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Dynamic Warpage on the Solder Joints of Large Plastic Ball-Grid Arrays
The dynamic warpage of large BGA packages enhances the risk of assembly solder joint defects when the assembly is subjected to temperature incursions during reflow, testing, or service.
Analysis Lab
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Authored By:
Francis M. Mutuku, Ph.D., Dominic Lu, Ph.D., Hongwen Zhang, Ph.D., Huaguang Wang, Ph.D., Tyler Richmond
Indium Corporation
NY, USA
Summary
The dynamic warpage of large ball grid array (BGA) packages enhances the risk of assembly solder joint defects and/or overstressing when the assembly is subjected to temperature incursions during reflow, testing, or service. Low-temperature soldering was thought to effectively reduce the resultant joint defects due to thermal incursions. However, results from this testing shows that a combination of a wide paste range and solidification of the solder joints at low temperatures may coincide with severe dynamic warpage and produce effects even homogenous solder joints might not withstand.
The current industrial supply chain for BGAs is dominated by components populated with tin-silver-copper (SAC) solder balls or their siblings. When a low-temperature reflow profile (<200°C peak temperature) is used, the solder joints built with SAC/low-temperature solder (LTS) are still at high risk of forming malformed joints due to the insufficient molten solder volume and the coincidence of the dynamic warpage with the solder joints solidification.
In such circumstances, mitigating warpage risks on joint formation relies heavily on the solder metallurgy (both BGA and solder paste), the volume ratio of solder pastes relative to the BGA ball, and the reflow temperature. In this study, we investigated the effects of reflow temperature and the paste-to-ball volume ratio (PBVR) on the solder joints formed with the plastic ball grid array (PBGA) 928 package, bumped with SAC305 solder balls, and reflowed on SnBi1Ag paste. Thermal fatigue performance was also studied for 500 cycles (cy) on a -40/125°C thermal profile.
The peak reflow temperature ranged from 185°C to 220°C, while the PBVR varied from 0.14 to 0.5. Results from this test showed that peak reflow temperature plays a major role in the production of quality solder joints in these large PBGA board assemblies. For the reflowed boards with a low PBVR of 0.14, which were reflowed at 220°C, the quality of the resultant solder joints was good. For the boards reflowed at 185°C, the quality of the solder joints was poor, even when the PBVR was increased to 0.5. Combining any PBVR options with a 220°C temperature produced good-quality solder joints.
Any lower reflow temperatures combined with all PBVR had solder joint quality issues, such as head-in-pillow (HiP), non-wet open, open solder joints, large voiding, shrinkage voiding, elongated and necked solder joints, hot-tearing, and phase segregation at the boundary between the LTS and SAC305 ball, especially after thermal cycling for 500 cycles. Although the 220°C reflow temperature is lower than the typical reflow temperature for the SAC family of alloys, the temperature is not low enough to be referred to as “low-temperature reflow” or to attract the benefits associated with low-temperature reflow.
To achieve low-temperature soldering for these large PBGA packages with good quality solder joints, PBVR and alloy metallurgy need to be optimized, bearing in mind that the width of the processing window is small. Alternatively, to reflow at low temperatures successfully, the components and boards must be designed to lower the dynamic warpage that comes with these large PBGA components.
Conclusions
Low peak reflow temperatures and low paste to ball volume ratios have major consequences on the solder joint quality of the PBGA928. Most of the solder joints defects and severity are observed for this set of parameter combination. The following solder joints defects were observed for this package as shown in figure 13 below; Head in pillow, Necking, voiding, shrinkage voiding, hot tear, phase separation and non-coplanarity.
For different packages of different sizes and made of different materials, the processing window of each is unique and has to be customized according to the thermal dynamic warpage of each of the packages. The solder alloy selection has to be in line with this processing window for the best yield results.
For the hybrid solder joint, the paste to ball volume ratio has to take into consideration the processing window and the solder alloy composition. The hybrid solder joint quality can be improved by the choice of the paste to ball volume ratio, the solder alloy composition and an optimized reflow profile.
For the hybrid SAC305/SnBi1Ag, PBGA928 will produce good quality solder joints with paste to ball volume ratios ranging from 0.14 to 0.5 if the peak reflow temperature is kept at 220oC. However, for the 0.5 PBVR, a low melt phase is observed, pasty range is wide, solder joints are necked, large voids are seen and possibly issues of co-planarity may arise. 0.14PBVR exhibits better solder joints quality; drum shaped or spherical healthy solder joints, and a small pasty range.
Initially Published in the SMTA Proceedings
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