LMZ31707: LMZ31707 Produced issue

Hi,

After SMT, measure the PCBA, found LMZ31707 some pins are short,   VIN to Vout or VIN to GND.  2 pcs in 10pcs at the first lot, 9 pcs in 40pcs at second lot.

So took down LMZ31707 form PCBA, measure chip and PCB, there are not issue in both, and then solder on PCB, the failure again, and the chip never work.

So, could you help me,

1, What is the probable cause chip short and how to avoid?(PVIN,VOUT and PGND are very close )

2,  Are secondary welding there any  announcement？

Thanks。