microjoining technologies

Micro-Resistance Welding
Micro-resistance welding processes use the heat generated by the passage of electrical current through the two parts to be welded to form the joint. The heat may cause the materials to fuse locally and form a nugget at the interface or may be just high enough to produce a solid-state bond.

In some cases, the heat generated is designed to melt only the plating (usually tin or an alloy of tin), which flows and bonds with the two materials to be joined; the process is then referred to as resistance soldering or brazing, depending on the temperature reached. The materials to be welded have to be held together with a clamping force, a job which is usually done by the resistance welding electrodes themselves.

A modification of the resistance welding process is percussion welding, a process during which heat is generated by an arc between the two parts. With this process, we are able to join dissimilar materials.

EWI has many power supplies to choose from including AC welders, DC welders, capacitive discharge welders, and high-frequency inverter power supplies.

Examples of EWI micro-resistance welding projects include the joining of:

• Stranded copper wire to an automotive airbag igniter
• Fuel injector stems to ball tips
• Be-copper foil to spring steel for gold leaf lettering printer
• Seam welding of nickel screen for a battery application
• Platinum coil to stainless steel wire for a medical device
• Tungsten wire to brass plate for electrical contacts

Micro-Plasma Arc Welding
Plasma arc welding (PAW) using low current has been utilized for many applications in the microjoining lab. PAW works in two basic methods, transferred and non-transferred arc methods. A flowing plasma gas is provided through the center of the torch and exits through a copper nozzle. When an arc is established between the tungsten electrode positioned within the body of the torch and the copper nozzle, the gas is ionized, forming a high-temperature plasma. The arc can be transferred to the workpiece where the intense heat causes fusion and a weld is produced.

In the microjoining lab, EWI has a microPAW system capable of as much as 50 amps. Pulsing is also available, with pulses from 1 to 999 Hz. The pulses can be configured using ramp-up, hold, and ramp-down controls. The system has excellent shielding and flow gas control and can be configured to run with a variety of gases to simulate the member's configuration.

Micro-plasma welding applications often overlap with laser welding applications, especially for thin tubes and sheet. Thicknesses of as little as 0.1 mm can be welded with this process, and the current can be as los _ as 0.05 amps. The process can be used to seam weld a variety of materials. The focused spot heat source of this process lessens the overall heat input to the assembly so that temperature-sensitive electronics and metals may be successfully joined.

EWI has used the plasma arc process to consolidate stranded wires and fabricate electrodes, sensors, thermocouples, and hermetically welded thin sheet materials. Because EWI has a variety of joining systems in one location, processes can be compared to each other and the best process selected for a given application.

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Ultrasonic Welding
As the name suggests, ultrasonic vibration energy is used to form the weld. The parts to be joined are held together between the horn, or the sonotrode (the 'electrode' which vibrates), and the anvil (the stationary 'electrode'). The ultrasonic energy is transmitted from the transducer through the horn to the parts to be welded. The parts rub against each other at a high frequency, in the range of 20 to 40 KHz. The rubbing action causes the materials to undergo localized plastic deformation which results in breakup and dispersion of the surface contaminants and oxide films. The bare metals, which are now brought into intimate contact with the applied load, form a strong metallurgical bond.

The advantage of ultrasonic welding is that it welds materials without passing electrical current through them and without additional external fluxes or fillers. The temperature rise can be localized to the bonding interface to prevent bulk transformation of the materials whose properties may be temperature sensitive.

Also, because the welding materials do not melt, the formation of harmful intermetallics is avoided. Dissimilar metals can also be joined with this method. The welding process, with proper selection of the horn, is able to convert up to 90 percent of the input energy into welding energy.

Because the process requires localized plastic deformation of the materials, soft metals such as aluminum, copper and their alloys are well-suited for ultrasonic welding. High thermal and electrical conductivity is not a deterrent in ultrasonic welding. Materials that are difficult to deform plastically, such as the refractory metals, are more challenging to weld ultrasonically. For such materials, a thin layer of slightly more 'giving' material, for example a platinum foil between molybdenum components, produces a satisfactory weld.

EWI's ultrasonic welding equipment includes two units which operate at a nominal frequency of 20 KHz, one with 1.5 kW power and another with 3.5 kW power, as well as a 1.2 kW unit which operates at 35 KHz. We have attachments for wire splicing and tube-end sealing.

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Low Power Lasers
Low power lasers have a variety of uses in microjoining applications. The controlled energy density and non-contact aspects of the process are ideal for joining wires, cutting plastics, and sealing electronic packages. The laser can be used to reflow existing platings (solder coatings) or to form in-situ eutectics such as gold tin.

The laser is used primarily to produce fusion welds where both materials in the joint are melted and fused together. This fusion can be accomplished in a single spot or in overlapped spots in a pulsed mode of operation or as a seam in a continuous mode of operation. The narrow heat affected zone and minimum overall heat input into the assembly allow laser processing to be completed within 0.050 inches of glass seals and temperature sensitive components.

EWI has three lasers in the microjoining area. These systems include a 150W pulsed carbon dioxide laser, a 50W pulsed ND:YAG with dual fiber-optic feeds, and a 300W pulsed ND:YAG. All systems are connected to computer controls and three axis machining tables to provide a flexible work surface for convenient fixturing. EWI also has the capability to bubble test and helium-leak test sealed components.

Laser processing is applicable to high-volume applications where fast cycle time and high machine up-time are required. The non-contact nature of the process is ideally suited to thinsheet materials, small diameter wire, and electronic packaging. The infrared wavelengths are also quite useful in plastics processing, especially hole drilling of thin sheet materials.

Numerous clients have used EWI to assess laser welding for specific applications, including automotive electrical components, pressure sensitive bellows, and joining small diameter wires for medical devices.

Often, EWI performs referee evaluations, where laser processing is compared to resistance welding and plasma arc welding processes.

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Wire Bonding
EWI has the equipment and capability to perform ultrasonic and thermosonic wire bonding using gold and aluminum wires. In this process, small diameter (0.0007- 0.002 inch) wire is clamped between a tool and a bonding site under a specific force. Ultrasonic energy is applied parallel to the joint interface and a solid-state weld is formed. This is the common interconnect technique for silicon and gallium arsenide integrated circuit devices.

EWI currently has two wirebonding systems, a gold ball bonder and a wedge/ribbon bonder. Each system is manually operated and has flexible fixturing to accommodate a variety of part configurations.

The wirebonding process is very mature with strong support from industry OEMs. The process allows a designer to compress an electrical circuit design and often can result in improved performance due to elimination of extra circuit elements.

EWI has performed a variety of research on the microstructure of thermosonic bonds and the effects of process parameters on shear strength and wire breaking strength. EWI has also performed DOE evaluation of the process and defined the "bonding window" for difficult substrates such as copper.


Micro-induction heating
A computer modeling capability is being established to assist coil design and braze joint design to allow maximum energy concentration at the joints and minimum heat effect on the device to be joined. This capability allows member companies to submit CAD or Pro/E drawings of a device and a coil to EWI who can then perform fast turnaround calculations.