SMT007 Magazine

SMT-Aug2015

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14 SMT Magazine • August 2015 fect with similar symptoms to HiP and NWO is insufficient solder paste volume transfer during printing. In this case, it can appear that no wet- ting has occurred to the land and mimic NWO (especially if no paste has been transferred). If a small amount of paste has been printed, the resulting connection can initially appear to be consistent with HiP. Troubleshooting of HiP and NWO defects should include steps to en- sure the solder paste printing process is properly controlled and performing well. Another defect that can be confused with HiP is cold solder, which is characterized by a lack of coalescence of the solder paste deposit. HiP is a defect that occurs in the presence of a well-defined and controlled reflow process, which ensures coalescence of the solder paste deposit. Diagnosis of a defect as HiP should in- clude an examination of the reflow profile to ensure that the process is not at risk of causing the occurrence of cold solder. Non-wetting to a PCB land has many causes that should be familiar to most with experience troubleshooting solder defects. These defects can easily be confused with NWO since both defects share a symptom: poor wetting of a co- alesced solder bump to the PCB land. The key difference is that NWO defects result in a solder bump that is spherical along the PCB side. A de- fect that is solely caused by poor PCB solderabil- ity will generally demonstrate the same shape as a typical area array solder connection: flatter and wider than a sphere and generally sharing the contour of the land along that interface. Testing the solderability of the PCB lands is an important step when attempting to determine if a wetting defect is a result of NWO defects. Area Array Soldering and Defect Formation Mechanisms During the soldering process, area array packages can appear to "drop" twice during a full reflow cycle. Each of the apparent drops by the package corresponds to an occurrence of coalescence of solder during the reflow process. The HiP and NWO defects represent the result of uncontrolled variation during one of those coalescence events. THE WAR ON SOLDERING DEFECTS uNDER AREA ARRAY PACkAGES continues FeAture figure 1: Head-in-pillow defect example, cross-section and endoscopic views. figure 2: Head-in-pillow defect example, X-ray computer tomography view (courtesy nordson Dage).

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