What is the Shelf Life of Solder Paste?

Phil
Welcome to Board Talk. This is Jim Hall and Phil Zarrow, the Assembly Brothers. Pick and place. Jim, what's our inquiry today?

Jim
It comes from D.P., and it has to do with the shelf life of solder paste. "What is the practical shelf life of a syringe of solder paste, and what symptoms would one see when the syringe is getting too old?"

Phil
The answer really lies with - if you really want to know - talk to your solder paste manufacturer. They will have specs.

Some are longer than others, but I tell you you shouldn't have it out for more than a week. Guys come in and say, "Well, we've done it for a month," or, "No, no, no; only a day." Check with the manufacturer. They know their formulation. They know what they have and that's what you want to follow.

I want to congratulate you for getting syringes because that certainly helps, as opposed to when you have a jar that you're constantly opening and exposing larger amounts of the past to the ambient humidity and temperature.

But even in a syringe, there are probably max recommendations that vary among the different manufacturers. They may also have recommendations in how you store it, whether you're storing it vertically or horizontally. So, check with them.

The other thing, look at the quantity you're buying. If you're going to have it out for long times, you might be buying them in too large a quantity. Penny-wise, pound foolish. If you're using only very small amounts, order them in smaller syringes.

If you're ordering 500 gram syringes, consider 100 gram syringes or even 50 gram syringes. Another trick you could do is place a blanket order. Basically, forecast how much solder paste you'll be using in, say, a year, and then tell them how you want it allocated per month. That way you're not carrying huge inventories of solder paste long-term. You're turning your inventory, and that's a key thing there, too.

Jim
Phil, you mealy-mouthed conservative! Darn it, solder paste is the most critical material you deal with in your process. Don't fool around with it! Don't take chances on saving a few pennies here and there by trying to use it longer, because what are the defects? The bottom question – it's not going to work right. It's not going to wet properly.

It's going to give you solder balls and voids. Worst case, it's going to settle. If it settles in the syringe, you know it's bad. But a lot of times it'll look fine, but you put it on the board and you get poor soldering. You get voids. If you're doing BGAs, you may not see it. Don't take a chance. It's too critical a material. Buy it in the sizes that you're going to use it, even if it costs you a little more up front. The longer-term economics are much better.

Phil
Absolutely. My brother speaks wisdom, and on that note I say thank you for listening to this installment of Board Talk.

Jim
Whatever you do, don't solder like my brother.

Phil
And don't solder like my brother.

Suppliers of solder paste will work with you and if you're supply builds up you can slow delivery. Some suppliers will even pull some of the paste back and give you a credit.

I agree with Phil. Ask your suppliers, they know better than you the chemistry of the paste. But also take into account your ambience: too cold/hot, humidity, self-lif, climate control, and also your team.

For my experience I would prefer syringe. I have been using cartridges for a few years but we have had lot of problems and we have decided to go back to syringes.

This is a very common question, with a lousy answer. It gets a bit esoteric here so bear with me... When solder guys refer to syringes we are usually referring to dispense paste (35-100g per syringe) and shelf life is the product unopened, properly stored. 'Open time' is the time in process. Is this a dispense application? If so, open time will depend on the environment, the dispense method/tip size and the paste formulation.

We refer to the 6oz Semco packaging as a cartridge. The open time for these packages is also determined by the environment and application. Generally speaking, the more challenging the application, the shorter the open time. To Jim's point, fresh paste is ALWAYS better and paste is cheap in comparison to defects! It's the last place one should scrimp in an assembly process.