Ultrasonic welding of thermoplastics encapsulates electronics

Ultrasonic welding of thermoplastic lidded enclosures around electronic devices on lead frames and printed circuit board material has been demonstrated as a potential technique for encapsulation and protection. The technique may overcome stress problems associated with traditional transfer injection moulded and liquid resin encapsulated devices caused by the continuing increase in Si chip size and reduction in package thickness.

Problems with contact encapsulation

In transfer injection moulded and liquid resin encapsulated electronic devices, stresses are imposed on the Si chip and substrate due to the encapsulant having a coefficient of thermal expansion greater than 10 times that of the chip or substrate. The stresses are most severe during soldering and may be exacerbated by the presence of moisture vaporising which may lead to package cracking and swelling.

Non-contact encapsulation by ultrasonic welding

The enclosure of devices by thermoplastic lids eliminates contact between encapsulant, integrated circuit and interconnect systems and reduces stresses. The localised lidding of electronic devices by thermoplastics can be performed by ultrasonic welding where no general heating is required and joints can be made in less than one second on equipment which may be easily automated.

Protection for chip-on-board components

The feasibility of using ultrasonic welding to produce non-contacting encapsulation with thermoplastic materials has been demonstrated at TWI on epoxy/glass and polyimide printed circuit boards. This work showed that the thermoplastic material flowed around the conductor tracks without damage and had joint to the epoxy/glass circuit board.

Potential benefits for the electronics industry

If this technique can be developed successfully it should improve device reliability through the elimination of stresses caused by contact between the encapsulant and device. The technique should also improve the cost effectiveness of the packaging system in certain applications as a standard lid could be used to encapsulate components on printed circuit boards, lead frames and flexible circuits. It may be particularly suited to the protection of small batches of different components. Copyright © 2004 TWI Ltd