Thread: Attempting to Join a Thin Sheet of Stainless to a Slightly Thicker Sheet?

Okay, just a fair warning, I'm an electrical engineering student, so take everything I say with a grain of salt because my metalworking knowledge is pretty sparse.

I'm working on this project of mine that I'm trying to get to fit into a wallet, so it needs to be very thin. The main component is a circuit board (which I'm having manufactured very thin, 0.6mm thick, or about 24mil). The batteries that I'm using don't last terribly long (a half hour of constant use), so the goal is to create some sort of way to replace the batteries in this thing. I'm using these CR2016 coin cells that are 1.6mm thick.

Here's a rendering of my current idea.

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Basically, the pcb will have these tiny magnets (1/32" thick) glued to it, with a small steel backing plate to prevent the magnetic fields from erasing credit cards and such in wallets. The batteries will go through the PCB and up into the stainless steel faceplate, and then be capped on the back with copper (or steel) discs that I'll solder to the pcb. So effectively the two halves form a magnetic battery holder. The yellow sheet is insulating tape.


My only problem is that I'm going for extreme thinness on this thing, so I'd like for the steel faceplate to be just over 1mm thick, or about 40mil. The batteries and components will stick at most 1mm above the pcb, and with the insulating tape being 5mil thick, that leaves about 5mil of thickness that I have at the back of the cavities for the batteries in the steel.


I know this is super thin. I've contacted some machine shops and they've all basically told me I was being ridiculous for trying to machine so deep into a material.


So, I've determined that a more realistic solution would be to use two sheets, one that was ~35mil thick and one that is ~5mil thick, cut the holes for the batteries and other components to stick up into the faceplate all the way through the thicker sheet, and then somehow adhere the thinner sheet to the surface of the thicker sheet. I need some holes to be through both, so I'll just cut whatever those are in both.


I've looked into rivets, but I'd really like the surface to be flat, and I'm also worried that the 5mil sheet will bow and not be flat in the middle of the surface. I've found some flush-on-both-sides blind rivets, but they're just a little too thick. So a friend suggested brazing. I have access to a kiln, and was curious if I could use the kiln on a low temperature to do an oven braze? I'm not sure if it's really that simple though, can I really just put some strips of lead free solder between the two surfaces and then weigh them down and then turn the kiln on?


Also was looking at silver-filled conductive epoxies (the connection needs to be conductive because the front surface is effectively 1/2 of the battery holder), and they're expensive, but it seems simple enough.


I'm looking at using 17-4 PH stainless steel, since I need it to be relatively strong but also magnetic, and I'd like it to be corrosion resistant, since it's something that people will be handling often.


Anyway, I'm really scratching my head here. Thanks in advance for any advice or direction you may be able to provide. Let me know if anything needs clarification!

Mattegan,
If you have time, it would be worth watching some of the videos put out by Dan Gelbart https://www.youtube.com/user/dgelbart/videos and I reckon most of your questions will at least be partly answered or lead to a solution. All the best with your project. Alan.

Flux filled solder may do in a pinch but you would be better off trying to find brazing paste.
Rather than brazing, spot welding may be a better way to go. There are techniques that minimize electrode marking - see Resistance Welding Manual by Resistance Welder Manufacturer's Association. My copy says they are at 1900 Arch Street in Philadelphia. (IBSN 0-9624382-0-0)

Michael

I'd second C-47's recommendation of watching Dan Gelbart for some inspiration.

You can actually machine 1mm SS and leave 5 thou thickness, but you need to make vacuum fixtures to hold the parts.

Another idea is to make the batteries slide in edgewise, between the top and bottom contacts, that way you don't need the halves to come apart.

Or yet another, make one side rotate, or a portion that rotates, that exposes the battery compartment.

Ray

Hey guys, thanks for the advice!

I actually just spent an hour or two watching Dan Gelbert's videos, and they're great. The best advice I think I got was this : "You have to think is there any place where I can insert precision parts into what I'm designing without having to machine it to precision." Which I think rings true with regards to trying to mill the entire thing. It really simplifies my construction if I can just mill the thin pockets and the deep pockets can be cut through the entire sheet, and then the 5mil backing can be brazed on.

I also watched his video on brazing, in which he mentions that for two sheets you can join them together using solder foil and some flux between the two surfaces, I think I'll give this a try. I'm kind of in a bind as I'm in California at an internship and don't have access to the tools at my school for about another month. I need to start layout on the PCB so I'm really just trying to make sure whatever solution I have for the front is viable before I devote too much time to a single solution.

I thought about the idea of the batteries coming in the edge, but I don't know if it really solves the problem of needing a small wall on the steel cover size.

I do have access to a standard sized CNC mill and also a really tiny desktop circuit mill and I probably could whip up some form of vaccum holder so I may try that as well when I get back to school.

Kind of a bummer there's nothing I can take action on at the moment with lack of equipment, but I'll try and keep you guys updated when I get back to school.

Also, I was looking for brazing alloys, and unfortunately with the stainless steel that I'm using (17-4 PH), it can only keep it's corrosion resistant properties up to 1100F, and I can't find any brazing alloys (on McMaster at least) that have a lower melting point. Any ideas? I've found some stainless that can withstand a higher temperature... but either I can't find it in the thicknesses I need (35mil and 5mil), or it's not magnetic.

Originally Posted by mattegan

Any ideas?

Silver solder?

Will magnets work on your chosen SS alloy?

Stuart

Epoxy or glue?