1. Field of the Invention
The present invention relates generally to the surface mounting of ceramic leadless chip carriers to printed wiring boards. More particularly, the present invention relates to methods and apparatus for increasing the reliability of the solder joint integrity between the printed wiring board and the chip carrier.
2. Description of Related Art
The surface mounting of electronic devices, such as ceramic leadless chip carriers, has become popular due to the ability to save weight and volume in electronic equipment when such surface-mount technology is used. In addition, the need for increased circuit density makes surface mounting of chip carriers even more desirable.
Typically, the ceramic leadless chip carrier is mounted to the printed wiring board surface by way of solder fillets or joints. It is important that the integrity and reliability of the solder joint be maintained for long periods of time and be able to withstand numerous temperature and power cycles. In addition, the solder joint must be able to withstand shock and vibration which may occur in many electronic systems such as those used in avionics.
The increasing popularity of surface mounting of ceramic leadless chip carriers has brought with it concerns over the reliability and integrity of the solder joints between the chip carriers and the printed wiring board. Cracking and other disturbances to the joint integrity have been noted. Joint cracking is believed to be due mainly to differences in the thermal coefficients of expansion between the printed wiring board and the chip carriers. Various different approaches have been taken to reduce the possibility of solder joint cracking. These approaches have included making the printed wiring board out of materials having thermal coefficients of expansion which are close to the thermal coefficient of expansion for the ceramic leadless chip carriers. In addition, attempts have been made to replace the solder joint with a joint which is less likely to crack, such as electrically conductive epoxy compounds. Although the various approaches used to date have experienced different degrees of success, there still is a continuing need to provide a method for surface mounting ceramic leadless chip carriers to printed wiring boards wherein the possibility of solder joint cracking is eliminated.