SMT - how much flux is too much?

[rob_gould]

Afternoon all,

Here's a question for the surface mount soldering gurus...

I'm struggling to get a sensible amount of flux onto my board (RKN80's DAC project), despite using a syringe and needle to apply the flux paste.  I end up with huge (relatively speaking) blobs of flux despite being careful as I can when applying it.

Actually this helps, because the component 'sticks' to the flux and stays in place more easily.

It's leaving more dirty marks on the board than I'd like, but cosmetics are frankly a secondary consideration compared to this project working properly.

Hunting round t'internet, people seem to say that the cosmetic issue is the main problem, but I'd like to hear any resident SMT experts' opinions on whether I might run into more serious problems...

Cheers

Rob

 

[Kingston]

You're using the wrong tool. Forget the needle and just use a cheapo q-tip and paint that rosin in. Not sure what exact paste you're using but mine is a little brown bottle with rosin dissolved into isopropanol. Works like a charm with q-tip.

As for the cosmetic issue of the boards being a yellow-brown mess, give it a nice long isopropanol bath with a toothbrush. You can even use some water. Will be good as new and shiny.

And since you're going to be cleaning the boards when finished, the amount of rosin needed is up to you. I use so much that the boards become a nasty sticky mess. This way the solder finds its way easiest.

 

[rob_gould]

Thanks for the reply.

The stuff I've got is called 'Brown Flux Jelly' by a UK manufacturer called Warton Metals.  It's more like spreading butter than dabbing a thick liquid when you apply it so doesn't sound as easy as the stuff you have.

I'll continue to apply the paste liberally then.  Washing it all off at the end is the easy part compared to keeping the components in place as you solder...

Presumably SMT components are pretty much waterproof like the majority of their through hole counterparts?  Anything to be careful of when washing the board down once it's stuffed?

 

[Kingston]

Yeah I sometimes wish I had a third hand to keep the parts in place while soldering.

Anything to be careful of when washing the board down once it's stuffed?

Nothing I can think of. I don't think you can exert enough pressure with a toothbrush to damage anything.

 

[gemini86]

It used to be that some caps were open on the ends, allowing fluids in them, but with modern manufacturing the way it is now, everything is encased in watertight coating that a usually resistant to iso-alcohol, so a bath isn't out of the question anymore. Hell, I've heard of people running motherboards and keyboards through the dish washer, (YMMV).

 

[mnats]

When soldering BGAs I find that even applying flux onto the opposite side of the board to the one that you're soldering can be helpful. It helps prevent oxides from forming on already soldered components.

For the flux itself I've found that some of the fake Chinese "Amtech" syringes are actually pretty good though they tend to be much thicker than the real Amtech flux. The big advantage to me is that they seem to be less irritating to the eyes and nose than real Amtech flux, though I'm sure there are many different manufacturer involved in making the fake stuff.

As for removal of the flux, Electrolube FLU is far more effective than isopropyl alcohol. It's hard to imagine in what way this stuff is "environmentally friendly" though (as stated on their main page).

 

[rob_gould]

Thanks all for the replies.

Seems like I oughn't be too worried as log as I'm cleaning the flux off

 

[Andy Peters]

Presumably SMT components are pretty much waterproof like the majority of their through hole counterparts?  Anything to be careful of when washing the board down once it's stuffed?

We don't use alcohol to clean PCBs. Use a low-/no-phosphorous detergent and scrub. Take a chip brush (small narrow paint brush), cut off all but about 3/16" of the bristles, go to it. The proper detergents will take the flux away, no problem, especially if you use the water-soluble sort (and you should). Rinse with deionized water. This is important, since you don't want your tap water to leave those white deposits everywhere. Then dry with some blasts of shop air.

-a