Apparatus and method for bonding silicon wafer to conductive substrate
A system and method is disclosed for bonding a substrate to a semiconductor die that is prone to curling when subjected to an elevated temperature in a solder reflow oven, for example, thereby improving the electrical and mechanical bonding for large dies, wafers, chips, and photovoltaic cells. In one embodiment, the substrate is adapted to curl to the same degree as the die to form a uniform gap between the substrate and die across the boundary there between. In another embodiment, solder used to bond the die and substrate is applied such that the volume deposited varies based on the expected gap between the die and substrate when heated to the melting temperature of the solder.
1. A method of bonding a plurality of semiconductor dies to a substrate, each of the dies susceptible to curling when subject to an elevated temperature; the method comprising the steps of:
providing a semiconductor die having a center and a plurality of side edges;
providing a substrate;
applying to the substrate a layer of the solder paste; wherein the layer of solder paste comprises a plurality of beads, wherein the volume of each bead is selected based on the magnitude of a gap between the substrate and die at the point of application for a temperature substantially equal to the reflow temperature of the solder paste; wherein the volume of said beads increases from the center of the die to the plurality of side edges; and
heating the substrate with solder paste and semiconductor die to the melting temperature of the solder paste to couple the die to the substrate;
wherein the semiconductor die comprises an upper layer and a lower layer proximal to the substrate; wherein the semiconductor die curls above 160 degrees Celsius due to thermal expansion mismatch of the upper layer and lower layer.
2. The method of bonding in claim 1 , wherein the semiconductor die is a photovoltaic cell.
3. A method of bonding a semiconductor die to a substrate, the die exhibiting curling due to coefficient of thermal expansion mismatch; the method comprising the steps of:
applying a plurality of beads of a solder paste to either the substrate or semiconductor die, the plurality of beads comprising a first bead at a center of the semiconductor die and a second bead at an edge of the semiconductor die; wherein a volume of the second bead is greater than a volume of the first bead; wherein the plurality of beads further comprise a third bead interposed between the first bead and the second bead; wherein the volume of the third bead is greater than the volume of the first bead and less than the volume of the second bead;
assembling the substrate and semiconductor die with the plurality of beads interposed between the semiconductor die and the substrate; and
heating the substrate, beads of a solder paste, and semiconductor die to a solder reflow temperature of the solder paste to couple the die to the substrate.
4. The method of bonding in claim 3 , wherein the semiconductor die is a photovoltaic cell.