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7-9 October 2009, Leuven, Belgium Warpage (µm) 500 400 300 200 Heating curve Cooling curve (+) Warpage Z-height (µm) 1600 1400 1200 1000 800 Sample @22°C after 168h in 85%-85°C chambre Dried sample @22°C 100 600 0 0 50 100 150 200 250 -100 (-) Warpage Temperature (°C) Fig. 6: Warpage measured by INSIDIX equipment during a reflow process (red curve is when temperature goes up, the blue curve refers to the cooling down of the package). The warpage during cooling is very similar to the warpage measured during heating. This indicates that the deformation is fully reversible and elastic (no plasticity neither creep effects). D. Measurement result for a “REF” sample Warpage (µm) Although the “REF” samples haven’t been stored in dry package and could take up moisture for more than one year, it has a very similar warpage vs. temperature behaviour as the dried one. With the “REF one, there is a delta warpage of about 50µm after the solder reflow, which could indicate that the absorbed moisture cause additional pressure during the heating up, resulting in a small permanent deformation after the reflow process. 500 400 300 200 100 0 0 50 100 150 200 250 -100 -200 (-) Warpage "DRY" (+) Warpage "REF" Temperature (°C) Fig. 7: Warpage measured by INSIDIX equipment during a reflow process for “DRY” and “REF” sample (see description in Table 1). E. IMPACT of moisture on warpage profile Even after being in the moisture oven (168h in 85% humidity, 85°C), the initial warpage is already much different from the dried and reference sample, as shown in Fig. 8. The additional bowing is caused the expansion of the overmould due to the moisture uptake. 400 200 0 0 10 20 30 40 50 Distance from one corner (mm) Fig. 8: Z-profile of a dried sample and a sample which has been in the moisture oven for 168h (both at room temperature) It was also found that there is no difference between “WET1” and “WET2” conditioned samples. Probably, in both conditions, the mould materials are fully saturated by moisture. The warpage measurement trend in a reflow profile for the “WET” samples are shown in Fig. 9 and compared with the “DRY” and “REF” samples (the graphs show the average of 5 measured samples). The “wet” sample is a clrealy different story. Due to the moisture uptake, the component has already an initial warpage of 200µm, which is caused by expansion of the overmould due to moisture uptake. After the reflow process, the warpage does not come back on the same curve, but will return to warpage , even with different sign. This is caused by a strong plastic deformation, strengthen by the vapour pressure. Warpage (µm) REF DRY WET 700 600 500 400 300 200 100 0 -100 0 50 100 150 200 250 -200 (-) Warpage -300 Temperature (°C) (+) Warpage Fig. 9: Measured warpage during a reflow temperature profile (from room temperature to 230°C and back) ©EDA Publishing/THERMINIC 2009 115 ISBN: 978-2-35500-010-2
III. FINITE ELEMENT MODELLING OF BGA WARPAGE A finite element model is built for this package (Fig. 11). Non-linear material properties are applied both for the overmould and BT laminate, in order to be able to include the T g for both materials. 7-9 October 2009, Leuven, Belgium Out-of-plane deformation (µm) 350 300 250 200 150 100 Experiment FEM - no expanding strain FEM - 0.05% strain FEM - 0.1% strain FEM - 0.15% strain 50 0 0 5 10 15 20 25 Distance to center over the diagonal (mm) Fig. 12: Simulations of warpage due to moisture uptake (vs. experiment) IV. CONCLUSIONS Warpage has been successfully measured using the INSIDIX equipment. For the particular PBGA package, a significant impact of moisture uptake and the Tg of the overmould has been found. It was shown that FEM can be used to estimate warpage of large package under reflow conditions. Fig. 10: 3D Finite Element Model simulating the thermally induced mechanical deformation during a reflow process Warpage (µm) 500 400 300 200 100 0 0 50 100 150 200 250 -100 -200 DRY Temperature (°C) FEM Fig. 11: Comparison between FEM analysis and measurement of the warpage of the 35by35 mm 2 PBGA The authors would like to thank Veerle Simons at IMEC for her support in the measurements. This work has been also supported by the ELIAS project, funded by the IWT and MEDEA+. REFERENCES [1] IPC/JEDEC Joint Industry Standard, J-STD-020C, “Moisture/Reflow Sensitivity Classification for Plastic Integrated Circuit Surface Mount Devices”, July 2004. [2] B.T. Vaccaro, I.I. Shook, E. Thomas,J.J. Gilbert, C. Horvath, A. Dairo and G.J. Libricz, PBGA package warpage and impact on traditional MSL classification for Pb-free assembly, SMTA International conference, 2005. [3] R.L. Shook, J.J Gilbert, E. Thomas, B.T. Vaccaro, A. Dairo, C. Horvath, G.J. Libricz, D. L. Crouthamel, and D.L. Gerlach, “Impact of Ingressed Moisture and High Temperature Warpage Behavior on the Robust Assembly Capability for Large Body PBGAs”, Proceedings of Electronic Components and Technology Conference, 2003, pp. 1823-1828. [4] Richard, I.; Fayolle, R.; Smyth, A.; Lecomte, J.-C, New experimental approach for failure prediction in electronics: topography and deformation measurement complemented with acoustic microscopy, EuroSimE 2005, pp 305 – 310. [5] M. Hertl, D. Weidmann, and J-C. Lecomte, Process Optimization: Influence of Heating and Cooling Rate on the Thermo-Mechanical Stress Generated in Components, EMPC2009, June 15th – 18th 2009, Rimini, Italy . With the same simulation model, the out-of-plane deformation over the diagonal of the package was calculated for different expansion strains. Comparing the results with the experiment, it was found that the moisture uptake by the overmould causes an expansion of about 0.12%. ©EDA Publishing/THERMINIC 2009 116 ISBN: 978-2-35500-010-2
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International Workshop on THERMal I
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