Author: D. R. Frear Company: Sandia National Laboratories Date Published: 4/28/1997
Surface Mount International
Abstract: Package designs for microelectronics devices have moved from through-hole to surface mount technology in order to increase the printed wiring board real estate available by utilizing both sides of the board. The traditional geometry for surface mount devices is peripheral arrays where the leads are on the edges of the device. As the technology drives towards high input/output (1/0) count (increasing number of leads) and smaller packages with finer pitch (less distance between peripheral leads), limitations on peripheral surface mount devices arise [1-2]. A solution to the peripheral surface mount issue is to shift the leads to the area under the device. This scheme is called areal array packaging and is exemplified by the ball grid array (BGA) package. In a BGA package, the leads are on the bottom surface of the package in the form of an array of solder balls. The current practice of joining BGA packages to printed wiring boards involves a hierarchy of solder alloy compositions. A high melting temperature ball (e.g., 90Pb- 10Sn that melts at 300”C) is typically used for standoff. These balls are made by either a spray forming operation or by cutting wire to specific lengths then reflowing them in flux. Both techniques result in a range of solder ball size that requires an expensive sieving operation to produce solder balls of uniform dimensions. The process to create the balls represents a significant fraction of the overall BGA package manufacturing cost, and it is estimated that solder balls alone make up 13% of this cost. A lower melting temperature solder (e.g., 63 Sn-37Pb near eutectic solder) paste is used to join the balls to the package substrate and board. One method of attaching high melting temperature balls to the substrate involves loading a graphite fixture, drilled with holes of the same diameter as the balls, in the desired areal array pattern. The package substrate is patterned with the lower melting temperature solder paste using either a dispensing, screen printing, or stenciling method. The substrate and fixture are placed together so that the balls make intimate contact with the solder paste. After the ball placement operation, the assembly goes through a reflow furnace that melts the solder paste and joins the paste to the balls and substrate. The substrate is cleaned to remove flux residue. Final assembly involves aligning the substrate/ball assembly to the board having the lower melting temperature solder paste on its lands. This assembly then undergoes reflow and cleaning. A promising alternative to current methods is the use of jetting technology to perform monolithic solder ball attachment. This paper describes an meal array jetter that was designed and built to simultaneously jet arrays of solder balls directly onto BGA substrates .