Conventionally, semiconductor devices in the form of semiconductor dice or integrated circuits are housed into packages. A package serves various important functions such as protecting the device from mechanical and chemical damage. It is also a bridge that interconnects the device with a next level of packaging. Die attachment is one of the steps involved in the packaging process during which the die is placed on and attached to a die pad formed on the carrier or substrate. There are various methods for attaching the device onto the die pad, such as by using epoxy and adhesive resin as an adhesive to stick the device onto the pad or stamping flux on the pad and placing a die with solder on its back surface onto the flux before performing a solder reflow process.
An increasingly popular approach is to directly mount a die with a back surface of the die coated with solder onto a heated substrate. The solder melts when it comes into contact with the heated substrate, and a bond is formed to the substrate. This method is conventionally termed as eutectic die bonding, since the solder on the die is usually made from a composition of eutectic alloy. Eutectic die bonding takes advantage of the lower melting point of eutectic alloys as compared to pure metals. The temperature of the substrate should be raised to above the melting point of the solder on the back surface of the die so that the solder melts immediately when the device is in contact with the die pad. When the substrate is subsequently cooled down, a metallurgical bond will form between the back surface of the die and the pad on the substrate. Some advantages of eutectic bonding over epoxy bonding include a higher service temperature capability for the die, good thermal/electrical conductivity between the die and the substrate and higher reliability.
When packaging devices like light-emitting diodes (“LED”), a non-metallic material such as a plastic housing may be present adjacent to the die pad for facilitating certain mounting functions. The plastic housing normally has a glass transition temperature of lower than 280° C., with a typical recommended process temperature of less than 260° C. With the global trend being to adopt lead-free solders in eutectic bonding, one dilemma emerges that constantly frustrates equipment manufacturers and packaging process engineers. Currently, the most popular lead-free solder comprises a Sn—Ag or Sn—Ag—Cu compound which has a melting temperature of around 220° C. As the process temperature should normally be 30 to 40° C. higher than the melting temperature of the solder, it is difficult in practice to find a process window that ensures effective die-bonding while preventing the plastic housing from overheating. An example of another type of packaging presenting such a problem is a composite substrate consisting of polymer and metal. The glass transition temperature for the polymeric part of the substrate is even lower, typically in the range of 180˜230° C., and it is a problem to prevent the polymeric substrate from overheating when performing eutectic die-bonding.
A prior art heating system for eutectic die-bonding makes use of a heat tunnel system consisting of several heating zones. In each heating zone, there is a heating block embedded with several heating elements for heating a carrier or substrate. As the substrate is transported through the heat tunnel, heating is performed on the substrate and the temperature of a considerable portion of the substrate is made to rise to the temperature necessary for eutectic bonding to take place, even though only the die pad which receives the die should preferably be heated to the said bonding temperature. The other parts of the substrate are preferably not heated or should receive less heat. The problem is that due to a much lower thermal conductivity of plastic material, the temperature on the plastic housing or some part of the polymeric substrate could be even higher than that of the die pad. In such circumstances, the temperature of the plastic material may be higher than its glass transition temperature. As a result, the plastic housing or the polymeric substrate may be deformed or damaged.