SMD solder now or later?

Question

I'm soldering up some boards with smd parts, using a hot plate to reflow, and I'm (inevitably) realizing that I got some of the wrong parts. Now, is it better to leave the boards with smd components stuck on them (sitting in wet solder paste) until I can get the other parts, and then do the reflow all at once, or to reflow now, and then paste and heatgun (or just solder iron) the other parts in later?

I only have a few parts for which I need to do this, and I can stick the boards in a quiet refrigerator until I get the parts. I might not have ready access to a heat gun later, which is why this isn't a no-brainer (I'm imagining soldering iron+paste isn't the best way to do things, since I can't get to pads under the parts). Personally, I think the real question is whether reflowing twice negatively affects smd components, but context might prove it otherwise.

Answer 1

I've left wet paste on a board overnight in cool weather without problems, but I'd be worried it would dry out over a few days - especially if left uncovered in a refrigerator (although I'm not sure if this would adversely effect the reflowing).

If there are pads under the parts, then soldering with an iron is out of the question, but a hot air rework gun should work fine. Depending on the complexity and the number of missing parts, I would either:

• Flow now and solder the additional parts later (you can place a cube of metal under specific parts of the board to reflow individual parts on the hotplate).
• Clean the board and restart when you've got the new parts

Answer 2

Solder paste drying out is not the problem. It's the fact that the flux is hygroscopic!

Now, this may not apply to all solderpaste, But I have left some unsoldered boards with paste on them lying about for varying amounts of time, and they become progressively more difficult to solder as time passes.

Basically, what happens is apparently the water flash-vaporizes, and literally causes the parts to go flying. I wound up having to use an old (dry) sponge to hold the parts down as they reflowed (this was the hot-plate reflow method).

I'm a bit mystified by the whole affair, as I would imagine the water would gradually boil off, not create a percussive effect powerful to actually launch 1206 parts about, but it is what happened.

This is with kester paste (variety is from digikey, don't know which specifically (not at my workstation at the moment).

Edit - It looks like I'm currently using Kester R500. I prefer a more aggressive flux because I'm often doing unusual things to prototypes, and the boards are often a bit abused (the place I work has a terrible feature-creep problem). Other fluxes may behave differently.

Don't forget to wash your boards if you use a stronger flux.

Answer 3

Personally, I'd solder all the parts now and add the missing parts when they arrive. The paste might dry out.

Answer 4

To the best of my knowledge, solder paste is kept chilled not because the flux dries out quickly, but to help keep the very fine bits of solder in suspension. You're probably okay waiting a few days to solder.

Answer 5

I recall reading on some paste datasheet that the shelf-life of stenciled boards was about 12-18 hours, but the life of boards with placed parts was significantly longer, about 2-3 days. I would surmise this is due to the parts physically containing the paste's flux, reducing dry-out where it matters most, between the part and the board.