![]() ![]() The ramp rate is the slope of temperature versus time for the heating portion of the reflow profile. The preheat phase preconditions the PCB assembly prior to actual reflow, removing flux volatiles and reducing thermal shock to the PCB assembly. The SMT reflow profile can be broken down into 4 phases or regions: preheat, pre-reflow, reflow, and cooling. A soak profile displays some “plateau” within a limited temperature range, before the alloy reflows. A ramp-to-peak profile is a linear ramp to the peak (max) temperature. The two most common types of reflow profiles are the ramp-to-peak profile, also called ramp-to-spike or tent profile and the soak profile, or ramp/soak/spike profile. This paper will discuss techniques to optimize the reflow profile to minimize such defects. The resulting surface oxide can lead to a number of solder defects such as voiding, beading/balling, graping, and head-in-pillow. Both the small solder paste deposits and smaller particle size result in a large surface area-to-volume ratio that challenges the solder paste’s flux to effectively perform its fluxing action. Decreased pad size might also require the solder paste to have solder powder with smaller particle diameters. Not only are the electronics components and the PWB at risk due to the higher reflow temperatures associated with lead-free processes, the components themselves can restrict the peak temperature that the process can use, making it difficult to achieve a robust solder joint, especially if the PCB is thermally massive. The introduction of higher lead-free process temperatures and a reduction in solder paste deposit volumes require narrower process windows to optimize the reflow profile. Key words: solder defects, reflow profile, tombstone, solder beads, solder balls, voids, head-in-pillow, graping. It also discusses trouble-shooting of the most common defects in lead-free reflow, such as tombstoning, solder beading/balling, residue discoloration, voiding, graping, and head-in-pillow. This paper is a summary of best practices in optimizing the reflow process to meet these challenges of higher reflow temperatures, smaller print deposits, decreased powder particle size, and their affect on the reflow process. Smaller components are also more susceptible to tombstoning and defects related to solder paste slump. The possible resulting surface oxidation can lead to voiding, graping, head-in-pillow, and other defects. Both the small solder paste deposits and small particle size result in a large surface area-tovolume ratio that challenges the solder paste’s flux to effectively perform its fluxing action. In addition, the constant miniaturization of electronic components, hence smaller solder paste deposits, may require the use of smaller particle-sized powders. Not only are the electronic components and the PWB at risk, but the ability to achieve a robust solder joint becomes difficult, especially if the PCB is thermally massive. The combination of higher lead-free process temperatures, smaller print deposits, and temperature restraints on electrical components has created difficult challenges in optimizing the reflow process. ![]()
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