Hi, we're currently running a design with a fairly simple BGA. The problem we're running into is poor solderability (cold looking joints, high voiding in the BGA). All of this on a know good oven, with a known good thermal profile.
I'm guessing it's due to the mis-match in surface finish and process alloy. I used to run into a similar issue in a past life with defense and aero parts moving to RoHS while still needing to be processed with 63/37 paste, and I know how to overcome those issues. I've never had it the other way around.
Any pointers or ideas on how to get some workable PCBAs out of this debacle?
What type of flux is in your solder paste? Active or No-Clean? With lead free solder you really should be using an active flux. The lead free oxidizes during reflow if you are not using a Nitrogen or Vapor Phase reflow. So a more robust and active flux will help over come this with an open atmosphere reflow process. We have been installing lead free BGA's with Amtech NWS-4400-HA-TF tacky paste flux for over 12 years now with pretty much 100% yield. By not using solder paste, you should get a void free solder joint. However we have found some brands of BGA's that have a defective copper finish. If you desolder the BGA pads and look at them under high magnification. You may see speckled area's of copper where the solder will not wet. I think this is from the copper not being properly cleaned or possibly contaminated in the fab process. This will cause a void where the solder does not wet to the pads.