gas for extending solderpaste print-to-place time?

[WayOutWest]

Okay, I know this is a long shot and maybe this isn't the most appropriate forum, but I didn't really know where else to ask.

Is there some sort of gas (nitrogen?) in which solderpaste-printed PCBs can be stored that will extend the amount of time allowed between printing and placing? Obviously the boards would be refrigerated too.

I'm pretty sure a vacuum is a bad idea; the main goal here is to keep the flux activators, adhesives, and solvents from evaporating, and a vacuum would just accelerate that process.

I have a somewhat complex solderpaste printing arrangement because of the extremely high-aspect ratio apertures needed for LGA mounting. I can't use my favorite solderpaste (Loctite GC10) for this job -- it has an insane abandon time (manufacturer says 24 hours, in practice I've gone four days with no degradation) but it jams up inside the stencil apertures very badly if you're pushing the limits on area ratio. And it's hard to clean out of thick stencils -- GC10 is strictly for 0.100mm stencils, no exceptions. I also need a lower reflow temperature for the first pass through the oven, and GC10 doesn't come in a PbSn version. So I have to use Kester EP256, which is an outstanding solderpaste, but it has a very very very short process window -- quality starts to degrade very quickly after being exposed to air for only 4-8 hours. It's great paste, but very "high maintenance" during all the steps up to reflow.

Anyways the setup and cleaning time for my printing setup is really killing throughput here... I could probably print the entire job (which is huge) in one day if I focused on it, but it'd take at least a week to finish placing all the boards even assuming everything goes 100% smoothly during placement (which it never does). Right now I only print as many boards as I'm sure I can place in one day, and I'm spending way too much time setting up and cleaning up the stenciling situation. I also have a somewhat elaborate ventilation system since I've become sensitized to the flux fumes, and that adds startup/shutdown overhead as well.

Any ideas?

[Picky]

I have played with lots of different paste varieties but never actually tried to maximize the abandon time. My guess will be based on general knowledge of basic physics. You can try a few things in addition to refrigerating it. First is to minimize contact with oxygen by as much as possible. Purging with nitrogen will probably help. Second is to pressurize the airtight vessel to minimize evaporation. Then also try to saturate the atmosphere (nitrogen) in the vessel with volatile components of the same paste. For example, smear some healthy amount of fresh paste somewhere inside the vessel and let it "dry" after you close the lid. Basically you want to replicate the conditions inside the jar of paste. It does dry up a little bit after you close the lid, but quickly saturates the small volume of air under the lid and evaporation stops.

[Mark Harris]

Build/buy a screen printing machine to print on demand?



I don't think anything much in the solder reacts with oxygen, the real issue is the flux drying out as the solvents evaporate. Perhaps check the MSDS of the paste and find what the solvents might be - then store the boards in a chamber with a pool of that solvent at the bottom to keep the vapour level high enough that the solvent does not want to evaporate from the paste.

Refrigeration is really your best bet though, it may be worth some experimentation to see how low you can go on the temperature, you may be able to get well below 0C which would dramatically increase your life.

[Pixopax]

For cleaning the stencils, I soak them in Acetone and then blow compressed air trhough them. Makes them clean in seconds!