC) solders. Also, the simple binary lead-free composition makes it amenable to develop custom ternary lead-free solders; such ternary compositions may be cost-effective compared to quarternary and multi-element solders that may be based on existing ternary SAC solders.

Through EWI’s Cooperative Research Program, the suitability of using the fluxless ultrasonic soldering for applications aimed at, but not limited to, those requiring structural integrity of the solder joint such as electronics packaging was evaluated (Fig. 2). Results from this project were very encouraging and showed that the shear strength (4-5 ksi) of ultrasonically soldered Sn-3.0Ag-0.5Cu (SAC305) lead-free solder was similar to that of SAC305 fabricated by conventional flux-based soldering obtained under identical conditions on copper substrates with Immersion Silver and Electroless Nickel Immersion Gold surface finishes. The study also showed that the shear strength of ultrasonically soldered EWI SonicTMSolder was also similar to that of SAC305.
In summary, EWI has demonstrated expertise in fluxless ultrasonic soldering, as well as in developing lead-free solders that could be valuable for the automotive industry in their quest to meet ELV requirements

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