Chip underfill in flip-chip technologies

A semiconductor structure fabrication method, comprising:

• providing a semiconductor structure which includes (a) a substrate, (b) a chip which includes N chip solder balls, wherein N is a positive integer, wherein the N chip solder balls comprise a solder material, and wherein the N chip solder balls are in electrical contact with the substrate;
• after said providing is performed, forming a first corner underfill region, a second corner underfill region, a third corner underfill region, and a fourth corner underfill region which are respectively at a first corner, a second corner, a third corner, and a fourth corner of the chip, and which are sandwiched between the chip and the substrate, wherein each corner underfill region of the first, second, third, and fourth corner underfill regions is in direct physical contact with at least one chip solder ball of the N chip solder balls;
• and
• after said forming the first, second, third, and fourth corner underfill regions is performed, forming a main underfill region sandwiched between the chip and the substrate, wherein the first, second, third, and fourth corner underfill regions, and the main underfill region occupy essentially an entire space between the chip and the substrate, and wherein a corner underfill material of the first, second, third, and fourth corner underfill regions is different from a main underfill material of the main underfill region.