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Solder Paste - what kind?

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fuseless

New Member
Hello,

I just need to know a good solder paste use for electronics, RF In this case.
What alloy?
Is flux needed?
Is silver bearing solder paste ok?

Thanks...
 

dknguyen

Well-Known Member
Most Helpful Member
Solder paste contains flux already. That's what makes it solder paste and not just solder powder. Won't just regular 60/40 or 63/37 tin-lead solder oaste work? Or lead-free (mostly high tin alloys) solder paste, like SAC? I don't think special solders are really used for RF work...I could be wrong but I'm confident that if it is, then it's only at the crazy high end of stuff that mere mortals will never touch...like those 110GHz oscilloscopes.

You mention paste specifically so I assume this is for reflow work?
 
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fuseless

New Member
Thanks dknguyen, most helpful. Forgive me though, I'm not sure on your terminology, by reflow work do you mean doing surface mount components with a heat source other than a soldering iron?

That's what I plan on doing but due to my lack of experience with SMT, I'm hoping you could also recommend a heat source that will not break the bank.
 

dknguyen

Well-Known Member
Most Helpful Member
You only need solder paste for reflow work which is where you squeegee solder paste across the entire PCB through a stencil, place all the components on it, then stick it in an oven so it solders everything in one go. Or when you use hot air.

But if you use a soldering iron, then you don't need paste at all (it's a PITA and needs to be refrigerated with a shelf life due to the high surface area of the powered metal resulting in rapid oxidation). For using an iron, you use wire solder which almost always has a flux core. You will still want separate flux though for surface mount work, but if you are doing through-hole only then you don't need a separate bottle of flux unless you need to make corrections.

Remember you still need to specify rosin flux in all cases.
 
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Sujo Mathew

New Member
Flux is designed to improve electrical contact and mechanical strength in solder joints. There are mainly two types of flux cores. Acid core and rosin core. Acid core is used for plumbing and rosin core is used for electronics.
 

gophert

Well-Known Member
Most Helpful Member
Same thing really...just primary or secondary effects.
No, "not the same thing really" at all...
...unless you are the type of person that would explain that soap and antibiotics are the same thing.
 

dknguyen

Well-Known Member
Most Helpful Member
No, "not the same thing really" at all...
...unless you are the type of person that would explain that soap and antibiotics are the same thing.
No, because those two work with different mechanisms and have different goals. In this case it's just the goal and how you achieve that goal. Would you say someone is wrong for saying "soap is designed to clean your skin" and correct them with "soap is designed to adhere water molecules to oil and grease molecules"?
 

rjenkinsgb

Well-Known Member
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Ahh, not really. It is to clean the metal and help the solder flow.
I've got to agree with Shortbus.

No flux = NO JOINT.

It's not a matter of improving the joint or lowering the resistance, it's down to the fundamental action - soldering (or brazing etc.) actually being properly carried out, with genuine metal to metal bonding, or not.
 

shortbus=

Well-Known Member
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Same thing really...just primary or secondary effects.
Funny but after doing a Google on this they all come back to what I said and couldn't find one agreeing with you. But I'm willing to learn, can you link something to what you said?
 

dknguyen

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Funny but after doing a Google on this they all come back to what I said and couldn't find one agreeing with you. But I'm willing to learn, can you link something to what you said?
Not written anywhere. It's just more logic and the way language works more than anything else. What does flux do? Clean the metal and help the solder flow. Why do you use it? Improve electrical contact and mechanical strength. As far as I'm concerned, both are correct when you tack them onto the end of "Flux is designed to _________________". The only difference is one is the objective and one is mechanism to accomplish that objective.

Just like saying a 747 is designed to fly vs a 747 is designed to transport people. The former is the mechanism/method and the latter is the ultimate objective. Neither is wrong. One is just more technical.
 
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gophert

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So, is your claim that flux makes the plane fly, or that the flux transports people?
 

gophert

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facetious
"Facetious" would require this topic to be serious, while my comment must be both humorous AND inappropriate. I doubt anyone would claim we have the necessary trifecta.
 

shortbus=

Well-Known Member
Most Helpful Member
Not written anywhere. It's just more logic and the way language works more than anything else. What does flux do? Clean the metal and help the solder flow. Why do you use it? Improve electrical contact and mechanical strength. As far as I'm concerned, both are correct when you tack them onto the end of "Flux is designed to _________________". The only difference is one is the objective and one is mechanism to accomplish that objective.

Just like saying a 747 is designed to fly vs a 747 is designed to transport people. The former is the mechanism/method and the latter is the ultimate objective. Neither is wrong. One is just more technical.
No, your, " Improve electrical contact and mechanical strength." is the result of my, " Clean the metal and help the solder flow". Without the clean and help flow you won't get the improved electrical and mechanical part. :banghead:
 

gophert

Well-Known Member
Most Helpful Member
Flux can be completely by-passed if you use a non-silicone buffing compound or silver polish to buff the copper before soldering, the solder flows great and bonds extremely well. If you rough up the copper with steel wool or scotchbrite pads, solder will bond well but the scratch marks prevent the solder from flooding over the pad as it melts.

Flux can be competency avoided and still get a strong bond between solder and copper.
 

rjenkinsgb

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Flux can be competency avoided and still get a strong bond between solder and copper
There will be some oxidation as the copper heats up to the solder flow temperature.
The joint can never be as good as with a flux to remove that.

Or is it that the chemicals in the buffing compound/polish happen to work as a flux? Many chemicals can be used other than conventional flux..
 

gophert

Well-Known Member
Most Helpful Member
There will be some oxidation as the copper heats up to the solder flow temperature.
The joint can never be as good as with a flux to remove that.
Ok, I'll tell the chemists and material scientists who worked on the project.
I may have left out the glove-box with an argon atmosphere. So, you'll have to explain the mechanism for the oxidation.

Or is it that the chemicals in the buffing compound/polish happen to work as a flux?
Well, that was the point of the research project - proving that cleaning the metal surface works just like a flux. [/quote]

Many chemicals can be used other than conventional flux..
So what do you call a flux that is not a flux?
 
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