Structure and method for stress reduction in flip chip microelectronic packages using underfill materials with spatially varying properties
View Patent ↗A structure for a flip chip package assembly includes: a flip chip die with solder attach bumps; a substrate for receiving and solder attaching the flip chip die; an underfill material with spatially varying curing properties applied to fill voids between the flip chip die and the substrate, and for forming a fillet around the perimeter of the flip chip die and extending to the surface of the substrate; and wherein the portion of the underfill material forming the fillets is cured prior to curing the portion of the underfill material that fills the voids between the flip chip die and the substrate.
1. A method for utilizing an underfill material in a flip chip package assembly, the method comprising:
applying an underfill material with spatially varying curing properties to fill voids between a flip chip die and a substrate of the flip chip package assembly, and for forming a fillet around the perimeter of the flip chip die and extending to the surface of the substrate; and
curing the underfill material forming the fillets prior to curing the portion of the underfill material that fills the voids between the flip chip die and the substrate.
2. The method of claim 1 , wherein the portion of the underfill material forming the fillet is cured at a temperature T 1 , which is lower than the temperature T 2 for curing the underfill material that fills the voids between the flip chip die and the substrate.
3. The method of claim 1 , wherein the portion of the underfill material forming the fillet is ultraviolet (UV) cured, and the underfill material that fills the voids between the flip chip die and the substrate is heat cured.
4. The method of claim 1 , wherein the portion of the underfill material forming the fillet is cured with infrared (IR) radiation, and the underfill material that fills the voids between the flip chip die and the substrate is heat cured.
5. The method of claim 4 , wherein shields are used to concentrate and direct the IR radiation to selectively heat and cure the fillets.
6. The method of claim 4 , wherein cold plates are placed above the flip chip die, and below the substrate in the proximate footprint of the flip chip die, to dissipate any heat related to the IR radiation.
7. The method of claim 1 , wherein the underfill material obtains its spatially varying properties through a first application of a first underfill material to fill the voids between the flip chip die and the substrate, and a second application of a second underfill material to form the fillets; and
wherein the first underfill material and the second underfill material are formed from different chemical compositions.