Hi, I have been given a task to solder thermistors on 22awg wire for work in my spare time. I will need to solder and epoxy them into a brass component at about 1000 a month.

Any hints or tricks on the soldering end? The wires need to be close to the actual thermistor as possible because the brass housing is shallow. About 3/8" deep and we do not want the solder joint to show after it is epoxied.

 

A good iron, 63/37 solder (depending on the final temp, another alloy), a fan and duct to exhaust fumes outside would be good ideas. You might want to research how much lead fumes you'll be generating. Perhaps attach the wires using a small resistance welder instead of solder.

Fab up fixture(s) to hold the thermistor in place while soldering and create a system for handling that minimizes how much you actually handle them. Have a system that counts the output e.g. full box = 30pcs

Consider outsourcing to an assembly house in your area and act as the agent with suitable markup.

 

I think soldering is the way to go. wrapping looks like it may be a little difficult do to the size of the thermistor.

My company did have this sensor manufactured in China for years and has now decided to pull all manufacturing back to the USA. This will actually be a project for my wife to do in her spare time.

The jig i was thinking about uses a few alligator clips to hold the wire in place while soldering. But what I really want is to be able to set up multiple thermistors, say 25 at a time and just run down a line soldering. but I am having trouble decideing how to hold the thermistors to the wire for soldering.

 

Nice to know the production is coming back here.

What do the thermistors look like? A picture or link to a datasheet would help.

 

We used to make a similar assembly where I work. The thermistor was mounted inside a stud eyelet terminal. We tried jut soldering direct. Twisting the thermistor lead around the wire. Poking the thermistor lead into the wires. The biggest issue we had was the thickness of the thermistor lead.

We added heatshrink to the thermistor to give it some strength and rigidity for potting in the terminal too.

Hope that helps.

 

Check if the terminals or thermister contains any magnetizable alloy (iron) so you can hold them in place with a magnet. If not, the flexible magnetic strips can be configured to hold a row in place. Flexible allows the parts to be placed one-by-one as the flex magnet is payed down (a rigid "gate" will require all parts to be perfectly set before the gate comes down which is difficult).
Your wife should be able to track down magnetic strips at a craft store.
Good luck.

 

The part number we are using is from Digi Key. BC2309-ND.

The legs of the thermistor are not an an iron alloy, but the actual thermistor is.

 

In my humble experience, two things could make this job miserable: Improper metallurgy and bad ergonomics. Using flux and solder that are appropriate to the metals you are connecting will allow a fast and clean "one-touch" job.

Anything done repetitively can really wear you out if you're not sitting straight (avoid hunching over) or if your have to strain to see what you're doing. Use good lighting and maybe a magnifier if it helps.

Definitely vent your fumes to get the lead out of the house and to keep fumes out of your eyes. Set up your workspace to make safe working easy, otherwise 8 hours in you'll be tempted to take shortcuts. Wash your hands and face at the end of each session.

 

We don't know what your specs re: wire type, gauge and insulation are so working in the dark but..

I'd consider wire-wrapping followed by solder (because the thermistor lead is round). Presumably, the joint is covered by heat-shrink tubing for insulation. See Step 5 on page 7 of the .pdf. Wire wrap uses solid or in a pinch tinned stranded (that holds the strands together). If you must use stranded, strip the ends and tin in a small solder pot by dipping. Most ends will not have so much solder on them that you could not wrap them with the gun. Tacky but would work with the right tip. The idea is not to have to twist the wires yourself to avoid repetitive motion injury issues.

To avoid a lump where both wires turn 180deg to exit the joint, cut one of the thermistor leads to stagger the joints.

Make a fixture out of a stick of Delrin or Al with holes for the thermistor that leaves the leads sticking out pointing at you at 45deg. That would make it easy to wrap the leads, slide on the shrink and shrink many connections at once. GopherT suggested magnets to hold the work in place. Sounds like a magnet at the end of the drilled hole would be just the thing to hold the thermistor head while you work on the leads then just grab a handful of completed units and pull them out of the hole. Choose the magnet that doesn't hold too strongly.

I don't know if this would work for your particular spec but its a way to avoid much handwork. At 2000 joints/mo. you'll need to be looking at ways of reducing the number of operations.

What do you think? If this won't do, post a spec or pic of the unit to be built and we'll be able to take more of a direct shot at it.

 

Here are some factors that occur to me in no particular order; you might want to think them through.

Is the wire solid or stranded?
Is the wire pre-tinned or bare copper?
What are the characteristics of the wire insulation, i.e., melting temperature, strippability, emission of noxious fumes, etc.?
How will you cut and strip the wires?
Do the wires have to be twisted together into a cable after they are attached?
Is a physical connection between the wire and the leads (e.g. hooking or twisting) required or just soldering?
Will there be insulation over the solder joint and wires, and if not, how will you keep the solder/wires from shorting to the brass housing?
How will the epoxy be mixed and applied? (It can be really messy.)

And the most important question of all: are you being paid enough? Have you estimated the amount of time, including "get out," "put up," and "clean up" it will take to do each piece?

Note that I don't need answers to these questions. They are just questions I would ask myself if I were preparing to do this job.

Good luck.

ETA: The first thing I noticed on the thermistor datasheet is that it's not recommended for potting.

 

Well that's what I get for assuming. I never knew there was a tool for wire wrapping. This will make it much easier. I see that in the recommended terminals none of them are round. Like what would be on the thermistor. But since it will be soldered after the fact that should not be an issue.

The wire I am using is a stranded wire un-tinned.
Not sure what the wire characteristics are. But what is being used is rated at 120 volt speaker wire at 22awg.
The wire will be manually cut to length and stripped with a stripping tool that has a die in it for that gauge wire.
The epoxy will be applied with a large catheter syringe. We have worked with this epoxy a lot in different applications.
I spoke with the engineer that built the unit for us and he did say that the thermitor will be ok potted. And I had the unit field tested on Grave Digger Monster Truck.
The price per piece has been determined to be adequate enough to get the little lady up and doing something other than shopping on eBay LOL.
Can stranded wire be used? I see they use solid core in all the examples I have seen.
Below is an example of how the finished product should turn out.

 

I guess I messed the picture up. Here is another try.

 

The price per piece has been determined to be adequate enough to get the little lady up and doing something other than shopping on eBay LOL.

If you have any extra work, send it my way. My wife needs a job. (Just kidding.)

 

Nice try but I need to dust off the old crystal ball..

This looks like the small wires of the thermistor get butt spliced to some substantial stranded wire. I might be thinking along the lines of a crimp/solder ferrule like this or this.

A ferrule slipped over the thermistor lead would guide the stranded wire in and hold the wires together while soldering. If you get a crimper $$$ would provide a good mechanical connection as well.

Other than that it might be twist, solder and shrink. You'd get tired of your wife being tired of doing that..

Perhaps others know of a better way..

 

...
The price per piece has been determined to be adequate enough to get the little lady up and doing something other than shopping on eBay LOL.
...

I would get back to basics. Assemble a few (like 10 or 20) and time how long it takes, individual time per unit, and then factor in setup time vs batch size (especially for things like the epoxy).

And don't forget time for things like unpacking the components and packing the final devices!

When you have some real hard figures on how long it takes, you can work out what your wife will be getting as an hourly rate vs how much of a pain the job is, and how much of her life it will take up.

And just as a ball park figure, if you are not making at least $1 clear profit on each unit (in my opinion) you are getting ripped off...