Steve Riches has been employed at TWI since 1982, where he has spent five years working in the Microjoining Section and two years in the Resistance Welding and Adhesives Section. He is now Advanced Planning Co-ordinator and Project Leader on a DTI/TWI contract on 'Interconnection and packaging techniques for electronic devices' which covers wire bonding, tape automated bonding, large area die attach, solder point inspection, laser reflow soldering and plastics packaging.
Earlier publications have covered development of Cu wire ball/wedge bonding and a study of the ultrasonic Au wire bondability of aluminium alloy thin film metallisations.
Steve Riches outlines the potential of four techniques.
There is increasing interest in welding insulated copper wire without removing the insulation as an alternative to soldering and resistance-heating techniques. This is of particular relevance to welding fine (< 100µm diameter) insulated Cu wires where difficulties are encountered with automating traditional techniques and in the consistency of joining to microelectronic components such as thick films on Al 20 3 substrates. This article reviews the application of four techniques (ultrasonic, resistance, laser and arc spot welding) to welding fine insulated Cu wires of different diameters and with different insulations to various substrates.
Insulated copper wires are extensively used in manufacturing transducers, transformers, coils and motor windings ( Figure 1) either as a fundamental element in their operation or for internal and external connection. The need to make these transducers ever smaller and more sophisticated has led to microelectronic components - such as silicon devices, hybrid circuits and printed circuit boards - being introduced into assemblies ( Figure 2). The persistent requirement for higher production rates and automated systems has placed increasing demands on traditional soldering and resistance-welding techniques.
Soldering techniques normally require the insulation to be stripped first. Some lower-temperature insulations are removed by heat from the soldering process before the solder joint is formed. However, as wires decrease in diameter (<100µm), insulating materials of higher temperature resistance ( eg polyimide) are required, making soldering more difficult. Moreover, handling the wires and removing the insulation before soldering is itself a problem.
Resistance heating uses modified resistance-welding machines which apply heat and pressure to crimp the Cu wire between tags. Unlike resistance welding, the formation of the joints depends on pressure which forges the materials together, and heat conducted from heated electrodes - usually tungsten or molybdenum based - to the joint. Advanced microelectronic components may be susceptible to high temperatures and involve terminations where no mechanical securing is present. This may limit the use of this technique for joining small diameter insulated wires to them.
There is therefore a need to develop welding techniques for insulated Cu wires - especially those of small diameter ( ie <100µm) - which can offer advantages in joint quality and production considerations over the present soldering and resistance heating processes. This is particularly relevant for connecting Cu wire to thick-film conductors on Al 20 3, Cu-plated tracks on printed circuit boards, and tags and pins where mechanical crimping of the wires is not possible.
Welding techniques
Ultrasonic welding
Ultrasonic welding has been used widely in the manufacture of integrated circuit devices for welding Au, Al and, more recently, Cu wire between 25 and 500µm diameter.
The ultrasonic process has a number of advantages over other welding techniques, including the fact that it is solid phase, which allows joining of dissimilar materials; it can easily be automated, especially when joining to flat substrates, owing to the guaranteed contact between wire and substrate.
The main drawback is that deformation of the wire is needed to form a weld, which restricts the materials which can be ultrasonically welded to ductile metals.
In trials performed on 25µm diameter Cu wire with various insulations (polyurethane, polyimide, polyesterimide), using ultrasonic wedge bonding equipment to weld the wire to Ag-Pd thick film on Al 20 3, consistently acceptable welds could be produced (>25% parent material tensile failure force) for all insulation types ( Figure 3). However, the bondability of the insulated wires was dependent on the wire-bonding equipment.
Other work has shown that two pulses of energy are necessary, the first to remove the insulation and the second to form the weld. Although acceptable welds can be produced for all insulation types, the higher-temperature insulations (polyimide) give welds with lower pull-failure forces than other insulations.
Resistance welding
A resistance-welding, as opposed to resistance-heating, technique has several advantages: it reduces heat input, the equipment is flexible, and production rates are improved. The difficulties of applying the technique to insulated Cu wire include the high electrical and thermal conductivity of Cu which hinders generation of heat at the joint interface and removal of the insulation.
For a current to flow, the top insulation must first be broken or removed. This can be accomplished by using either a sharp-edged electrode to cut through the insulation or a heated electrode to soften it, causing it to flow and enable electrical contact to be made. A high electrode force or vibrating electrode may aid breakthrough.
