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Profiling Primer
Are you throwing profits away? If you're not profiling your PCB assemblies, you might want to rethink your assembly process.
Classic printed circuit board (PCB) profiling involves connecting thermocouples from your PCB assembly to a data recording profiling instrument and running the assembly through your reflow oven. Profiling has two primary objectives: 1) determining the correct process settings for a given PCB assembly and 2) verifying process consistency to ensure repeatable results. By viewing the actual temperatures (thermal profile) of a PCB while it travels through the reflow oven, one can verify and/or correct oven settings to achieve optimum quality for the finished product.
Classic thermal profiling of PCBs will ensure optimum, consistent quality of finished PCB assemblies; substantially reduce PCB scrap rates; improve PCB production rates and yields; and improve overall profitability.
The Reflow Process
During the reflow process, heat is applied within the oven to bring the assembly to the proper soldering temperatures without damaging the product. To verify the process of reflow soldering, a profiling device is used to determine process setup. A profile is a visual data log of the time and temperature of each sensor as it progresses through the heating process. By reviewing this profile, one can visually see exactly how much energy, and where the energy, is being applied to the product. The profile allows the operator to make appropriate changes to optimize the reflow process.
A typical reflow profile contains several different stages-- initial ramp, soak, spike to reflow, reflow and cool down of the product. As a general rule, temperature slopes in the range of 2 to 4 deg C are desirable to prevent damage to the board and/or components caused by heating or cooling too rapidly.
During heating of the product, many factors can affect the quality of the assembly. The initial ramp is a quick rise in temperature as the product enters the oven. The objective is to bring the paste to the desired soak temperatures required to begin the activation of the solder. The most desirable soak temperature is just under the melting point of the paste material-183 deg C for eutectic solders, with a soak time of between 30 and 90 seconds. The soak zone has two...