Microjoining on wheels - the quest rolls on for smaller and smaller, tougher and tougher
TWI Bulletin, May/June 1999
John Goward is a Senior Project Leader in the Microtechnology and Reliability Centre. John joined TWI in 1996 and is responsible for planning and leading projects in the field of electronic packaging and assembly. He is currently managing TWI's research programme, looking at the addition of EMC shielding materials into plastic packages. He is also a course manager and lecturer for training in die attach, chip-on-board and adhesives technologies.
Similar joining techniques can be used to fabricate both automobiles and the electronic systems inside them, but on a smaller scale says John Goward. Miniaturisation is a key future issue, requiring the small joints in the electrical and electronic assemblies to be extremely robust.
Both current and future automobiles can be considered as rolling platforms, supported by numerous electrical and electronic systems. These systems have many functions to perform including emissions control, performance monitoring and deployment of safety equipment. The systems can be passive, active or intelligent. There is however a common thread to all of these systems, they are held together through a myriad of small joints. These small joints are manufactured using techniques that can currently be found on a larger scale in the assembly of bodies in white goods as well as for mechanical parts. These processes include welding; by ultrasonic, resistance and lasers as well as soldering and adhesives.
There are many factors driving the development of the automobile. There is the need to reduce the weight and size of the vehicle to make it more efficient. However the addition of more electrical/electronic componentry to the body could do the opposite. Therefore the parts being manufactured today (and in the future) will, out of necessity, continue to shrink in size, whilst increasing in complexity due to new demands. Additionally, the space to mount these components and systems is decreasing. The move to mount the electronics near the unit being monitored will lead to the development of smart components/systems. An example of this is electrically power assisted steering that incorporates the sensors and actuators directly onto the steering rack.
All electrical and electronic parts in an automobile use microjoining techniques. The actual technique adopted may enable a new feature to be added to a vehicle, for example, electrostatic bonding to enable a delicate accelerometer to be packaged. It may enable an existing part to be made more robustly, an example being the replacement of crimp joints in wire harnesses via ultrasonic welding. It may even allow joints to be made at a lower cost; for example by removing the cost of filler material by moving from a soldered joint to a TIG weld when joining formed lead frames. The use of newer materials may also necessitate a change in how these small joints are made. This article reviews some of the joining techniques used for the assembly of small joints for electrical and electronic components.
Ultrasonic welding
Fig.1 Typical ultrasonically welded assemblies (parts supplied by Amtech).
Ultrasonic welding is a solid phase welding technique and has been used in the manufacture of automotive parts, such as relays. Typically copper wire is attached to copper or aluminium lead frames. Ultrasonic welding results in low heat conduction into the bulk material and the resultant robustness of the final joint can impart better joint integrity to external factors such as vibrational loading than by making a crimp joint. The benefits of using ultrasonic welding include cost as well as good joint quality. Typical examples of ultrasonic joints are shown in Fig 1.
The majority of connections on to silicon chips, used as either bare die or sealed in plastic packages, are made using ultrasonic welding. The silicon chip can be thought of as the heart of all electronic systems. Originally the joints were made through thermocompression bonds ( ie heat and pressure), but the majority are now made with either thermosonic ( ie a combination of heat and ultrasonic energy) or ultrasonic welding. The smallest joint can be made with a wire 13µm diameter, for signal applications, whilst at the other end of the scale, 500µm wires are welded for use in power applications.
Fig.2 Ball/wedge wire bonded die assembly.
Although aluminium ultrasonic wire bonding is the predominant technique, copper wire has also been investigated and may eventually supersede it. When connecting a power transistor to a substrate, typical pairs of joints consist of an aluminium wire joined to a thin aluminium film on the silicon device bonded to either a direct bonded copper (DBC), FR4, thick film or thin film substrate. The tracks on FR4 are generally 1/ 2 ounce copper that has been flash-coated with nickel then gold. Thick film circuits are made by firing tracks that are generally silver/palladium or gold, onto a ceramic substrate. Thin film layers are generally aluminium, deposited on silicon substrates. A generic term for ultrasonically wire bonded bare die assemblies, assembled on either FR4, ceramic or silicon substrates are multichip modules (MCMs). Figure 2 demonstrates Al ultrasonic ball/wedge bonding for a power die application.
