Poor mans bonding - Page 1
Forum section: electronics / general

author: justanothername
author_rank: Regular Contributor
timestamp: June 05, 2020, 10:18:40 pm
content: Hi.So this is purely theoretical for now but I'm wondering:Let's assume I may get an experimental IC with pure Silicon contacts to bond wires to. These contacts are fairly big, 100x300µm, so this is maybe a task that is doable by hand. They are also a few of them with reasonably wide spacing.I guess I have to bond aluminium wires to it to get ohmic bonds. So what would you do if you do not have a bonding machine? Any ideas to modify existing ultrasonic equipment to achieve an improvised ultrasonic welder?Is this insane or a valid thought experiment?

author: Weston
author_rank: Regular Contributor
timestamp: June 06, 2020, 12:27:06 am
content: What do you mean by "pure silicon contacts"? In everything I have worked on, pads are a metalized layer, typically aluminum. Can you even directly bond aluminum bond wires to bare silicon?Bond wire machines are pretty finicky, you might be better off trying to do something like a flip chip connection where you solder the contacts directly to a carrier PCB. 100x300um is close to the same size as a normal WLCSP / BGA part.

author: justanothername
author_rank: Regular Contributor
timestamp: June 06, 2020, 11:16:29 am
content: According to the data I got the surface is Si + SiO2 to bond to. I guess thats what aluminium bond wires are for, to produce an ohmic connection if the die is not sputtered with aluminium?Soldering wont work in my understanding. Mainly because of the oxide layer, also if i can get the solder to diffuse into the silicon (by scratching off the oxide layer somehow while soldering) it will still result in a schottky barrier.

author: wraper
author_rank: Unknown
timestamp: June 06, 2020, 11:35:32 am
content: Quote from: justanothername on June 06, 2020, 11:16:29 amAnd where did you get it? Written on a fence? SiO2can be used for passivation, not bond pads. It's not even conductive.

author: justanothername
author_rank: Regular Contributor
timestamp: June 06, 2020, 11:49:01 am
content: So in all cases there will be a metal layer on the bonding pads? I guess I have to request further information on this to clarify. In the most cases it will be aluminium which is unsolderable. Am i right?So what now if I do not own a bonding machine? Use conductive glue? I am not able to add another metal layer without sophisticated equipment or is there a way?

author: coppercone2
author_rank: Super Contributor
timestamp: June 06, 2020, 07:52:56 pm
content: you can use conductive glue, the resistance is higher, so it depends on the current leveldeoxidize/clean and glue would be a viable option for a prototype I think

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 07, 2020, 01:29:16 am
content: The surface should at least be heavily doped to ensure an ohmic contact. Usually that's immediately followed up with metallization, as shown above.If this is like a chunk of wafer measured for Hall effect, it might suffice to just stick spring loaded probes on the thing. Make a fixture.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: June 07, 2020, 03:06:36 am
content: i thought to make a X-y table with two micrometers with a tiny displacement so you can dip a bent wire into silver epoxy and position it on the IC and cure it with little blobs connecting the pad to the wire

author: justanothername
author_rank: Regular Contributor
timestamp: June 07, 2020, 07:04:38 am
content: Quote from: coppercone2 on June 07, 2020, 03:06:36 amThis could actually work! I then just have to make a dipping fixture that lowers the wire into the glue. Any idea how to precisely dispense the glue in such low quantities?Edit: i guess the easiest way would be to dip the wire into glue first.

author: coppercone2
author_rank: Super Contributor
timestamp: June 07, 2020, 07:05:06 pm
content: the mg chemicals silver epoxy is not like a typical epoxy that flows well, it is kind of a pasty putty.The only conductive glue I have seen that you could really dispense is the radioshack conductive stuff, its liquid. But the conductivity is pretty bad, but it might not matter.Silver epoxy is like tomato paste. You would need to make a holder for the wire, that is able to attach to a micrometer jig (with magnet perhaps), align it vertically, clean it with acetone drip, then under magnification dip it into a pile of conductive epoxy, check if it adheres and possibly mess with the mixture, it might be too pasty, and I had bad luck with thinning glues, on a small scale you might need a micropipete to do it reliably. This is the part that will suck.Then when both ends are dipped, take it to your alignment jig, press it, and wait 4 hours or bake it to cure it faster. Still I expect this to be the easiest and most economical way to bond a die at home, anything with heat or ultrasonics is going to be really hard. You will either need precise optics, or precise mechanics (micromachining). The only easy electrode I can think of would be a needle/pin for thread.You might want to do research in conductive glues to find something other then silver laden epoxy because it might be too viscous. What is the AWG of the wire you plan on using? Also, you might want to warm the glue, it might be less viscous at some temperature for a short amount of time.Another idea might be to try to put the glue on the pad by very very carefully touching it down with a needle, then placing the clean wire on top. It needs to be really clean. In this case, high viscosity would help.https://www.permabond.com/resource-center/electrically-conductive-adhesives/

author: wraper
author_rank: Unknown
timestamp: June 07, 2020, 07:13:56 pm
content: Maybe simply try finding someone with wire bonder who will do the job for you or company which offers prototype bonding service.

author: justanothername
author_rank: Regular Contributor
timestamp: June 07, 2020, 08:11:22 pm
content: @wraper : Finding a third party company definitely is plan B, I may get samples to practise. For now I'm just sounding out if this is doable by myself and then how.@coppercone thank you a lot for these ideas! I plan to use 50 to 75µm gold wire for this if the die actually is coated with aluminium this should be OK. On the outside I want to bond it to a flex connector end, here i thought about traditional hand soldering with low temperature solder like bismuth paste or indium solder.

author: coppercone2
author_rank: Super Contributor
timestamp: June 07, 2020, 08:33:31 pm
content: hmm 40 awg might be doable. I think I have a spool of 36 and its pretty stiff stilldon't know if you want to use gold with glue, its going to be much more nimble