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16 SMT Magazine • August 2015 possible. Water-soluble pastes tend to be less ro- bust to this condition than no-clean pastes, and water-soluble chemistries should be avoided if possible when HiP risk is a concern. Inert reflow can prevent this condition by preventing the solder bumps from oxidizing during preheat. As the reflow profile is within the control of the assembler in most cases, it is important to ex- periment with the process to determine if HiP for- mation can be mitigated through reflow profile ad- justments. It is important to understand that HiP can be low-occurrence defect, so testing should be performed on a sizeable sample of product in order to increase confidence that process perfor- mance has been improved. It is also important to understand that HiP can be caused by multiple factors, so process alterations can demonstrate improvement without complete elimination of the defect in some cases. Having a well-defined and robust reflow process is an important step to take first when troubleshooting HiP, as the other factors can be difficult to troubleshoot when pro - cess causes confound the solution by remaining an undiagnosed secondary cause of HiP. FeAture THE WAR ON SOLDERING DEFECTS uNDER AREA ARRAY PACkAGES continues land and does not ensure a continuous, reliable electrical connection. Strategies for Prevention of HiP: Process Factors An improperly controlled process can be a significant contributor to HiP defects, particu- larly with the type that form due to an inability of the solder paste and solder bump to coalesce. The reflow profile, if defined in a way that ex- ceeds the limits that a solder paste flux can with- stand, can be a root cause of HiP. An extended preheat length or excessive soak temperature can exhaust the activator in a flux chemistry, thereby preventing the flux from performing its primary function: removing oxides from the surface of a solderable material (in this case, the solder bump). If the solder bump is oxidized, upon reaching liquidus temperatures it will not allow the molten solder deposit to coalesce across the oxide barrier. The robustness of a solder paste to extended and hot preheat phase of a profile is specific to that formulation, but some general guidance is Figure 5: non-wet open formation mechanism (courtesy intel/SMTA) [1] .