There is no problem associated with running Lead-free HASL boards using leaded paste. Nihon Superior, the patent holder on the Tin/Nickel/Copper (Sn100c) alloy has done some reliability studies mixing leaded and SN100c alloys. The results are available from NS or either of their licensee’s, FCT Assembly and AIM.

There is considerable test data available on this one of the sites is Vitronics Soltec under 5 steps to lead free.

Concern should be raised and confirmed on the site when soldering BGA in reflow and with the wave pot temp at 240 deg C. Incompatible materials should be avoided, However in the wave the movement of the wave will remove /replace most of the alloy on the surface In reflow that will not be the case.

The lead-free finish on the boards sounds like a SAC type alloy. This is a metallurgical issue which has been discussed since the introduction of backwards and forward capability soldering with lead-free solders and leaded solders.

To get a good alloy formation of both solders, the minimum temperature has to be higher than the liquidus temperature of the highest melting alloy. In this case the soldering temperature should be in the vicinity of at least 250C for the reflow and 255 to 260 for the wave. At this point the fluxes in the leaded solder may cause some soldering issues as it is probably going to break down and not be effective in preparing and protecting the parts for the soldering operation.

Many people are working this issue but I would check with Werner Englemeir as he has conducted many reliability trials with these materials. His information is used quite extensively in the IPC documents for soldering and solder joint design.