Tiny joins, severe demands - microjoining at Abington

TWI Bulletin, January/February 1991

Sue joined TWI in 1979 with an honours degree in metallurgy from UMIST. Her first post was as a Research Engineer working in diffusion bonding and friction welding, gaining experience in the solid phase bonding of like and dissimilar materials. Other technical activities have included work in the NDT, resistance welding, surfacing and ceramics fields.

Now Head of the Ceramics and Precision Processes Department, Sue is responsible for technical and administrative aspects of the Department's activities. These include microjoining, ceramics bonding and coating, brazing and diffusion bonding. Sue also holds the post of Technical Secretary to the British Association of Brazing and Soldering (BABS). Sue is a Chartered Engineer, a Member of the Institute of Metals and a Member of The Welding Institute.

Head of the Microjoining Section, Norman is responsible for precision joining techniques applicable to small scale components for the electronics, electrical, instrument and aerospace type industries. This involves work on a wide range of welding, brazing, soldering and adhesive bonding processes. He has 11 years experience in microelectronic packaging on a wide variety of projects including wire, ribbon and tape bonding, die attach, package sealing, surface mount assembly and optical packaging. He represents TWI on ITIC (the Interconnection Technology Industrial Consortium) and is a member of the Editorial Panel of the International Materials Reviews.

Successful service performance of micro and miniature devices depends critically on microjoining technology. Sue Dunkerton and Norman Stockham review the available joining techniques and their applications.

The increasing complexity and miniaturisation of micro and miniature devices are placing more stringent demands on the assembly techniques for, and the performance of, microjoints. The full range of welding, soldering and adhesive bonding techniques is being pushed to its limits to cope with increasing miniaturisation and rapid change in design. Microjoining has therefore emerged as a major element of the micro-assembly and micro-mechanics technologies receiving significant interest today.

Microjoining is a growth area at TWI, see the Table, and the Microjoining Section encompasses the above joining techniques for all classes of materials: metals, plastics, glasses and ceramics. Prime areas of application have been the electronics and the transducer and sensor fields, but this is being expanded to cover the telecommunications, bio-engineering and jewellery industries.

Typically microjoining covers joining of all materials in sheet or wire form with dimensions below 1mm in thickness or diameter. Recently this has reached minimum levels of 7µm diameter wires, with the industry poised to go to even smaller diameters.

Interconnection and packaging

The continuing increase in device complexity and packaging density has led to a requirement for devices with higher pin counts (i.e. 200-500 leads) and finer lead pitches (i.e. <625µm, 25mil) for surface mount assembly. This trend is presenting a number of process assembly problems, including a general reduction in process tolerances, and specific solder and wire attachment problems due to coplanarity, solder placement, lead fragility, contact bridging, flux cleaning and stress due to thermal expansion mismatch. Several projects are directed towards these areas.

Small diameter wire bonding

Techniques available for wire bonding include ultrasonic, thermosonic, and thermocompression bonding. Ultrasonic bonding uses ultrasonic energy generated at bond interfaces by contact with a tuned ultrasonic tool; thermosonic involves ultrasonics with the addition of heat, usually to the substrate; and thermocompression is a hot pressure technique.

Commercial ultrasonic bonding equipment generally has a minimum force capability of 150mN which is too high for bonding wires of less than 10µm diameter to small (e.g. <20µm square) bond pads. Considerable overdeformation can occur with weakening of the wire above the bond. However, by modifying commercial equipment to operate at lower loads, gold wires of 7µm diameter have been successfully bonded. Further modifications to the wire feed and tool design also enable close pitch bonding to be carried out. Al-1%Mg wires of 7µm diameter have also been welded, although deformation is greater than for the gold wires.

For wires of 4µm diameter, thermocompression bonding has been successfully applied and this technique can also be used for larger diameter wires (10µm).

Ribbon bonding

Manual bonding of 30 x 12µm Al-1%Si ribbon to various interconnect systems has been demonstrated, see Fig.1. Typically this has involved bonding to Cu tracks on A1 ₂O ₃ and FR4 printed circuit board at 50 and 60µm conductor track width and 100-120µm pitch.

Wire/ribbon TAB

Wire/ribbon TAB is a new development currently being investigated as an interconnection technique for electrical devices. It involves a device attached to a flexible tape or rigid carrier using wire or ribbon bonding techniques, see Fig.2. This allows a pre-testable TAB format with wires or ribbons which can subsequently be excised and ultrasonically bonded to a substrate or package. The advantages of the technique include the flexibility and economics of wire bonding with the pre-testability/burn-in and high lead count capability of TAB. This offers particular benefit for pre-production trials and low volume production.

Wire bonding to soft substrates

Wire bonding to soft substrates is becoming increasingly important with the emergence of chip on board technology, the development of thermoplastic substrates and the use of low dielectric constant materials (PTFE) for high speed, high frequency devices. These substrates possess different wire bonding characteristics because of damping within the more compliant substrates, lower stiffness and temperature restrictions on certain of the substrates.

