3. Commonly seen reflow soldering defects' relationship with profiles (only based on consideration of reflow process)

I.C.T is a manufacturer of SMT machines. It mainly provides customers with SMT production lines including SMT Stencil Printers,  Pick and place machines, Reflow Oven, AOI Machine, Wave Soldering Machine and PCB Handling Machine etc. I.C.T has more than 25 researches on SMT and DIP technology,  for the world Customers provide SMT total solutions. There are successful cases of SMT technICal team in Asia, Europe, AmerICa, AfrICa, and Australia.

Bridging (short circuit) is caused by solder's hot melting down which only happens in the soldering stage when temperature is below the melting point. Due to molecules' thermal motion, viscosities of the materials with fixed compositions and chemical structures rise along with the fall of temperature, viscosity fall in higher temperature causes more serious hot melting down. Temperature rise, on the other hand, often causes flux to release more solvent and this leads to the increase of solid contents and thus viscosity rise. Due to the fact that the former is only related to temperature and the latter, total solvent's reduction, is a function of both time and temperature, so under the known temperature, solder paste's viscosity with lower temperature rise rate is higher than that with higher temperature rise rate. So, we require that temperature rise rate is generally lower in the preheat phase, thereby reducing the occurrence of short- circuit.

Tin sweats: Tin sweats are formed this way. Along with the process of volatile solvent being removed from the solder paste in preheat stage, solder paste gasification occurs. If the cohesive force among metal powders in solder paste is smaller than the force created by the gasification, a small amount of solder paste will leave the soldering pad and some of them may hide under the chip components. When reflow soldering, these solder paste will melt and then be forced out from under the sheet-type resistor-capacitor units, thus forming the tin sweats.

It can be seen from its formation process that the higher the preheat temperature is and the faster the preheat speed is, the more seriously will the solder paste be splashed during gasification, so the more easily tin sweats are formed. At the same time, the higher the temperature is, the faster the solder oxidation will be. The oxide film on the surface of solder powder can prevent solder powder from melting together well, thus forming tin sweats. But, this phenomenon can be controlled effectively by using proper preheat temperature and warm-up speed.

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