[Athrax]

Hi!
 
One of my hobbies is electronics design. Because of logistics issues it is highly
impractical for me to order circuit boards at one of the common board houses,
so I'm spinning my own. To improve solderability, I'm tin coating the boards,
using an electroless process.
The tinning solution consists of 5g of SnCl2, 50g of thiourea and 10ml of H2SO4
in 1l of distilled water, and can be reused multiple times. Unfortunately it has a
relatively short shelf life of just 4-6 weeks.
 
Other that cool, air-tight storage, is there any way I can extend the shelf life?

[Borek]

I doubt there is much that can be done, perhaps just prepare much smaller quantities? Divide everything by 10, or even 20, so that the batch is for a single use?

That's interesting. I never felt a need to try tin coating, I was soldering SMD elements (PCB-s etched with persulfate after thermal toner transfer, just covered with ethanol colophony solution) without any problems. I do agree PCB-s from JLCPCB took to long to arrive for prototyping (but fast enough for making final versions of whatever I was working on). Good times

[Athrax]

I was giving the amounts for my tinning recipe normalized to a 1l amount, I normally don't mix up more than 200ml per batch, and keep that in a brown glass bottle for as long as it keeps.

As for the tin coating...well...different processes, different quality requirements, maybe. I never could get the toner transfer method to work reliably, despite trying out a number of different methods (acetone wet transfer, ironing on, laminating) and paper types. Nowadays I'm spraying the boards with primer paint, then use a laser engraver to selectively remove the paint, and then etch in CuCl2+HCl, since I can pretty much indefinitely regenerate that etchant. Then the boards are tin-coated. And then I'm even applying solder mask. The UV-exposure mask sold in syringes on ebay is horrible, I've found. It doesn't stick well to the boards, it's not very heat resistant, and it's just a mess to work with. My solution is Pebeo Vitrea 160 glass paint. The boards get coated, the paint baked on, and then I'll selectively remove it with the laser engraver where I want copper exposed.

[Hunter2]

Unfortunately it has a
relatively short shelf life of just 4-6 weeks.

The shelf life is driven by the copper content and the decomposition of thiourea.