Flux cleaning regime - how often?

[RogerRowland]

As a relative newbie to SMD work, I've been hand-soldering a few boards and typically do something like this:

1. Tack all fine-pitch IC's (like TSSOP-20's) on, say, ten boards.
2. Change to hoof tip
3. Drag solder all tacked IC's on all boards
4. Clean the boards
5. Change to fine tip
6. Mount all SOIC's and 0805 R's and C's on all boards
7. Clean the boards
8. Solder all through-hole connectors
9. Clean the boards

Should I skip steps 4 & 7? I sort of feel like I'm wasting time, but I don't know if it's easier to clean "fresh" flux (from an SMF flux pen) rather than waiting until all is done.

What do you guys do? Clean after every component? Clean after every board? Something else?

[orbiter]

I'll usually just clean a pcb at the end of a soldering session, however if the board is to be left for any length of time between soldering sessions I'll clean it.
So that when I come to start soldering again it's not all sticky to work on.

[Ian.M]

It depends:

Double sided or single sided board?
If double sided, are the SMD parts topside, bottom side or both?
Which specific flux and what process temperature?
How do you clean the board, and with what solvent?
Is the flux likely to cause contact dermatitis?

In general, I would manually clean up flux as I go on double sided boards with top side components, because its easier with better access before taller components and through hole parts are fitted.  However if you are set up with the right board washing equipment and flux, intermediate cleaning is a waste of time, money and effort.

[Simon]

I tend to use a small reflow oven so that it all happens at once.

[RogerRowland]

It depends:

Double sided or single sided board?
If double sided, are the SMD parts topside, bottom side or both?
Which specific flux and what process temperature?
How do you clean the board, and with what solvent?
Is the flux likely to cause contact dermatitis?

In general, I would manually clean up flux as I go on double sided boards with top side components, because its easier with better access before taller components and through hole parts are fitted.  However if you are set up with the right board washing equipment and flux, intermediate cleaning is a waste of time, money and effort.

On this project (just hobby, not commercial), I have double-sided boards with SMD parts on both sides. There are two types of board, I have 10 of one type and 30 of the other, so 40 boards to do all told. I'm using an Electrolube SMF Rosin Flux Pen, "process temperature"? I guess you're talking reflow ovens? I'm just soldering with my Hakko at 400 degrees C. I clean the boards by hand with IPA from a spray can and an ESD-safe brush.

[Ian.M]

http://www.farnell.com/datasheets/1776697.pdf
Its a halide free no-clean rosin (RMA) flux.

Are you seeing significant tip blackening or any flux blackening around the joints?  400°C (750°F) although mid-range for Pb-free hand soldering is a little on the high side for rosin fluxes.   If you can run even 10°C or 20°C  cooler, its worth doing so.  Otherwise, if you are getting *ANY* charring, consider a synthetic flux.

If you aren't charring the flux, you certainly shouldn't need to clean as you work, even with extended storage between sessions. Mopping up excess liquid flux as you go to avoid working in a sticky mess is a different matter.  Any dried residue should soften up with no trouble if you spray the board with IPA in a tupperware box or similar, then put the lid on for 10 minutes or so to reduce the evaporation rate.

[RogerRowland]

http://www.farnell.com/datasheets/1776697.pdf
Its a halide free no-clean rosin (RMA) flux.

That's the one.

Are you seeing significant tip blackening or any flux blackening around the joints?  400°C (750°F) although mid-range for Pb-free hand soldering is a little on the high side for rosin fluxes.   If you can run even 10°C or 20°C  cooler, its worth doing so.  Otherwise, if you are getting *ANY* charring, consider a synthetic flux.

Nope, no significant blackening, only on the tip if I leave the iron on but unused for too long but joints are fine. I'm using 60/40, not lead-free, but my Hakko may be running cooler than 400°C anyway after I cocked up and adjusted the calibration rather than the temp, so I reset to factory default and just manually raised the temperature until I got satisfactory results. I don't have the means to measure the actual tip temp.

If you aren't charring the flux, you certainly shouldn't need to clean as you work, even with extended storage between sessions. Mopping up excess liquid flux as you go to avoid working in a sticky mess is a different matter.  Any dried residue should soften up with no trouble if you spray the board with IPA in a tupperware box or similar, then put the lid on for 10 minutes or so to reduce the evaporation rate.

Ok, maybe I'll only clean if I make a sticky mess then, otherwise leave it until I'm done. Thanks for the advice.

[Ian.M]

If you are using SnPb at the minimum effective working temperature then you are well within the range Rosin fluxes are good for.  You are probably well below 400°C, and may even be closer to 370°C (700°F).

If you are mounting large TQFPs and wide SOICs, you *MAY* wish to clean early, immediately after soldering, to reduce the usage of IPA, by flushing out excess flux under the chip before it's had a chance to fully dry. OTOH an IPA saturated board may cause its own issues with gassing disturbing through-hole joints so early cleaning may be more trouble than its worth.