ELECTRICAL PERFORMANCE AND RELIABILITY OF A TBGA WITH A GROUNDED STIFFENER
Author: R.D. Schueller Company: 3M Electronic Prdcts Division Date Published: 4/28/1997
Surface Mount International
Abstract: As an increasing number of complex functions are integrated into individual die, the number of i/o increases as do the requirements for better electrical and thermal performance. Packages such as the ceramic pin grid array and ceramic ball grid array are capable of meeting these requirements but at a fairly high cost. Various forms of tape ball grid array packages have been developed to satisfy these requirements in a more cost effective manner. In many instances, a single metal layer TBGA is adequate to meet the needs and is the lowest cost method of doing so. For the higher speed applications in which a separate ground plane is required, the natural tendency is to use a two metal layer flex circuit to create a TBGA, however, this can be fairly costly. The strategy at 3M is to create a lower cost version of a two metal system by utilizing the stiffener as the ground plane. A simple deposition process is used to fill vias with a 10/90 Sn/Pb solder paste which is then reflowed to the surface of the copper stiffener. The via locations are placed next to the desired ground balls and connected to them with a copper trace. The resulting structure has been proven reliable through the various environmental tests standard in the industry. Electrical measurements and modeling have shown this package to have very low inductance and cross talk properties due to the close proximity of the grounded stiffener to the circuit traces. This package has, therefore, been deemed suitable for many high speed applications.