Modeling of the Infrarec Retlow Process for Solder Ball Connect (sbc)

A thermal model of the infrared reflow process has been developed for an FR-4 card populated with an array of Solder Ball Connect (SBC) modules. The analysis of the threedimensional, transient, finite element model accounts for radiative exchange within the infrared oven and for the heat conduction (nonisotropic) within the modules and card. Transient temperature profiles of selected points and three-dimensional temperature distributions at selected times are presented to describe the primary heat-transport mechanisms. Numerical predictions and empirical data indicate that the SBC modules are relatively isothermal throughout the infrared reflow process. Therefore, every solder ball within the array exhibits a nearly identical thermal profile. This result is fortunate, since the inner solder ball connections cannot be visually inspected. The influence of module spacing and the ability to improve the reflow process by use of a highemissivity cap coating are demonstrated. Introduction Solder Ball Connect (SBC) is an area array surface mount technology (SMT) in which a ceramic substrate containing one or more chips (a “module”) is connected to an FR-4 card by the use of an array of high-temperature-melting 90%Pb/lO%Sn solder balls and eutectic solder [l]. Interconnection between the card and the SBC module (as well as other surface mount components, in general) is typically accomplished with an infrared (IR) reflow process. In this process, a printed circuit card with solder paste applied to its pads and with modules placed thereon is passed through an infrared reflow oven. The primary purpose of this reflow process is to melt the solder paste, wet the surfaces to be connected, and solidify the solder into a strong metallurgical bond [ 2 ] . The reliability of this bond is inherently dependent upon the thermal profile the bond experiences throughout the IR reflow process. A radiation-dominated IR reflow oven is shown schematically in Figure 1. Since the oven is nearly 100 times as long as the spacing between the top and bottom heater panels, Figure 1 is not drawn to scale. A conveyor belt, moving at constant velocity, is employed to transport the card populated with modules through the oven. Forty