Soldering
Fig.3 Soldered assembly (courtesy of Motorola).
Soldering is probably the second most commonly used interconnect technique, (compared to ultrasonic welding), in the assembly of electrical and electronic components. Figure 3 shows a typical soldered automotive assembly. The majority of connections between a printed circuit board (PCB) and the component are made with solder joints; primarily surface mount although through-hole soldering is still routinely used. Solder joints are also used in the attachment of bare die, mainly of power diodes and transistors because they enable a thermal as well as an electrical interconnection to be made. Typical die sizes can be up to 100mm 2. The solder joint may also be seen as a replacement joint for the wire bond, enabling flip chip assemblies to be made (where the die is inverted and the wire bonds are replaced with small solder joints). The benefits of attaching a bare silicon die using flip chip techniques is the reduction in substrate area, enabling a smaller system to be made whilst saving on cost. The joint size in this instance could be as small as 100µm 2.
Resistance welding
Many electrical connections are made using resistance heating. The types of welds that can be made include fusion welds, solid phase welds and brazed joints. A typical automotive example of the use of resistance welding would be the projection welding of lead frame assemblies used in electronic control unit (ECUs) and power modules.
Fig.4 Resistance welded coil connect, without removing the insulation.
Other applications include the joining of copper wires to pins during the manufacture of coils. The coils can subsequently be used in either relays, motors or alternators. Resistance welding has the benefit of not requiring expensive jigging as the parts are clamped together prior to the weld being made. Normally the copper wire would have its insulation stripped prior to welding, but TWI has developed joining techniques that make the joint without having to remove the insulation, see Fig.4, further simplifying the welding process. There is a wide choice of power supplies that will enable close control, in conjunction with a suitable electrode pair, of the joining process. A wide range of materials can be joined using this technique. Typical joint diameters range from 10µm to 2mm.
Lasers
Fig.5 Laser welding of lead frames.
Many uses can be found for laser technology in microjoining applications. The use of lasers to enable small parts to be machined in silicon for sensors found in a vehicle, is growing. Copper vapour lasers are well suited to this task. The characteristics of the laser weld, including low heat distortion as well as ease of manipulation of the laser beam, are beneficial to the fabrication of small parts; although smaller power ranges (50-300W) are required for these applications. The laser energy can be guided into restricted access areas, where other joining techniques can not be used. Nd:YAG lasers are routinely used to produce welds between similar and dissimilar materials. They can produce mechanical joints, see Fig.5, as well as hermetic seals between materials. Lasers can also be used as a source of thermal energy to reflow solders. In this instance, a diode laser may allow better process control than a Nd:YAG laser because of the more efficient absorption of the laser energy by the solder. The importance of laser soldering is now becoming apparent with the proposed removal of lead from solders. The replacement solder materials have higher reflow temperatures, that could result in damage to the parts being soldered. Lasers allow the application of heat to the solder joint only, decreasing the risk of thermal damage to the surrounding parts. Typical applications of lasers include the joining of lead frames and light bulb filaments.
Adhesive bonding
Fig.6 Electrically conductive adhesive used for surface mount assembly.
The use of adhesives can allow the manufacture of either conducting or non-conducting joints. Conductive adhesives are generally used for die attach prior to wire bonding, although they have been investigated as replacement materials for surface mount soldering, see Fig.6.
Adhesive materials can be used to package electronic components as well as enable interconnections to be made to them. In general applications, components can be potted using a number of resin types. For bare die assemblies, such as ECUs, although the unit is packed in a plastic/metal box, the internal electronics are generally protected using a silicone gel. This enables a cheaper, non-hermetic package type to be used, as the silicone protects the circuitry from any water vapour/hydrocarbon that may have entered the unit, (which could in turn cause corrosion problems). For discrete die assemblies chip-on-board (COB) techniques can be adopted, where the bare die is protected with a resin glob top after wire bonding.
Conclusions
Although the physical scale of assembly between an automobile and the electrical and electronic parts that go into it are quite different, it can be seen that many similar joining techniques can be used to fabricate them. As with the structural integrity required from the body shell, the smaller joints must be made robust enough to last the lifetime of the vehicle, or the warranty period of the respective part. These joints, and the way they are made, will face an ever increasing challenge as the parts inevitably decrease in size, increase in complexity whilst having to survive longer warranty periods.
