Nat, I figured after Dave Hillman posted his response that the question would be coming, so I started to look over the situation. 1. Since you have all those PTHs, you really have a mixed technology board, not an SMT board. Is it truly a double sided board or a multi-layer? This can make a difference if we are talking about entrapped flux. 2. Personally, I like LPIs. When I was with Naval Avionics, I cursed dry films and the demon-on-earth that invented them. I have heard from some that there are LPIs which can tent vias, but I don't have any of the formulations which supposedly can do so. Perhaps some of the other TechNet individuals could help in that respect. Use a high quality mask though. One that will withstand at least 4-5 passes to molten solder. Anything less and you risk the situation of a cheap mask being highly absorbing of HASL fluxes (can you say Metal Migration?!). 3. I think you are wise to be concerned about the HASL residues. I have lost track of the number of no-clean assemblers we have helped that have been bitten by HASL residues on the bare boards. Find out from your fabricator what HASL flux or fusing fluid they are using. Is it mostly chloride-activated, bromide-activated, a mixture of the two? What is the cleaning operation? In-line, batch, tap water, DI water.....? That kind of information can better help me determine how likely a fabrication process is to leave detrimental residues. 4. In most cases, HASL flux residues are more in the laminate and mask, the areas between traces, than in the PTHs themselves, although that is a possibility. Your assumption that most of the HASL flux/fluids vaporize during HASL is only partly correct. Part is vaporized, part is absorbed. Usually, the higher the solder mask quality, the lower the absorption. 5. You state that you are using a leave on flux, which I would term a low residue flux or a no-clean flux. Is the flux halide free? I would recommend it be so. Some flux vendors have marketed no-clean fluxes which contain halide elements. Not a no-clean flux in my opinion. 6. Ion chromatography (see IPC-TM-650, method 2.3.28) may be able to do what you are looking to do. Our internal method is much more detailed than the TM-650 method, but still covers most of the board surface, not the individual PTHs themselves. IC is the only method readily available that might be sensitive enough to detect the low levels of corrosive elements. Send me a couple of the boards and I'll see if I can adapt our method to PTH only. 7. Can you share what the assembly will be and what end-use environment the assembly will face? Jungle environments drive corrosion and metal migration far more than office environments. Hope this was helpful. Doug Pauls Contamination Studies Laboratories (CSL)