My first concern would be the choice of solder paste you have. There are better chemistries out there that far exceed the capabilities of the 609
2. Yes, the part will partially float on the solder (assuming plastic qfp). Cross sectioning would show a gap between the base metal and the lead form. The difference is made up of intermetalic layers and tin/lead solder.
3. 2 of 1000 errors. Thats 2000 defects per million, 4.4 sigma process or a yield of 99.8 % If this is the only issue you are having you are doing quite well. This is a typical failure on QFP devices. Lead planarity is and always will be an issue. If the design could be migrated to BGA your yields would be even better.
4. If it was a solderability issue a more aggressive flux might be able to power its way through the contaminent. My experience with ws609 was very poor. Low stencil life, clogged aperatures....and so on. Evaluated many different solder paste...ended up recycling 12,000 dollars in (un-opened)solder paste inventory because the cost of defects due to solder paste issues was many times greater than the cost of the solder paste. This was 4 or 5 years ago...maybe its better now?
5. More aggressvie flux will not solve your lead planarity issue. Test / inspection can "cirlce the wagons" around the problem in the short term. If this is high reliability applications I would push back on the IC house. You may end up with "special" restrictions on lead form and trim requirements...undoubtedly this will cost you with increased component pricing. Most typial responses are test and inspection.
6. You did not mention the lead finish. You may want to look into this. How old are the parts? Grey market parts or new? Makes a diference...especiall, with the choice of solder paste.
Hope this helps. Keep us posted on your findings.
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