ABSTRACT The fundamentals of reflowing solder balls for the BGA ball attach process is discussed. The content is divided into three major sections: modes of heat transfer, convection technologies used in reflow ovens and mechanics of temperature profiles. There are three basic modes of heat transfer: conduction, convection and radiation. All three modes are utilized in achieving desired temperature profiles. For most of the reflow ovens in the industry, forced convection is the dominant mode of heat transfer. Understanding the modes of heat transfer assists process engineers in troubleshooting reflow applications. In the reflow oven industry, there are two basic methods of achieving forced convection: fans and pressure sources. Typically, fans are used to achieve convection and recirculate process gas. Examples of pressure sources are compressors and nitrogen tanks. Benefits of both methods are discussed. In convection heat transfer, temperature and speed of the gas provide quantitatively different effect in heating the product. Adjusting the temperature difference between the process gas and the product is a more effective method of controlling heat transfer. In general, higher gas speed or volumetric flow rate improves the temperature uniformity across products. There are four different elements in reflow profiling: preheat, dryout, reflow and cooling. Profiling is no longer a ‘shoot in the dark’ task. There is a definite procedure in performing temperature profiling. Understanding the relationship among heater set point temperatures, desired profile temperature, gas speed and conveyor speed, one can achieve desired temperature profiles with improved efficiency and accuracy. Understanding and practicing the fundamentals of the reflow process for the BGA packaging can enhance the productivity and lower operational cost.