During the past decade flip chip technology has emerged as a popular alternative to wire bonding for interconnecting semiconductor devices such as integrated circuit (IC) dies and microelectromechanical systems (MEMS) to substrates such as printed circuit boards, other interconnect boards and other dies. 
“Flip chip,” is also known as “controlled collapse chip connection” or its acronym, “C4.”  With flip chip technology, solder balls/bumps are attached to electrical contact pads on one face of the die/chip.  The dies are usually processed at the wafer level, i.e., while multiple identical dies are still part of a large “wafer.”  Solder balls are deposited on chip pads on the top side of the wafer.  The wafer is sometimes “singulated” or “diced” (cut up into separate dies) at this point to provide a number of separate flip chip dies each having solder balls on the top face surface.  The chips may then be “flipped” over to connect the solder balls to matching contact pads on the top surface of a substrate such as a printed circuit board on which the flip chip is mounted.  Solder ball attachment is usually provided by reflow heating. 
Sometimes the wafer is further processed after solder bumping and while still in wafer form to connect it to another large substrate, such as a second wafer, which may comprise for example another flip chip wafer or a wafer of another type of die.  Regions on the first wafer corresponding to later formed dies are attached to corresponding areas on the second wafer.  Thereafter individual die on die units or “flip chip assemblies” are singulated from the attached wafers.  In this situation the other substrate usually has interconnect structure on the face thereof opposite the face that is connected to the flip chip die.  This other side face may then be connected to another substrate such as a printed circuit board.  As IC dies have become more complex, the number of solder bumps/balls on flip chips have increased dramatically.  Whereas in the past the solder balls were usually provided by relatively large round solder balls attached to the chip contact pads, more recently copper pillars (“CuP's”) have been used in place of the solder balls.  A CuP is an elongated copper post member that is attached at one end to a contact pad on the die.  The CuP extends outwardly from the die in a direction perpendicular to the face of the die.  Each CuP has a bullet shaped solder piece attached to its distal end.  The CuP's are soldered by this bullet shaped solder piece to corresponding contact pads on a substrate as by reflow heating.  CuP's are capable of being positioned much more densely, i.e., at a “higher pitch,” than conventional solder balls/bumps.