How do you solder-paste it?

[LoveLaika]

So, I'm working with BGA components which have a relatively fine pitch. Given the part, I ordered a stencil from Osh Stencils for the occasion. What I usually do with stencils, I use a solder-paste spreader (AKA, a plastic card) and run the paste over the stencil, fill in the holes, lay out the parts, and bake it in my oven. Pretty straightforward. I was wondering if you guys might have any techniques and whatnot that would make the process easier. Is it best to have the solder-paste be a bit runny, or should it be clay-like/solid?  When I use my solder-paste, it's kept in a chemical fridge, and it's essentially solid when being used making it hard to actually get paste and smear it over the stencil.

[fourfathom]

I keep my paste in the refrigerator, but it needs to warm up before you squeegee it through the stencil.  I squirt out enough paste for the job and let that blob warm for a while.

[LoveLaika]

Yeah, it would be easy if I could syringe it. My paste is in a jar, and I can't put it in a syringe for easy transferring.

[tooki]

Yeah, it would be easy if I could syringe it. My paste is in a jar, and I can't put it in a syringe for easy transferring.

So... scoop some out and let it warm up?

fourfathom’s point was that by taking out just the amount you need, it won’t take as long to warm up, since you don’t need to warm up the entire container.

As far as texture, once at room temperature, it should be similar to toothpaste. If it’s clay-like at room temp, it’s dried out and should be replaced.

[LoveLaika]

I found on YouTube that mixing it with liquid flux (from a flux pen) helps when it is dried out. For me, it's definitely dried out, but it's still kind of workable. It's just solder; there doesn't seem to be anything like flux mixed within it.

Actually, thinking about it, where are good places to buy flux and solder-paste, and how do you find good flux/solder-paste?

[tooki]

Solder paste is literally powdered solder in flux paste. (Indeed, some flux pastes are exactly the same as used in the solder paste, so as to guarantee compatibility.)

You’re in USA, so you have tons of sources, including Digi-Key and Mouser.

As far as what’s good: name brand. The finer (smaller solder particle size) the better. (Higher number = finer. I prefer T4 or T5.) Buy solder paste in tubes for home use. You can’t put paste back in, but it stays fresher. Store it in the fridge and take it out a few hours before use.

For what it’s worth, I have MG Chemicals solder paste (they only make T3, sadly) that’s exceeded its shelf life by several years now and still dispenses easily. 

I also like ChipQuik’s selection of pastes. They have low-temp lead-free paste that’s a joy to work with. They also have what’s probably my favorite flux gel, called SMD291. It’s smooth as silk, and when heated smells like hazelnuts, rather than the acrid smoke from most fluxes. And it’s not even particularly expensive.

[LoveLaika]

Thanks. I'll have to look into that.

For the time being, I'm using Kester EP256HA from a jar. Looking at the price of all this stuff (the solder paste), it's expensive. Doesn't seem like I'll be able to buy new solder for the time being. Considering how it is, would the way I've been doing it now work, mixing it with flux to 'toothpaste' consistency? Your comments made me remember that I used to do that.

[tooki]

For the time being, I'm using Kester EP256HA from a jar. Looking at the price of all this stuff (the solder paste), it's expensive.

The amount used per board is tiny. I consider the cost of solder to be negligible.

The only real issue for the hobbyist is solder paste's comparatively short shelf life, forcing us to buy small containers, forgoing volume discounts on larger containers.

Doesn't seem like I'll be able to buy new solder for the time being. Considering how it is, would the way I've been doing it now work, mixing it with flux to 'toothpaste' consistency? Your comments made me remember that I used to do that.

It's not a recommended practice, but for home use I'd say it's fine.

[fcb]

I use T5 normally. T6 for those really fine pitches and BGA.  Alot of modern pastes keep for at least a year at room temperature.  I keep tubs of expired paste, no idea why - haven't tried reconstituing it.