What is reflow soldering?
Reflow soldering is a process whereby electronic components are connected, both electrically and mechanically, to a printed circuit board (PCB). The process begins by applying solder paste to the PCB in a specific pattern using a purpose-built stencil printer. The paste consists of a metal alloy suspended in mixture of solvents and other materials. The board is then heated by use of a reflow oven according to the specification for the solder paste being used, including heating/cooling ramp rates, time above liquidus and peak temperature (max/min).
Reflow ovens come in many lengths and varying numbers of zones allowing for more throughput (boards/min) depending on production needs. The quality of a reflow oven is generally measured by the thermal uniformity and repeatability. The thermal uniformity is measured by the “deltaT” which is the difference between the hottest and coolest thermocouple (TC) as measured on a test board processed through the reflow oven. Process with the lowest deltaT are superior to those with higher deltaTs – as this allows all components to stay within process specifications throughout the entire process and with stand normal process variations. Process repeatability is also key to a good reflow soldering process. Repeatability can be evaluated in terms of boards that are processed within one reflow oven or SMT line, or within the context of an entire factory or even within the context of global manufacturing output.