Hermetic Sealing

Wandering through the references to indium metal on the internet, I sometimes see it referred to as, "that 'rare' metal."  But is it really so rare?  I recently talked to my colleague, Claire Miko, Director, Metals and Chemicals for Indium Corporation and asked if the reports of the rarity of the metal (like the death of Mark Twain) were greatly exaggerated.

Question:  The element indium is widely used today in many electronic (glass coating, low temperature solder, hermetic sealing and thermal interface material) and solar applications (CIG solar panels), but very little is known about it.  Can you tell us where indium metal comes from?

Claire:  Indium is a by-product of several base metals such as zinc, lead, copper, tin and other poly metallic ores. It is very abundant on the crust of the earth (much more than silver for example and the annual silver production is at least 40 times bigger than the annual indium production). Geographically indium is abundant in South America, Canada, Australia, China and the CIS, i.e. the reserves are widely spread.

 
Question:     Does indium have to be refined after it is mined?

Claire:    Indium is present in the base metal ores at ppm levels. It first needs to be separated from the base ore and concentrated. This is done at the base metal smelter (for example during the refining of zinc, lead, copper, tin etc). It is then further refined and purified at indium refineries.

Question:  Indium Tin Oxide (ITO) is the one of largest indium-containing products today.  How much of the indium mined goes to making ITO?

Claire:   About 50% of the indium refined is used for making ITO. A larger percentage is needed to start the ITO target productions but the sputtering process used (when putting the ITO layer onto the glass) is inefficient and generates a large quantity of indium which is reclaimed and is then recycled and put back into circulation.

Question:     Is there enough indium available to meet the current and future needs of the marketplace?

Claire:   The indium production has always expanded to meet growing demand. Indium production grew from 70MT (metric tonnes/year to over 500MT/year over the last 20 years. At the moment only one-third of the indium mined yearly is being refined in indium metal, another third accumulates in residues that are more expensive to treat but they remain available for future processing, and the last third is currently lost because it does not reach a base metal smelter which has the equipment to separate it from the base metal ore. Investments at these smelters would enable the extraction and refining of these quantities if the need arose.

Question:    Are there recycling programs in place to recover unused ITO from the targets used to deposit it onto the glass surfaces where it is used?  What is the rate of recovery?

Claire:   There is ample capacity to treat spent ITO targets (as per point 3) and the recovery process is now mature and very efficient. The cycle time of this process has also now become very short enabling a very quick return of the refined indium for new consumption.

Question:    Are there any viable alternatives to ITO?

Claire:   A far as we know ITO remains the best material for LCD and other flat panel displays applications. It offers the best performances in terms of optical transparency, electrical resistivity, uniformity of both transparency and resistivity, chemical and mechanical stability, resistance to corrosion, and, finally, uniformity of etching.

The cost of the ITO on 42” TV represents less than $2 and less than 1% of the display cost. It is a small cost to pay to ensure that the quality of the display is maintained. Alternative materials have shown significant process problems with resistivity, uniformity and chemical and mechanical stability.

• It stays pliable at cryogenic (-150C) temperatures.  This means that indium seals that are exposed to these cold temperatures will not crack or fail like other metals will.
• Indium is great for bonding non-metallics such as glass, quartz and certain ceramics.
• Since metals expand at different rates, this mismatch can cause a bond to break during heating and cooling.  Indium compensates for these differences.
• Indium cold welds to itself so you can apply an indium coating to two surfaces like glass, quartz or certain ceramics that cannot be soldered and cold weld them together.

If you look for indium on the periodic chart, you will see that it located right by tin (Sn) and lead (Pb) but it is a world away in terms of its properties.

Indium and indium alloys have some unique characteristics that make them ideal for a variety of usages including: soldering to non-metals, low temperature alloys, RoHS compliance, thermal management, battery manufacturing, cryogenic or hermetic sealing and many, many more.

Does your application require you to bond to glass, quartz or ceramic?  Then you know that traditional solders will not work.  But if you choose Indalloy #4 (pure indium) or Indalloy #1E (52In 48Sn) you will get excellent wetting.  If you choose an alloy that includes silver (like Indalloy #290 which is 97In 3Ag or Indalloy #3 which is 90In 10Ag) you will get slightly less wetting but a much stronger solder joint.

Surface preparation along with the proper tools and the proper process are key to acheiving the proper bond.  You can get all the details with our PDS, Bonding Non-Metallic Materials Using Indium and High Indium Alloys.
 

Today (March 3rd) I start my 25th year at Indium Corporation.  It seems like just yesterday that the minor league hockey team I was doing PR for folded and I was forced to look for a new job.

I knew after the first week that I had made a huge mistake.  Going from a world of writing and promoting and ticket selling to day after day of science and chemistry and math couldn't be right for me.  But I needed the job so I stuck it out.  For 24 years so far!  I'll bet I am not the only one with that story to tell.
And it turned out that the move was right for me.  Because Indium is a great place to work with great, smart people but also because every day is different from the day before.  I knew nothing about Indium or indium (the metal) before I started here (other than they were one of the hockey team's season ticket holders and our sports intern's mother worked here).  But I have sure learned a lot since that fateful first week.  Especially about indium metal.  Terms like Solder Preform, Burn In, Thermal Interface and Hermetic Sealing have replaced road trip, slap shot and overtime.

Over the next few weeks I will share with you some of the things I have learned during my tenure here at Indium.  I hope you find them as interesting (and hopefully usable) as I do.

