The present disclosure relates to a method for bonding substrates, and particularly, to a flip chip assembly apparatus employing a warpage-suppressor assembly, and a method of operating the same.
Connections employing an array of solder material portions, such as C4 balls or any other type of solder balls, are susceptible to mechanical stress created by a mismatch in the coefficients of thermal expansion (CTE's) between the semiconductor chip and the other semiconductor chip or the packaging substrate. Such mechanical stress may cause cracks in the solder material portions, back-end-of-lines in chips, or the semiconductor chip(s), causing the semiconductor chip(s) to fail during flip chip assembly process and/or usage.
The problem of mechanical stress caused by the mismatch between CTE's are exacerbated when an organic substrate is employed for a packaging substrate because the mismatch of CTE's is greater between organic substrates and semiconductor substrates than between ceramic substrates and semiconductor substrates. When an organic substrate is used as a packaging substrate for a fine pitch flip chip assembly, substrate warpage can occur in the conventional reflow process during which solder balls reflow. This warpage can result in non-wetting of solder bumps and/or bridging between solder bumps, thereby decreasing the assembly yield.
In general, organic substrates expand and contract more than silicon chips. For example, a silicon chip has a CTE of about 2.6 p.p.m./° C., and an organic substrate has a CTE of about 17 p.p.m./° C. Such a mismatch between CTE's can create thermally-induced stress and strain in a bonded flip-chip structure during the flip chip assembly process. Thermally-induced stress and strain in the flip-chip structure during a reflow process often results in a failure of back-end-of-line (BEOL) interconnect structures.
When a semiconductor chip is bonded to an organic substrate, a predominant fraction of the total heat energy employed to reflow the solder balls is transmitted through the semiconductor chip. Even if the organic substrate is maintained below 100 degrees Celsius and the heat flow from the semiconductor chip into the organic substrate is constricted at the chip area during the reflow of the solder balls, the thermal expansion of the organic substrate is sufficient to cause warpage of the organic substrate. Such warpage of the organic substrate tends to cause non-contacts between peripheral solder balls within the array of solder balls and bonding pads on the organic substrate, thereby causing electrical opens between the semiconductor chip and the organic substrate across the peripheral solder balls. Thus, there exists a need to provide a reliable flip chip joining method that can provide reliable solder bonding despite inherent warpage issues due to the thermal expansion of the organic substrate.