Following 10 years developing gas shielding arc welding techniques on aluminium alloys at the research laboratories of the British Aluminium Company, Mike joined the British Welding Research Association in 1967. He spent two years as an experimental officer in the Arc Welding Department before transferring to the School of Welding Technology as Senior Welding Instructor.
In 1978 Mike transferred to the Process Operation and Control Department to carry out process development on AC square wave and MMA welding and AC MIG welding. This was followed by investigations of pulsed MIG welding on very thin section aluminium and very thick section titanium alloys. On completion of a study of orbital TIG welding on stainless steel pipe Mike transferred to the Plastics Joining Section.
Over the past three years he has worked on friction, hot plate and ultrasonic welding of thermoplastics with the emphasis on process development. Current studies relate to encapsulation of electronic devices using ultrasonically welded thermoplastics in the automotive industry.
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 'Interconnention and packaging techniques for electronic devices' which covers wire bonding, tape automated bonding, large area die attatch, solder joint 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.
Using ultrasonic welding to encapsulate electronic devices can avoid problems found with conventional packaging techniques. Steve Riches and Mike Murch explain.
Plastics packaging of electronic devices has developed along two main routes: transfer injection moulding has been used for integrated circuit packaging while encapsulating resins have been used mainly to protect chips on circuit-board substrates (chip on board) or tape automated bonded (TAB) devices. However, both techniques can give rise to problems in packaging advanced electronic devices where increased chip size and reduced package size may cause problems with device functionality, cracking of the package/resin and failure due to moisture penetration.
Thermal stresses
In integrated circuit manufacture, dies are attached to leadframes, wire bonded and then transfer injection moulded with a thermosetting resin. Stresses are imposed on the chip and leadframe by the encapsulating resin which generally has a thermal coefficient of expansion greater than ten times that of the chip or leadframe. Thermal mismatch stresses are most severe during soldering operations where the whole package is exposed to temperatures of 215-260°C, and may be exacerbated by the presence of moisture vaporising during soldering.
Stresses may cause passivation cracking and aluminium conductor slide at the interface between the silicon and the resin. The stresses at the interface between the leadframe and resin in conjunction with the presence of moisture may lead to package cracking and swelling.
To alleviate the chip stresses, developments have focused on modification of the resin structure to reduce the thermal coefficient of expansion and modulus of elasticity, or on use of a stress-absorbing layer ( eg polyimide) between the moulding compound and the chip. Steps to eliminate the package cracking problem have included reducing the moisture absorption of the moulding compound, improving the adhesion strength between the leadframe and encapsulating resin or improving the high temperature flexural strength of the encapsulant. This latter factor is achieved by increasing the density of cross linkages in the organic resin and incorporating silicone dispersions into the resin structure.
Use of encapsulating resins in hybrid circuits, chip on board and TAB applications, ( Figure 1), poses similar problems to those of transfer injection moulding because of the stresses imposed on the devices by the contact of the encapsulating resin with the device.
Non-contact encapsulation
Enclosure of integrated circuits by thermoplastic lids and bases, or localised lidding of areas of circuit boards, has inherent advantages over transfer injection moulding and encapsulation (glob topping) as there is no contact between the plastics encapsulant and the chip or interconnect system. This eliminates problems caused by stresses on the die or leadframe. There also may be advantages in small batch production where a common lid and/or base could be used with a number of devices including those mounted on leadframes and circuit board substrates. This approach would make possible flexible manufacturing, reducing the lead time between design and production of finished components without incurring the tooling charges and delays associated with custom mould production.
Enclosure or localised lidding of electronic devices by thermoplastic materials ( Figure 2), can be performed by ultrasonic welding where the heat generated during welding is confined to projections on the lid and/or base structure. Ultrasonic welding is a widely used technique for joining plastics where high frequency vibrations of 20-40kHz are employed to generate heat by a combination of surface and intermolecular friction. Ultrasonic welding is fast - typical weld times are less than a second - and may be automated easily. The ability to weld a plastics component using ultrasonics is governed by the design of the welding equipment, the properties of the material to be welded, the component design and the welding parameters. The major process variables have been identified as ultrasonic vibration amplitude, applied force, time, near-field or far-field operation, and component joint design. The welding horn also needs to be tuned with the ultrasonic welding equipment to transmit vibrations in the optimum manner.
The feasibility of using ultrasonic welding to produce non-contacting encapsulation using thermoplastic materials has been demonstrated at TWI on leadframes, epoxy/glass and polyimide printed circuit board, polyimide tape (to house TAB devices) and thermoplastic substrates. This work showed that a number of thermoplastic materials (polyarylamide, aromatic polyester and polyphenylene sulphide) could be ultrasonically welded to encapsulate leadframes where joints could withstand internal water pressure of up to 15bar. Ultrasonic welding of the thermoplastic lids to epoxy/glass printed circuit board and polyimide circuit board and tape was also achieved.
For the thermoplastic/epoxy-glass circuit board joint, the thermoplastic material had flowed around the conductor tracks without damage and joined to the thermosetting epoxy resin.
Future work
Results from these trials are promising enough to justify a detailed programme of work. This will be launched soon as a Group Sponsored Project intended to provide details of materials, techniques and properties for industrial use. For further information please contact Steve Riches or Norman Stockham.
