1.8 Chips and Bonds

• The surface of the wafer is cut along the boundary lines between the chips. 
• Each chip is then attached to a metallized substrate (chip carrier), which is part of the final assembled component or package
• Heating is involved in the attachment (bonding of chip to substrate) since alloying or soldering processes are used and the metallized substrate must be chosen to have the same linear thermal expansion coefficient  as silicon to prevent thermal fracture
• The linear thermal expansion coefficient is defined as the rate of fractional change of length l with temperature T
• Silicon : 2.6 x 10-6 per degree Celcius (room temperature)
• The chip circuit-to-package connections are made by gold (Au) wires that are bonded to the pads (aluminum layers that are deposited during the metallization and patterning process) and to the metal lines on the package frame
• The fabrication of the package imparts mechanical stresses in the wire bond and pull tests (a 5g weight attached to the wire ) can be used to ensure good interface bonding
• Other technique: place solder balls on the aluminum pads that coated with thin films of chrome and copper-tin to provide a reliable joint. 
• The use of solder balls allow the connection to the copper leads on the chip carrier be made without wire bonds connection