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Entry  Sat Sep 9 23:21:04 2017, awade, Notes, Vacuum, New shield installation planning 2017-09-09_22.15.05.jpg
    Reply  Fri Sep 15 19:32:10 2017, awade, Notes, Vacuum, Solder in vacuum for Kapton heaters 
       Reply  Mon Sep 18 15:08:35 2017, rana, Notes, Vacuum, Solder in vacuum for Kapton heaters 
       Reply  Thu Jan 11 00:28:07 2018, awade, Notes, TempCtrl, Soldering Kapton heaters 
          Reply  Wed Jan 17 13:55:29 2018, awade, DailyProgress, TempCtrl, Soldering Kapton heaters 
    Reply  Wed Sep 20 22:22:37 2017, awade, Notes, Vacuum, Preparing for shield bake 2017-09-20_20.51.34.jpg
Message ID: 1922     Entry time: Fri Sep 15 19:32:10 2017     In reply to: 1908     Reply to this: 1927   2039
Author: awade 
Type: Notes 
Category: Vacuum 
Subject: Solder in vacuum for Kapton heaters 

I looks like the only feasible way to attach wires to our Kapton heaters in vacuum is to solder them on. I can't find any suitable clips/crimps.  There are two issues. First, I and Yinzi both found it hard to form a reliable solder joint to the Kapton pads in our initial tests: the surface type and the fact it is thin and holds no heat means that the solder beads onto the wire and doesn't wet to the flat surface of the heater electrical contact.  Secondly, solder has rosin and other contaminants that are potentially bad for outgassing and redepositing on optics in vacuum. Furthermore, it is difficult to bake because it has such a low melting point.

I did a quick search for UHV compatible solder, this is what I found

  • Accuglass UHV solder part 110796.  I believe the MIT people used this for some squeezer parts for their setup and stuff going into LIGO vacuum (although I can't find it on their elog).  The website lists this as $39 per inch of solder (not including Isopropyl soluble flux).  Its an 85% gold alloy, which might explain the price. Max bake temperature 250 C, melt point 280 C, contains no rosin or flux (this is applied separately). 
  • German company Allectra sells an Sn/Cu/Ag UHV solder part 315-SOLDER.  From their catalog 0.5 m is 32 Euro ($US 38) and comes with the flux solution, melting point is 225 C.  There is also a  Pb/Ag non-lead-free version that is 35 Euro with a higher melting point of 300 C, this one is bakeable at 250 C.
  • Lake Shore also sell some soldering materials. They sell a 'high temperature' Pb90Sn10 that goes to 227 C, this may not be good for the reasons listed below. Also they have Indium, presumably used for mechanical joints, although it does have an ok conductivity. Frank used indium for mechanical attachment for parts in the past (see PSL:786). One issue, as rana has pointed out, is that it can be a little unreliable for good mechanical contacts. Its very ductile which means that it sticks like stiff chewing gum, but also peels off too easily.  Lake Shore also sells something called Ostalloy 158 solder, melting point is 70 C, so only for cryo stuff.

From general reading, apparently lead based solders tend to outgas and should be possibly be ruled out. This has something to do with the vapor pressure of lead.  Antimony has similar issues. That said, other sources seem to have no issue with it. The tin-silver solder alloys tend to be a better choice and ones like Sn95Ag5 have melting points of order 230 C so can be baked up to 200 C.  Rosen free is better with a separately applied flux that is Isopropyl soluble.

I also wrote to Rich Abbott to see what they did for other LIGO.  He said they regularly use Kester 37/63 rosin core solder for a small joints within the LIGO vacuum envelope.  This solder is Sn63Pb37  and has a melting point of 183 C and has a 44 Resin RA activated rosin core.  The low melting point puts an upper bound on bake temperatures.  There is residue from the flux that can be cleaned away with isopropyl.  They made a technical note on using these Sn63Pb37 solders with rosin cores in vacuum along with cleaning procedures at LIGO-T1300040.  According to Rich there was also an experiment conducted to test its UHV suitability (see LIGO-T1200491) but he didn't have the outcome/results on hand.

So for our heater wire soldering attachment we might be able to get away with using Kester37/63 style solder, if we do a good job cleaning it. Or, we might like to opt for something that is silver/tin based and apply a separate soluble flux at the time of soldering.  

 

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