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Finish and quality


Quality depends on proper temperatures when heating and on properly treated surfaces.
DefectPossible causesEffects
CracksMechanical StressLoss of Conductivity
CavitiesContaminated surface
Lack of flux
Insufficient preheating
Reduction in strength
Poor conductivity
Wrong solder thicknessWrong solder temperature
Wrong conveyor speed
Susceptible to stress
Too thin for current load
Undesired bridging between paths
Poor ConductorContaminated solderProduct Failures
Solder types
Effects of cooling rate
Thermal profiling
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 and Sn63Pb37 (Sn63A) which is 63% tin, 37% lead. The latter combination is strong, has a low melting range, and melts and sets quickly. 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. 
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 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 fixture combined with analysis software allows the production engineer to establish and then control the wave solder process.

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