You didn't say what sort of soldering iron you are using. To solder pads to ground planes:
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Use a temperature controlled soldering iron with a heat output of around 50W or more, not a constant-power soldering iron (typical heat output of those is 18 to 25W).
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The bit on the soldering iron needs to have a large enough flat area to conduct sufficient heat to the pad. The fine bit that you probably use for soldering the other pins of the chip won't conduct enough heat when soldering pads to ground planes.
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Tin the flat area of the bit first to aid heat conduction.