Low melting point solder paste

[JacquesBBB]

I  am quite new to smd soldering  with solder paste.
I needed to solder some sensors with QFN package, so it was not possible to
use normal soldering iron.

I am using an average kitchen hotplate, with no regulation and  supervise everything
manually with the temperature sensor of a multimeter.

All this is rather approximate, but I used
low melting point solder paste (Alloy: SN42 + Bi58) with 138 C  melting point.
http://www.electrodragon.com/product/low-soldering-point-paste138syring-type/

I can thus do everything with a large  error margin.

All this seems quite nice,  and  I wonder if there is any drawback at using this
 low melting point solder paste vs normal one ? Any advices ?

Thanks

[helius]

According to the Wikipedia list of solders, it is brittle, and mixing with lead solder can make joints fail. So it would not be recommended for parts tinned with lead solder or HASL finish.
https://en.wikipedia.org/wiki/Solder#cite_ref-soljorel_30-4

[Jeroen3]

Just get 63sn, 37pb, melts at 183, and if you add additional flux paste it will work fine.
You can add a dip of flux paste and touch and drag with a fine soldering iron to rework the shorts. Make sure to not put too much paste on the center pad. Also don't heat too quickly because the paste can "explode".

[zapta]

IIRC, this allow is more brittle than normal solder.

[tooki]

SRA is manufacturing and selling these SnBiAg solder paste on Amazon. Warning -- very short shelf life at room temperature. Flux deactivates over time, so refrigerate this thing if you need it.

<off topic>Why is that, actually? I ask simply because fluxes themselves (be they the solid core of solder wire, or the pastes, or the liquids) have either indefinite or long shelf lives, yet solder paste has rather short life. And me being the curious fellow I am, want to know! </off topic>

[helius]

it's because any change in viscosity will make it not print properly with a stencil. If you apply it manually the shelf life is much less an issue.

[tooki]

Well yes, but that's not my question. I'm trying to figure out why it ages so fast, not what happens once it's aged.

[JacquesBBB]

it's because any change in viscosity will make it not print properly with a stencil. If you apply it manually the shelf life is much less an issue.

Thanks for this precision.  I have seen that one can rejuvenate solder paste by adding some flux if its too dry.  From what you say  I understand  that  if the paste is applied manually,
precise viscosity is not too much a concern and
one can store the paste many years without problems. Am I right  ?

[zapta]

I am not sure about the 'many years'. It's better to buy in smaller quantities so you refresh it once in a while.

[helius]

Well yes, but that's not my question. I'm trying to figure out why it ages so fast, not what happens once it's aged.

My uneducated guess would be polymerization reactions in the flux carrier that are catalyzed by metal surfaces. When the carrier gets slightly stickier, it adheres adjacent balls together, which causes problems for stencil printing (and also for dispensing through narrow needles). There is also the problem of slumping that makes the spacing of balls tighter at the bottom of the tube because they are not neutrally buoyant.