The current applications of semiconductor arts show the use of bumped solder interconnect structures, such as Controlled, Collapse Chip Connection (C4) technology as well as Evaporated, Extended Eutectic (E3), for connections to ceramic Ball Grid Arrays (BGAs). Ceramic BGAs (CBGAs) are known in the industry as an efficient apparatus for packaging semiconductor devices having large numbers of external interconnects such as input and outputs. FIG. 1 illustrates assembly of a CBGA. As illustrated in FIG. 1, a CBGA is formed by combining a ceramic substrate 120 and a semiconductor device 112 with solder interconnect structures 144, 162. The ceramic substrate 120 includes chip pads 122, which connect the ceramic substrate 120 to the semiconductor device/solder structure 110, and BGA pads 124, which connect the ceramic substrate 120 to BGA spheres 152 as illustrates in FIG. 1.
The chip pads 122 and BGA pads 124 are generally metallic and are electrically connected through the ceramic portion 121 by interconnect vias not (illustrated). The semiconductor device 110 comprises a semiconductor substrate 112 and bump solder structures 114. The bump solder structures 114 can have several different structural forms. One such structural form would be a Controlled, Collapse Chip Connection Technology (C4) bump as known in the semiconductor industry. C4 bumps are known to have high temperature melting properties. For example, it is common for such structures to have a 90% , or greater, lead content by weight with the remaining 10%, or less, being tin. In such a composition, the melting point needed during a manufacturing process is in excess of 300.degree. Centigrade. The actual process for forming BGAs with semiconductor devices attached is illustrated with reference to FIG. 2, and discussed in conjunction with the illustrations of FIG. 1.
FIG. 2 shows, in a flow diagram, a CBGA assembly process in accordance with the prior art. A prior art process 200 of FIG. 2 connects the semiconductor device 110 to the ceramic substrate 120. Next, a process 210 connects the resulting device of process 200 to BGA spheres. Process 200 comprises a step 201 for placing the semiconductor device in contact with the semiconductor substrate. As illustrated in FIG. 1, the semiconductor device 110 is placed in contact with the ceramic substrate 120 such that the bump solder structures 114 are aligned with the chip pads 122, and in contact with a flux 118. The semiconductor device is held in place by a small positive force generally on top of the substrate 112 which is not shown in FIG. 1. In addition, the flux 118, which was applied over the chip pads 122 of the ceramic substrate 120, provides a degree of adhesion. The purpose of the flux, as is well known in the art, is to reduce oxides and enhance "wettability" of the bump solder structures 114 to the chip pads 122. However, where C4 processes are being used, the flux 118 must have properties allowing its use in a high temperature environment.
Next, at step 202, the semiconductor device 110 and ceramic substrate 120 combination is applied to reflow step 202. During the reflow step 202, the semiconductor device 110 and ceramic substrate 120 are exposed to a high temperature furnace such that the solder bump structures 114 reflow to the chip pads 122 to form a semiconductor device 140 having a physical and electrical connection between the bumps 114 and the pads 122. The temperature needed to perform this reflow process is approximately 360.degree. Centigrade to assure proper melting and reflowing of the solder bump structures 114.
Next, at step 204, a batch cleaning process is implemented. The batch reflow process requires that the devices from step 202 be removed from their carriers and placed in a carrier appropriate for the batch cleaning process. During the batch cleaning process 204, the residue flux 118 remaining from the reflow process 202 is removed. The cleaning step 204 not only removes contaminants which may remain, but cleans and prepares the bump solder structure surfaces for a subsequent underfill step to assure appropriate reliability. Due to safety and environmental requirements and regulations, the chemicals and solvents used in the cleaning process are often a low level flux dissolver or reactant, in that they react slowly to remove the residue remaining from step 202. As a result, the total amount time required to perform an effective cleaning of the structure 140 is increased to the point where it is a significant time factor in the processing of the semiconductor device 140.
Following the reflow and cleaning of the semiconductor device 140, an underfill is dispensed between the semiconductor substrate 112 and the ceramic portion 121, as shown at step 206. Next, at step 208, the underfill is cured 208 to provide additional support and reliability. This curing process is generally accomplished through a temperature cycle process. The dispensed underfill surrounds the resulting attached bump solder structures 144 and their associated interconnections with the chip pads 122. The underfill dispensed serves to provide enhanced reliability by eliminating environmental effects on the interconnections, as well providing mechanical support for the interconnections. However, while providing additional mechanical support, the underfill has a limitation in its use with low temperature solder bump structures 114.
When low temperature bump solder structures 114 are used, the underfill dispensed will prohibit further expansion of the solder bumps during further processing steps. The effects of expansion of low temperature structures 114 is more pronounced than those of high temperature structures. Solder expansions have been shown to cause reliability issues as a result of the hydrostatic forces of solder material between the underfill and the semiconductor substrate 112 interface, or the ceramic portion 121 and underfill interface.
The previously discussed steps 201 through 208, which form process 200, are performed in order to attach the semiconductor device 110 to the ceramic substrate 120, and require the use of a high temperature reflow of the solder bump structures 114. In addition, where low temperature eutectic materials are used, such as with a plating or screened process, a flow similar to 200 is used. Whether using a high temperature version of bump solder structures 114 or a low temperature version of solder bump structures 114, the process of FIG. 1 is substantially the same, and requires the same cleaning processes.
In order to complete the BGA package, BGA spheres 152 need to be attached to the underside of the ceramic structure pads 124. A sphere loading process takes place. During the sphere loading, the BGA spheres 152 are placed into a boat 150. The boat 150 acts as a holder for maintaining the BGA spheres at precise locations that align to the BGA pads 124. Next, screen printing is performed 212. During screen printing, a solder paste or joining material 154 is applied over the BGA spheres 152. The solder paste generally comprises a combination of solder, flux, and other elements to form a ready-to-use mixture. Next, the semiconductor device 140 is placed in contact with the boat 150, sphere 152, and paste 154 combination 214. Again, positive pressure is applied to the semiconductor device 140 to maintain appropriate contact with the boat 152 and BGA sphere combination. Next, a sphere attach low temperature reflow process is performed. The placed structure is reflowed, resulting in the semiconductor device 160. The solder paste 154 has been chosen to be a low temperature solder to interface the spheres 152 to the pads 124. The use of a low temperature reflow process allows the integrity of the spheres to be maintained. Assuring integrity of the spheres allows a uniform height across the BGA to be maintained, thus allowing for subsequent placement on printed circuit boards. Next, the spheres 152 are cleaned, 218, to assure integrity of connectivity later.
As discussed previously, in order to assure the integrity of the connections of semiconductor device 110, the ceramic substrate 120, and the resulting semiconductor device 140's attachment to the BGA spheres 152, a two processes (200 and 210) need to be performed. Both require duplicate placement equipment, reflow equipment, flux cleaning underfill dispense machinery. In addition to the equipment, there are costs associated with operation and facilities to be considered. Therefore, a CBGA process that would allow for lower manufacturing costs would be desirable.
USPTO Backgrounds
USPTO Backgrounds