Reflow Process Recommendations for rework of Lead Free QFN on Alum-backed PCB

[pieterhk]

I work in a ASIC repair center where we use hotplates to manually reflow QFNs on aluminum PCBs with a hotplate preheater, high-activity flux and physically reseating chips. Quite fast for a skilled technician but still not ideal. On hotplate we use 260C to get the lead-free solder flowing easily, works very well if total contact time is short, our ChipQuick flux is very temp stable compared to a large variety of Amtech fluxes we’ve tried.

We're looking into use of reflow ovens, we’ve ran a series of tests using a T962 with the ES-Technical mod to get custom profiles and convection for even heating on our large PCBs.

We use the same flux (NC191) in the reflow oven and bump up the temps to make sure we get complete reflow.  The flux is not working very well, seems to evaporate/burn (quite extreme temps to get lead-free reflowing, 280C peak) but contact must be lost to the soldered data pins as PCB’s are coming out with cracked solder. Our profile is 0.8C/s ramp, 180C soak, 280C reflow and 3C/s ramp down.

With hotplate soldering the QFNs don't reflow unless we apply physical pressure on the center of the chips, each QFN has 10 data pins on two sides and Vin/out pads on the bottom, I suspect the reflow is causing the power pads to reflow drawing the solder in together and slightly lifting the whole QFN breaking the digital pin connections. Perhaps heavy use of solder paste may be needed to get a proper reflow.

We’ve tried a little with lower temps but don't have the exact specs on the reflow temps required for this manufacturer’s lead-free solder. It becomes soft at 240C and very low viscosity at 260C, we hit higher temps on hotplate reflow for brief periods to make fast work possible and to overcome the bulk heat capacity of the large aluminum PCB and the heavy voltage rails.

Totally new to reflow, any suggestions for profiles and fluxes that can handle the extreme temps and stay in contact with all the no-lead pins? Are there any reflow fluxes or perhaps heavy oil-based fluxes that could keep a consistent contact with the whole PCB as it runs through the profile?