benign after the soldering process. Typically no-clean fluxes are especially sensitive to process conditions, which may make them undesirable in some applications. Other kinds of flux, however, require a cleaning stage, in which the PCB is washed with solvents and/or deionized water to remove flux residue. Finish and quality Quality depends on proper temperatures when heating and on properly treated surfaces. Solder types Different combinations of tin, lead and other metals are used to create solder. The combinations used depend on the desired properties. The most popular combinations are SAC (Tin(Sn)/Silver(Ag)/Copper(Cu)) alloys for lead-free processes and Sn63Pb37 (Sn63A) which is a eutectic alloy consisting of 63% tin and 37% lead. This latter combination is strong, has a low melting range, and melts and sets quickly (i.e., no 'plastic' range between the solid and molten states like the older 60% tin / 40% lead alloy). Higher tin compositions give the solder higher corrosion resistances, but raise the melting point. Another common composition is 11% tin, 37% lead, 42% bismuth, and 10% cadmium. This combination has a low melting point and is useful for soldering components that are sensitive to heat. Environmental and performance requirements also factor into alloy selection. Common restrictions include restrictions on lead (Pb) when RoHS compliance is required and restrictions on pure tin (Sn) when long term reliability is a concern. Effects of cooling rate It is important that the PCBs be allowed to cool at a reasonable rate. If they are cooled too fast, then the PCB can become warped and the solder can be compromised. On the other hand, if the PCB is allowed to cool too slowly, then the PCB can become brittle and some components may be damaged by heat. The PCB should be cooled by either a fine water spray or air cooled to decrease the amount of damage to the board. Thermal profiling Thermal profiling is the act of measuring several points on a circuit board to determine the thermal excursion it takes through the soldering process. In the electronics manufacturing industry, SPC (Statistical Process Control) helps determine if the process is in control, measured against the reflow parameters defined by the soldering technologies and component requirements. Products like the Solderstar WaveShuttle and the Optiminer have been developed special fixtures which are passed through the process and can measure the temperature profile, along with contact times, wave parallelism and wave heights. These