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4hv.org :: Forums :: General Science and Electronics
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Lead free solder paste

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Conundrum
Fri Apr 27 2012, 08:57PM Print
Conundrum Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
Hi all.
I finally managed to source 10 grams of SnAgCu solder paste for about £7 UKP.
Came with liquid flux in another container, a syringe and no instructions.

Any ideas?
It seems that you need a stencil to dispense it, however the alternate method is to flux the whole board and then place paste on the PCB using a syringe where you want the solder to go, at which point you place the components and reflow.

There is some controversy about whether SMD LEDs should be reflowed with IR or modified toaster oven etc.
As I have next to no experience with this, some feedback would be appreciated.

Also having issues getting lead free solder out of through hole pads on motherboard.

-A
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tobias
Fri Apr 27 2012, 09:23PM
tobias Registered Member #1956 Joined: Wed Feb 04 2009, 01:22PM
Location: Jersey City
Posts: 172
Do you have a hot air rework station?
When I decided to go for SMD Ive bough paste, flux and really thin solder wire. And the solder wire worked very good for me, even for the PQFP packages.
What are you trying to solder?
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mikeselectricstuff
Sat Apr 28 2012, 09:20AM
mikeselectricstuff Registered Member #311 Joined: Sun Mar 12 2006, 08:28PM
Location:
Posts: 253
wrote ...
SnAgCu solder paste for about £7 UKP.
There's your problem - used leaded paste - much easier to work with.
Not sure about the seperate flux - maybe for some specialist application - normal solder paste is a mix of solder and flux so no need for any additional flux. Normal method is to either stencil or dispense blobs from a syringe.
wrote ...

Also having issues getting lead free solder out of through hole pads on motherboard.
Flood the joint with leaded solder - it will then be much easier to desolder as it flows much more readily
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Dr. Slack
Sat Apr 28 2012, 05:32PM
Dr. Slack Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Only use lead-free solder if you are forced to by regulatory authorities, ie you are a manufacturer and you are selling goods to the public. Hobbyists use leaded.

Lead free melts at a higher temperature, so there is a much smaller safety margin for damaging your stuff. And then it doesn't wet as well. A real PITA unless you have the $$$ to buy the right gear.
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Carbon_Rod
Sun Apr 29 2012, 06:31AM
Carbon_Rod Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155
Some cheaper pastes do not contain flux, and require mixing.
Typically, most people will use the "no-wash" flux as the board yields are better for smaller chips.

I find a few pastes definitely need a solder mask layer to prevent bleeding away from leads.
Some RoHs compliant brands are inappropriate technology to deploy in colder climates: Link2 Link2


I have tried the paste film dipping process like the JUKI machines:
1.) place two parallel strips of scotch-tape on glass
2.) use a rigid sharp edge of another piece of glass to spread uniform paste film between strips of tape
3.) use position locked chuck in drill press to hold vacuum pick-up tool, and pick up SOIC.
4.) vertical dip SOIC leads on flat film (film layer dip depth less than 0.1 clearance between IC body and lead PCB contact area)
5.) mount SOIC to PCB surface
6.) run PCB re-flow oven

It works for larger BGA chips, but it seems to depend on how tacky the paste remains as a film... Also, it doesn't work at all with things like QFNs etc...
The simple $25 stencil can mean less hassle in the long run.

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