TBGA bond process for ground and power plane connections

Abstract

General construction of TBGA developed at LSI Logic Corp. is discussed. Ground/Power plane connections are typically provided in double metal tape for Tape Ball Grid Array(TBGA) by using plated vias for connections. A unique process which allows ground connections for TBGA package in ASSEMBLY, rather than in TAPE is discussed. It further discusses a novel way to provide power plane connections also during ASSEMBLY of a TBGA package, instead of providing it in the tape. Providing such interconnections in the assembly makes the product more cost effective and TBGA tape becomes more manufacturable. Both equipment and process for these unique "down bond" technique are discussed. Such interconnection technique is currently being used in manufacturing at LSI Logic Corp.. .0 INTROB UCTIONTape Automated Bondmg (TAB) offers higher electrical performance and yet has not been a resounding success story in the IC packaging arena. One primary reason for lack of general acceptance for TAB has been its incompatibility with the surface mount technology. Custom equipment are needed to excise an inner lead bonded TAB package before it can be mounted to a board. Also, custom equipment and process are needed to perform outer lead bonding (OLB) to the board. In other words, TAB can not be surface mounted and reflowed in same manner as other surface mount components on the board.Tape Ball Grid Array (TBGA) answers the need of an advanced package to be compatible with other surface mount components, while keeping intact all the advantages offered by TAB. TBGA employs an Inner Lead Bonded tape (lLB) in the same manner as TAB. However, similarity ends with the ILB. Solder balls are provided for outer connections on the TBGA tape. These solder balls can be reflowed at the same time as other surface mount components on a board.TBGA eliminates the need for custom equipment and process for OLB required by TAB so the TBGA package is more cost effective for the board assembly house.Application Specific Integrated Circuits (ASIC) designs demand higher electrical and thermal performance from an IC package. TBGA can handle frequencies in excess of 600Mhz and lead counts as high as 800. TBGA is often used, if not always, in applications requiring high performance. Higher performance can be achieved by providing separate ground plane and sometimes even separate power plane The conventional method for providing a separate ground plane is to use a double metal layer tah. Signal layer and ground layer are then connected with the help of plated through vias. Accuracy of alignment of both layers and proper plating of the v i a s are among the critical criteria which determine the yield of the double metal layer tape.Typically, complexity in the tape due to double metal layer drives down the yield in tape manufacturing. Consequently, dou6le metal layer tape s sigmficantly more expensive than single metal layer tape. 1 Any additional metal layer, such as a separate power plane, make the tape both more complex and more expensive. Key for the success of any...