SMT007 Magazine

SMT-Aug2015

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August 2015 • SMT Magazine 15 As the PCBA reaches the liquidus tempera- tures of the solders, the solder paste will coalesce together into one large solder deposit. When this occurs, the package drops as the liquid sol- der is displaced by the mass of the package on the solder bumps. When the bumps and the paste have both reached a temperature where they are liquid, the separate solder volumes normally coalesce together and the package drops again. Upon cooling, the resulting solder connection has a characteristic shape with a flattened top and bottom (at package and PCB interfaces, respectively) and a rounded shape at the edges. One mechanism of HiP formation is a fail- ure of coalescence to occur between the solder paste and the solder bump. This prevents the joining of the solder volume into a single con- nection. Although the solder volumes share physical contact, they do not form a permanent connection and exhibit marginal electrical per- formance and no mechanical strength. Optimi- zation of soldering process and materials can be effective at eliminating this type of HiP defect. A second mechanism of HiP formation is also a primary driver of NWO defects. Both defects can be a result of warpage causing a co-planarity mismatch between the area array package and the PCB. This warpage can be de- scribed as having a "smile" or a "frown" warp- age profile between the package and the PCB. When a smile warpage occurs, the primary loca- tion for HiP and NWO defects to occur is closer to the edges of the package, where the package lifts away from the PCB. When a frown warpage occurs, the primary location for HiP and NWO defects is in the center of the package—again, where the package and the PCB have separated from each other during reflow. Mitigation of defects caused by warpage conditions is more difficult to troubleshoot and mitigate than pure coalescence failures. HiP occurs when the two solder volumes reach liquidus when they are not in contact with each other due to the relative warpage mismatch of the assembly. Upon cooling, the dissimilar warpage of the assembly relaxes and the solder volumes come in to contact with each other for a short period before solidify- ing. This can allow for the molten solder bump and solder paste deposit to displace each other without coalescing together. Upon solidifying, the bumps form the namesake head (package bump) in pillow (solder paste deposit) feature as they rest against each other. This contact does not ensure a consistent NWO defects also occur due to dissimilar warpage between the package and the PCB. This defect forms when the printed solder paste deposit has significantly more affinity for the solder bump than the PCB land prior to reflow. When the warpage mismatch separates the bump and land, the solder paste can remain in contact with the bump and reflow away from the PCB land surface. The solder paste and the package bump coalesce together, forming a larger bump at the package surface. The co- alesced larger bump will form a spherical shape due to the surface tension of the molten solder. When the assembly cools, the dissimilar warp- age relaxes and the bump typically comes to rest against the PCB land. The bump may form a small flat against the land or retain its spheri- cal shape. This connection is not wetted to the THE WAR ON SOLDERING DEFECTS uNDER AREA ARRAY PACkAGES continues FeAture figure 3: Typical area array solder connection cross-section. Figure 4: "Smile" and "frown" warpage profiles.

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