By varying bonding conditions, it has been shown that 25µm diameter Al-1%Si wire can be ultrasonically wedge bonded to Cu metallisation on PTFE and epoxy/glass circuit board, and to Ni and Au metallisations on PEI thermoplastics substrates. Good results have also been achieved for Au wires although this material is more susceptible to changes in the metallisation type and thickness.

Tape automated bonding (TAB)

TAB is now a well accepted route for chip interconnect, with particular advantages for high performance, high pin count packages and high volume production. For inner lead bonding, bumped tapes can be used or alternatively a flat tape is used and bumps are formed on the chip. Work in the Microjoining Section has included use of lasers and ultrasonic welding techniques to form bumps on the lead frame, see Fig.3, followed by single point ultrasonic bonding to Al metallisation on standard integrated circuits.

For outer lead bonding, reflow soldering is the predominant joining method and the techniques available include vapour phase, infrared, hot bar, thermocompression, laser, resistance and ultrasonics. As the pitch decreases to less than 0.5mm, the mass reflow techniques of vapour phase and infrared become less consistent and much work is now directed to hot bar and single point techniques.

Conductive adhesives

Adhesives are increasingly being applied in electronic packaging for attachment of Si chips, fixing and locating packages to surface mount circuit boards, attachment of connections on flat screens and package sealing. Their advantages include ease of application and low processing costs, no requirement for pre-plated components, low curing temperatures, ability to connect high lead count, fine pitch devices and no fluxing requirements. These advantages have led to the development of a number of conductive adhesives, which can be used as a replacement for solders. Electrical and thermal conductivity is achieved by addition of metallic fillers, typically Ag loaded to 80%.

TWI has recently completed a survey of commercially available adhesives, addressing their electrical and mechanical properties, processing requirements, and the future needs of the industry. The work has been carried out within a Group Sponsored Project and is shortly to cover an experimental programme comparing and contrasting the various adhesives for bonding specific components, e.g. chip capacitors and resistors, plastics and ceramic leaded chip carriers and quad flat packages.

Laser reflow soldering

Laser reflow soldering, see Fig.4, offers a number of advantages over alternative soldering techniques because of the ability to apply heat locally and accurately without causing damage to adjacent leads. Three systems can be used: continuous wave CO ₂ , continuous wave Nd:YAG and pulsed Nd:YAG. All three have been shown to be acceptable, with the shorter wavelength of the Nd:YAG laser offering the advantage of reduced reflectivity with solder alloy.

Lasers operate well on both reflow solder and solder paste, although less energy is required for reflowed solder.

Few microstructural and strength variations have been seen between laser soldered and standard vapour phase or infrared soldered joints. The advantages are therefore to be found in processing and the ability to laser solder at fine pitch.

Laser/ultrasonic bonding

Combined laser and ultrasonic bonding is a new technology only recently developed for reflow soldering. The laser is fed coaxially through, or adjacent to, an ultrasonic bonding tool, see Fig.5 and front cover. The laser energy produces very rapid localised heating and the ultrasonic energy agitates the molten solder to improve surface wetting. This combination is expected to reduce or eliminate the need for flux in this type of joint assembly. It has the additional benefits of being potentially the fastest (e.g. up to ten joints/second) single point contact reflow soldering system and can he applied to leads with pitches of only 100µm (4mil).

The process forms the basis of a Group Sponsored Project on fine pitch soldering which began in January 1991.

Plastics packaging

Plastics play an important role in all areas of electronics including thermosetting plastics (e.g. epoxy/silicones) being used for glob topping individual devices, and as encapsulating materials for plastics package. However, the thermosetting plastics are susceptible to the ingress of moisture, cracking of the plastic and cracking along plastic/metal interfaces possibly causing premature service failure. Other problems arise with use of transfer injection moulded thermosetting resins, where contact with the Si chips and interconnection wires imposes stress during both manufacture and service.

Recent work has led to development of a non-contact encapsulation technique, involving plastics packages. By using thermoplastics packages to contain devices, lids can he ultrasonically welded to the package to seal the device. Three thermoplastics materials (polyarylamide, aromatic polyester and polyphenylene sulphide) have been successfully welded to encapsulate Ni-Fe-Co leadframes and epoxy/glass printed circuit boards, see Fig.6.

Transducers and sensors

The transducer and control instrument manufacturing industries are crucial to many other high technology industries and service sectors including aerospace, defence, surface transport, power generation and banking. The requirement for increased monitoring and automation of systems has led to the necessity for smaller, more sophisticated (intelligent) transducers which can be produced cheaply at high production rates. Some of the assembly problems related to these areas are discussed below.

Strain gauges

Ni-Cr strain gauges are used extensively in pressure transducers. Interconnections to the gauges are normally made by Au plating the Ni-Cr thin film metallisation which is then thermosonic or thermocompression bonded with Au wire. It has now been demonstrated that by changing to an Al-1%Si wire, bonds can be made directly to the Ni-Cr metallisation using ultrasonic bonding. The joints are able to withstand long times at high temperatures (125-200°C) without a significant drop in tensile efficiency.