The second stage of the weld is to pass current through the wire and insulation at the interface to the substrate to form a joint. The success of this depends on metallic contact at the joint interface, and on the combination of materials being capable of passing the high current required for resistance welding. This may be a limiting factor for microelectronic components, where damage can be caused to the thin conductor layers on insulating substrates ( eg Al 20 3 or printed circuit board).
Parallel-gap resistance welding with both electrodes in contact with the insulated Cu wire could obviate passage of current through thin conductors on substrates. However, it would be more difficult to achieve metallic contact consistently at the joint especially with insulations resistant to high temperatures.
The options for resistance welding of wires have expanded with the development of controlled direct-current power supplies with facilities for double pulsing (first pulse to break through the insulation; second pulse to form the weld) as well as complete control over current time (including upslope and downslope), energy and power.
In trials performed on welding 75µm diameter insulated Cu wires to brass tags, consistently acceptable welds were achieved using a spot-welding arrangement. However, the application of a parallel-gap resistance spot technique to weld insulated Cu wire to conductor tracks on insulated substrates is made more difficult because of possible damage to the conductor, limited access and lastly, difficulties with breakthrough.
Laser welding
Laser welding is becoming more widely used for components in the electronic and electrical industries. It has the advantage of being a non-contact process and the potential to produce smaller, more attractive welds at high production rates.
Lasers have also been used for wire stripping. Their application to joining fine wires to flat substrates has been examined but difficulties in guaranteeing adequate clamping near the joint have led to inconsistent welds. At present, therefore, use of lasers for wire welding is confined to applications where contact between the components is ensured ( eg wire-wrap joints and butt welds).
One advantage of Nd-YAG lasers is the possibility of using fibre optic systems to deliver the energy to the joint. This is relevant for multiple-wire welding operations or applications where access might be difficult. Moreover, for Cu welding, a pulsed Nd-YAG laser is probably more suitable than a CO 2 unit as Cu is more highly reflective to energy at the CO 2 laser wavelength.
The pulsed Nd-YAG laser has been shown to be capable of welding Cu, and extending its capabilities to welding insulated Cu wires may require pulse shaping or double pulsing to remove the insulation before welding.
In trials performed on welding 75µm diameter insulated Cu wire to Ni-Fe-Co pins, laser welding has produced joints with consistently acceptable quality on all insulated types using a short, single pulse of energy with the laser aimed at the interface between wire and pin ( Figure 4). However, the alignment of the laser and wire-pin contact has proved critical.
TIG spot welding
With the recent development of stable arc power supplies operating at currents as low as 0.5A, the single-pulse arc has become more suitable for fine-wire welding. The technique uses a tungsten inert gas (TIG) torch connected to a direct-current power supply. It can deliver a single pulse with controlled arc current, pulse duration, upslope and downslope. However, because it is non-contacting, the wire must be satisfactorily clamped to the component to ensure acceptable weld quality. This technique is thought to give a more controlled heat pulse than microplasma welding and is therefore more suitable for welding small components.
The arc itself will strike across the shortest gap between the electrode and workpiece which means it is flexible in terms of joint design. Welding directly to pins is an ideal joint configuration as the arc can be directed at the end of the pin rather than having to be accurately aligned with the wire as is the case with the laser.
In trials performed on 25 and 75µm diameter insulated wire to Ni-Fe-Co pins, consistently acceptable welds were produced with 10A arc current and 1-7ms weld time. However, the TIG spot welding systems did not appear to be capable of removing polyimide insulation before welding, and this resulted in brittle welds.
Conclusions
- Ultrasonic welding can be used to join 25µm diameter insulated Cu wire to Ag-Pd thick film on Al 20 3 at room temperatures for all insulations examined. However, the results obtained were dependent on the equipment used.
- Resistance welding of 75µm diameter polyesterimide/polyamide-imide insulated Cu wire to brass with shaped electrodes produced acceptable welds. Trials were unsuccessful on polyimide-insulated Cu wire.
- Laser welding of 75µm diameter insulated Cu wire to Ni-Fe-Co pins produced consistently acceptable welds for all insulation types examined. TIG spot welding of 25 and 75µm diameter insulated Cu wires to Ni-Fe-Co pins produced acceptable welds except for polyimide-insulated wire. Alignment and contact between the wire and pin are critical for both laser and TIG spot welding systems.
Acknowledgements
This work was funded by industrial Members of TWI and the Information Engineering Directorate of the UK Department of Trade and Industry. R G Clements, A W Hillman and G Foord conducted the welding trials.
