Low melting point solder paste - Page 1
Forum section: electronics / projects

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: November 15, 2015, 08:30:48 pm
content: I am quite new to smd soldering with solder paste.I needed to solder some sensors with QFN package, so it was not possible touse normal soldering iron.I am using an average kitchen hotplate, with no regulation and supervise everythingmanually with the temperature sensor of a multimeter.All this is rather approximate, but I usedlow 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 thislow melting point solder paste vs normal one ? Any advices ?Thanks

author: helius
author_rank: Super Contributor
timestamp: November 15, 2015, 08:38:30 pm
content: 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

author: Jeroen3
author_rank: Super Contributor
timestamp: November 15, 2015, 08:50:34 pm
content: 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".

author: zapta
author_rank: Super Contributor
timestamp: November 16, 2015, 04:03:02 am
content: IIRC, this allow is more brittle than normal solder.

author: tooki
author_rank: Super Contributor
timestamp: November 16, 2015, 10:18:50 am
content: Quote from: blueskull on November 16, 2015, 04:23:22 am<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>

author: helius
author_rank: Super Contributor
timestamp: November 16, 2015, 04:34:56 pm
content: 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.

author: tooki
author_rank: Super Contributor
timestamp: November 16, 2015, 04:51:17 pm
content: Well yes, but that's not my question. I'm trying to figure outwhyit ages so fast, not what happens once it's aged.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: November 16, 2015, 07:11:01 pm
content: Quote from: helius on November 16, 2015, 04:34:56 pmThanks 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 andone can store the paste many years without problems. Am I right ?

author: zapta
author_rank: Super Contributor
timestamp: November 16, 2015, 07:57:20 pm
content: I am not sure about the 'many years'. It's better to buy in smaller quantities so you refresh it once in a while.

author: helius
author_rank: Super Contributor
timestamp: November 16, 2015, 08:57:27 pm
content: Quote from: tooki on November 16, 2015, 04:51:17 pmMy 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.