Silicon to glass bonding

A new generation of transducers is being designed in which metal/wire (Ni-Cr) strain gauge systems are being replaced with silicon diaphragm/load arms. These latter devices are cheaper to produce than the conventional systems and also offer technical advantages. The devices require bonding of silicon to glass, and this is accomplished by electrostatic bonding. Data have been developed on the conditions necessary to bond these materials, such that consistent bonds are now possible for 50mm diameter components, out of which many small size devices can be cut, see Fig.7.

Further work has demonstrated the feasibility of joining glass to metals (Fe or Ni based) by electrostatic bonding, where correct surface finish of the metal component is critical to achieve high quality bonds.

Diaphragms and bellows welding

Metal diaphragms, bellows and bursting have been welded for many years using electron beam welding and arc welding. Problems still arise due to the excessive heat generated during welding, causing distortion of the components. Also, there are additional problems when sealing heat sensitive components, as damage can occur because of the high temperatures. More precise welding techniques such as laser and micro TIG have been examined for this application. Both are capable of producing high strength, low distortion welds in 0.075 - 0.25mm thickness disc diaphragms and 0.5mm wall thickness tube, all in stainless steel, see Fig.8. For thicker components, the TIG process appears to be more tolerant whilst laser welding is more efficient for thin sections.

Co-operative research program

A new programme on joining techniques for assembly of transducers and sensors has started at TWI (November 1990). The programme aims to improve the transducer and sensor manufacturing capability through the development of new welding/joining techniques. The specific areas to be covered are outlined below.

MIAF welding

Magnetically impelled arc fusion welding is a relatively new and little exploited technique which has potential for edge welding thin sheet or tubular components with minimal distortion. The process works by rapid rotation of an arc struck between the workpieces and an encircling auxiliary electrode. A superimposed magnetic field causes the arc to move at high speed along the edges to be joined, see Fig.9.

The technique has previously been developed for welding sheet sections greater than 0.2mm thickness. The present programme will extend the capability of the process by developing equipment and welding procedures for sheet sections of 0.025 - 0.2mm, typical for diaphragms and bellows.

Silicon bonding

Electrostatic bonding of silicon to glass has already been demonstrated, and is in commercial application today. Interest is now being directed towards silicon to silicon bonding for which processes have not yet been demonstrated. A number of techniques offer potential for this material, and trials will be carried out using electrostatic bonding, diffusion bonding, eutectic bonding and adhesives. The use of glass and ceramic interlayers will be examined.

Further work is also planned on glass to silicon bonding because of a drive to use thinner silicon wafers (<0.5mm thickness) over large areas (up to 150mm diameter). This creates additional problems of material damage during cooling because of thermal expansion mismatch. Work will therefore be directed towards developing low temperature electrostatic bonding procedures.

Equipment available in the Microjoining Section

Stranded wire welding

Stranded copper wires of 0.5 - 2.0mm diameter are used extensively to connect sensing elements, potentiometers, solenoids and transformers. These wires are usually insulated, and the normal soldering is not appropriate for high temperature applications and gives the added complication of flux removal. Other techniques based on welding will therefore be developed, including ultrasonics, resistance, friction, percussive arc, precision TIG and laser.

Jewellery

Jewellery components present a wide range of applications where many different material types and joint configurations need to be joined. These include joints in rings, bracelets, earrings and fixing of semi-precious and precious stones to housings, where the materials to be joined can range from a few microns thickness for plated layers to greater than 5mm. The alloys used in jewellery are mainly precious metal based (Au, Ag, Pt, Pd) although materials such as Ti and plastics are gaining acceptance in certain market sectors due to their aesthetic appearance.

Traditionally, mechanical fixing, adhesives, brazing and soldering have been used in the jewellery industry. However, advanced joining techniques offer scope for improved product yield and productivity for traditional products and for the introduction of new materials and designs. Of interest are diffusion bonding and hot pressure/ultrasonic welding techniques which are particularly appropriate for jewellery materials. Also, use of lasers for drilling, marking and joining offers further scope for new manufacturing options for the industry.

In summary

In many of the projects described the authors highlight the activities of TWI's Microjoining Section, but increasingly there is interaction with other TWI departments.

Examples include Materials Department support in fracture analysis of soldered joints and, more recently, detailed characterisation of adhesive bonds using differential scanning calorimetry and Fourier transform infrared spectroscopy. The Engineering Department is expanding its activities into the stress analysis and fracture mechanics of microsoldered and microbonded joints for the derivation of joint reliability data.

Finally, and of great importance, is the review and comparison of available non-destructive testing (NDT) techniques by the NDT Department. The number of joints per circuit board can run into thousands and 100% yield is essential, so that there will be greater emphasis on quality control and inspection of micro welded, soldered and adhesively bonded joints.