Flux-less direct soldering of aluminum by ultrasonic surface activation

Nowadays aluminum utilization is fundamental for manufacturing in automobile industry as it is light weighted and good conductors of heat and electricity. The joining technologies for aluminum are categorized largely into fusion welding, solid-state joining, and non-metallurgical joining. In these types of welding there are many problems like aluminum’s intrinsic susceptibility to oxidation and use of flux which affects the environment.

Ultrasonic joining is preferred generally in electronic industry as it is an active, low temperature non-fusion joining process that best complement for spot welding of sheet and wire. In ultrasonic welding, no protective atmosphere is needed but an inner layer is frequently used for dissimilar joining of aluminum to another material. As reviewed, ultrasonic soldering is better than the ultrasonic joining and reduces distortion of aluminum parts.

While ultrasonic soldering requires a pre-treatment such as pre-plating, oxide removal with flux which adds to the cost and also impacts the environment. Moreover flux residue may cause corrosion in the joint. Through various examined methods like mechanical rubbing of aluminum surface with molten solder, ultrasonic bath soldering, and thermal spraying, Dr. Somayeh Gheybi Hashemabad, and Professor Teiichi Ando from Northeastern University and Professor Zhiyong Gu from University of Massachusetts Lowell observed that these methods taken with zinc-based solders guarantee good wetting in reflow over the aluminum surface, while it give rise to sagging, warping, softening, re-alloying and hot cracking of the base metal. Moreover, they are not suitable to heat-sensitive parts, like electronic chips and age hardened aluminum parts.

The research team presented a cost effective and impressive  method for the flux-less direct soldering of aluminum with tin-base soldering which fluxing is replaced with pre-reflow ultra-sonic abrasion to activate the aluminum surface for fresh metal-to-metal contact with the solder. Flux-less soldering consists of ultrasonic activation of aluminum surface at room temperature and ensuing reflow to produce well-bonded aluminum solder interface which is flux less.

By surface activation and reflow process, the aluminum sheet will be fabricated. F fabrication is done on two types of specimen for microscopic characterization of the solder-aluminum interface and for evaluating the strength of soldered aluminum sheets by tensile shear testing.

A sedimentation method was valuable in pre-sprinkling alumina particles on the solder sheet at controlled concentrations. Below the optimum concentrations, due to insufficient surface activation the bonding became partial, however above the optimum concentrations, bonding was restricted by alumina colonies which held at interface preventing the molten solder from wetting the aluminum surface. Tensile tests show joint strengths exceeding that of conventionally soldered joints. The fracture surfaces of the tensile specimens revealed fracture paths through the solder indicative of good bonding at the solder joints of these specimens.

This study provided an effective method of ultrasonic abrasive activation of aluminum surface accomplished at room temperature. Operating costs would also be low as the flux-less process is active, low-energy process that needs no pre-treatment of aluminum part surfaces or atmosphere control. Thereby flux less soldering is proved to be economical and environment friendly.