...and we're back with more nitrogen and inert soldering myths.

Myth 6: "I turned on the nitrogen flow, so my oven is now inerted"
Fact: It takes time to purge an oven down (and yes, I have an equation to determine this). The final equlibrium oxygen level, as we have seen in the previous discussion, will be somewhere between that found in the incoming nitrogen gas and the 209,000ppm oxygen level found in air.

Myth 7: "If I increase the nitrogen flow rate, I'll get a lower oxygen level"
Fact: Yes you will, to a certain point. But every reflow oven has an oxygen level below which you will not be able to go, due to turbulent mixing (see Myth 3 in the previous post) and you will never be able to reach the same oxygen level as that seen in the incoming gas, as by definition, it is impossible to hermetically seal an in-line reflow oven.

Myth 8: "I have 26 identical reflow ovens using nitrogen gas, with the same inlet pressure, and I know the ppm oxygen level in one of the ovens, so the ppm will be the same in the others."
Fact: I had a customer make this same claim to me, then tell me that he had soldering defects on some ovens and not on others. Why? Because when we went in and actually measured the oxygen level, it was different in each one - and not a small difference either. I measured from 80ppm oxygen up to 1% (10,000ppm) simply because the oven curtains were trimmed to different lengths. A power semiconductor customer was carrying out reflow with forming gas and his process started having huge voids over time. Why? He was not doing sufficient preventative maintenance, and flux residues were choking or diverting the gas flow, allowing oxygen to contaminate the process. He cleaned the oven out, and the problem disappeared.

Myth 9: "I measured the oxygen level in my ovens once, so now I know exactly what it is."
Fact: Reflow equipment "air tightness" changes over time. Amongst the factors causing this change are:
 - Flux residues can clog diffusers and block exhausts:
 - Operators can trim curtains and adjust the oven exhaust balance.
 - The copper-filled silicone gasket materials can lose pliability and harden so they no longer seal correctly.
One contract manufacturer I visited was having some reflow problems, and it was only when he took the external exhaust system to pieces, and removed the twofoot long piece of rosin that was almost completely blocking his 4inch duct that he realized what was going on. Remember, flux "volatiles" only remain volatile if they are hot.

Myth 10: "I can balance the reflow exhausts easily by having the inlet and outlet vents tied together".
Fact: This is one for the engineers amongst you. The face velocity (gas flow rate in ft/min or cm/min) into the exhausts at both ends of the reflow oven has to be the same (see diagram). Not the volumetric gas flow rates. Not the vent pipe ID (inner diameter). The reason is simple: if you have the same face velocity into the ducts at each end, you exactly balance the Bernoulli effect across the oven, so that the pressure is the same. The minute you have a difference in the face velocity, you have a difference in the air pressure, which will drive air into the oven, at a rate that is a function of the difference of the squares of the face velocities.

One final note: Inert or "nitrogen" soldering really should be renamed "low oxygen level" soldering to get the emphasis on the control variable here: the ppm oxygen (O2). From an engineering perspective it IS a critical control variable, and ppm levels within reflow ovens should be measured on a continuous basis.

Remember: a variable that is not monitored can not be controlled: just good engineering practise.

Next time: forming gas.

Cheers!  Andy

An image of a medium size indium preform. This form of indium is very popular as a thermal interface material to transmit heat away from a microchip in computer processors.

The term “indium bonding” refers to attaching two surfaces with the very unique element indium (In).  Indium is one of the few elements used in lower temperature solders, and out of those elements it is the softest.  Indium bonding is preferred in applications where:

• Cryogenic stability is needed
• Sealing requires high levels of hermeticity
• Maximum thermal transfer is required
• Bonding to not-metallic surfaces
• Flux can not be used (as noted by Amanda Hartnett)

Indium bonding can also be used to form a solder quality bond at room temperature.  This is called cold welding.  Here’s how it is commonly done:

1) Parts to be assembled are coated with indium through plating, sputtering, vapor deposition, pre-tinning or other methods.

2) The indium surfaces are etched with a light HCl solution.

3) The indium surfaces are aligned and pressed together.  The pressure needed to form the bond can be as gentle as a few psi depending on the purity of indium, dwell after etching, and angle of attachment.

4) The solder joint should be good for approximately 270psi tensile strength and although it was formed at room temperature, it will not melt until it reaches ~157C.

For more information on ‘cold welding’ click here.  

Compress indium between the arrows to form a hermetic seal

Have you ever tried to form a seal using an epoxy where you dispense the material, clamp the seal shut, and VOILA! It's attached!?

You can do the same thing with indium in a wire, washer, or frame form.  The process is no more difficult, it's cleaner, and with some control, the seal is hermetic.  

Pure indium or indium alloys are used to form hermetic seals that also conduct heat.  Indium washers are often soldered around lenses which are metallized with gold.

From my perspective, the key to hermeticity is to have very clean preforms.  The trouble is that indium readily forms a thin layer of oxides when it is exposed to air which should be removed prior to soldering a hermetic seal.

The recommended procedure by the Indium Corporation is to etch the indium preforms using a 5-10% HCl solution for up to 1 minute.  I have heard of other methods though such as etching in different acids or plasma cleaning and I'd like to hear feedback from those of you cleaning indium be other procedures.

Please let me know what you think!

_{Mr. Clean picture retrieved from http://www.croatia.org/crown/articles/9421/3/Croatians-in-America—-photo-collection-by-Vladimir-Novak,-part-1}