Use of the area-of-spread method for monitoring the stability of reflow furnaces

In this paper, we investigate the use of the area of spread (AOS) method for the assessment of wetting parameters which can be used as a guide to the solderability of PCB pad surfaces in reflow soldering. In the AOS method, the wetting (and hence solderability) of the PCB pad surface is assessed by measuring the area over which a given mass of fluxed solder spreads on a sample PCB pad at a given temperature (in excess of the solder melting point) and in a given time. Although the reflow soldering technique has been around for quite a number of years, and has been used in the assembly of surface mount devices, there are very few methods available for the on-line monitoring and assessment of solderability. There are a large number of off-line solderability assessment techniques, such as the solder dip method, the wetting balance method and the globule balance method. Although these methods are quite useful for process characterisation, they are not suitable for monitoring the process stability of the reflow furnace over time. This paper discusses the use of an area-of-spread experiment as a means of measuring the stability of the reflow process using statistical process control (SPC) methods. It is expected that this methodology will be applicable for process control and monitoring activities to expedite the development and efficient operation of reflow soldering systems. An improved method of measuring the area-of-spread of solder paste is used in the experiment from which quantitative results can be produced for reflow oven control data.