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Indium Solder Hermetic Seal Voiding

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#46571

Indium Solder Hermetic Seal Voiding | 9 January, 2007

I am soldering a Germanium Window with a metalized rim to a Ni-Au plated kovar lid. The solder joint needs to be hermetic. I am using 80In solder washer shape preform, but am experiencing lots of voiding when x-rayed, although the joint is sealing. I have to use Rosin flux for cleaning purposes, but could go to an acid flux that can be easilly cleaned. Any suggestions for fluxes or profiles when making a hermetic seal of this type?

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#46582

Indium Solder Hermetic Seal Voiding | 10 January, 2007

Brian,

There are a couple of ideas that you can use for this.

Slowing the ramp is the first one that comes to mind. I would try soaking at about 10 degrees "C" below the solidus of the alloy for about 45-60 seconds. Slowing the ramp to remove the volatiles from the flux.

Swaping fluxes to a less active flux or a less-voiding flux, and a few others.

Have you tried moving to a flux coated preform? Or, even reflowing without flux?

I would be happy to go over several other ideas...

Mario Scalzo mscalzo@indium.com 315-853-4900

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#46584

Indium Solder Hermetic Seal Voiding | 10 January, 2007

> I am soldering a Germanium Window with a > metalized rim to a Ni-Au plated kovar lid. The > solder joint needs to be hermetic. I am using > 80In solder washer shape preform, but am > experiencing lots of voiding when x-rayed, > although the joint is sealing. I have to use > Rosin flux for cleaning purposes, but could go to > an acid flux that can be easilly cleaned. Any > suggestions for fluxes or profiles when making a > hermetic seal of this type?

I once used 60/40 In/Pb NC solder paste for soldering to gold bumped flip chips. Used the In/Pb solder to avoid gold imbrittlement. I found that I had to use nitrogen to produce good solder joints when using In/Pb NC solder. I also had to get the ROL (residual oxygen level) to around 5 ppm in order to achieve good solder joints. An ROL of 20 ppm did next to nothing.

Are you using forming gas (N2/H2 90/10)for your lid attach process? Are you using a lid attach type oven or a typical SMT reflow oven? Something like a Sikama oven or an SST International oven would help out. You might be able to do flux free soldering in forming gas with these ovens.

Chris

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#46588

Indium Solder Hermetic Seal Voiding | 11 January, 2007

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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#46589

Indium Solder Hermetic Seal Voiding | 11 January, 2007

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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#46595

Indium Solder Hermetic Seal Voiding | 11 January, 2007

BE,

Flux coating helps as there is no solvent to volatize. Also, we offer vacuum-cast preforms that will eliminate the possibility of the voids comimg from the alloy itself.

I can email a PDF of some profiles, all I need your email address. You can send it to me offline...

Thanks, Mario

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#46601

Indium Solder Hermetic Seal Voiding | 11 January, 2007

The N2 would allow you to use less flux since the oxidation level will be reduced in an oxygen free environment.

How about using adhesive to attach the lid?

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#46609

Indium Solder Hermetic Seal Voiding | 11 January, 2007

it's a vacuum dewar so adhesives will eventually permeate and the vacuum would be lost. Sounds to me since I have the N2 capability, I definately should try it. I am obtaining Indium Flux and Solvent so I can adjust the activity when I start using N2.

Thanks B

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