Indium Solder Hermetic Seal Voiding

Thanks both of you for responding. I am using a protoype reflow oven. It uses a programable controller to ramp the temp up. It has convection and radiative heating. Ultimately yes, I would like to use a vacuum furnace like the sst but it is not cost prohibited. There are vendors who can solder these assy's without those systems. The oven is N2 capable, and that is one step I intend to take but was not sure if would help with the voids. The larger voids I am seeing have some residue in them, as if it is the remaining flux. If I went to N2 would it eliminate the voids, or would it allow me to lower the flux activity which in turn would reduce voids? We use forming gas here for some processes, but it creates a safety issue for us, so I would prefer to not go there. When I get the N2 installed, I will try your suggestions.

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Mario,

I tried a couple of profiles. In all cases the preheat ramps fairly slow, while the reflow utilizes the radiative heater to speed to reflow as quick as possible. I tried a quick profile and a slower one. The quick has about 15-20 sec at preheat which is 15C below reflow temp (I measure all temps with a TC on a sample) and reflow dwells about 15C above reflow temp for approx 30 sec. Then uses a rapid cool down. I tried another profile that preheated for 60 sec and dwelled at reflow for 60 sec. I used a slow cool down. Got the same results for voiding. I use a Kester RMA flux pen which I later found out from the vendor "approaches RA fluxing". I also did an RTS profile, very fast and hot. Same results, strong joint but lots of voids. I am thinking because this is a flux pen, it may be for rework, and the flux may like faster reflows. Anyway, I also tried Indium Corp. RMA flux which did not wet well at all. So I am going to order Indium Corp. RA flux and cut it with the solvent to see if the right flux helps. Since you work for Indium Corp. I was wondering if you had a profile that works well with the R fluxes and In80 solder? I have heard of pre-fluxed solder pre-forms, does this help?

Overall, I believe I need to get rid of the voiding to create a reliable hermetic joint. Leak testing does not seem to be the best test because the leak is convoluted. I would like to be able to reduce voiding to a minum and use x-ray to monitor the process.

Thanks

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