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Looking through videos of people soldering fine-pitch IC packages (like SOIC, SSOP, SOT), they usually use a large amount of rosin-core flux to make the joints nice and shiny. I have lead-free solder (leaded is not an option for me), with 99.7 Sn, 0.3 Cu, and 2% flux. The solder is 0.71 mm in diameter.

Do I need to use flux for drag soldering fine-pitch packages, or is the 2% flux in the solder sufficient? Usually I use 1 mm lead-free, but for fine-pitch packages I use 0.71 mm solder.

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  • \$\begingroup\$ Many of the people I know who complain about lead-free solder use too little, low quality flux or don't use flux at all. Do you need to use additional flux? Maybe not especially if you are fast enough to work with the core flux, but you will make your life a lot harder especially working with higher temperature lead-free solders where the core flux will burn away very quickly. \$\endgroup\$
    – user1850479
    Commented Feb 20 at 14:08
  • \$\begingroup\$ Is it absolutely necessary? Probably not but it could make your life easier, especially with that SnCu solder. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Feb 20 at 16:34

3 Answers 3

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Generally, the flux contained in your solder is enough for fast and direct-apply soldering.
With small soldering, where the pins are really close to each other, you really need the capillary action added by the flux to avoid bridge soldering.

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    \$\begingroup\$ What tin are you referring to? \$\endgroup\$
    – Andy aka
    Commented Feb 20 at 8:43
  • \$\begingroup\$ Well any typical Tin that you use for soldering I don't realy know all the possibilities. \$\endgroup\$
    – SteveninJ
    Commented Feb 20 at 8:59
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    \$\begingroup\$ @andyaka, Lemon's solder is 99.7% tin. and 0.3% not-tin. calling it "tin" is not a gross distortion of the facts. It's better than 99% pure! \$\endgroup\$
    – Jasen Слава Україні
    Commented Feb 20 at 9:13
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    \$\begingroup\$ @JasenСлаваУкраїні in the context it was written it could have meant "a tin" as in what native English speakers generally refer to as a metal container that contains something (e.g. a tin of paint or beans). I never said it was a gross distortion of anything. I was asking for clarification. \$\endgroup\$
    – Andy aka
    Commented Feb 20 at 9:46
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    \$\begingroup\$ Sorry I'm not a native speaker, in french we use the french word for Tin... \$\endgroup\$
    – SteveninJ
    Commented Feb 20 at 10:30
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It's kind of hard to find any decent solder for hand soldering without a rosin flux core. Also I wouldn't really call "SOIC, SSOP, SOT" fine pitch, these are typically 1.27mm/0.05'' or larger. You should be able to solder these without any problems at all using standard practices and no external flux.

However, 0.7mm solder is used for soldering elephants! :) That's way too thick still, you must use 0.25mm or thinner when dealing with fine pitch. The most common mistake by far when doing SMD by hand seems to be too much solder. Use some RoHS solder with "no clean" rosin core flux.

As for actual fine pitch (0.65mm or smaller), there's basically 3 different hand soldering techniques:

  • Pin by pin. I posted some advise for the first one of these here: https://electronics.stackexchange.com/a/502238/6102, with 0.5mm pitch QFP as the example.
  • "All at once", which means align the part and solder outer pins as told in my link above, but then just spam lots of solder all along one side of the part. This will typically work very poorly just with the rosin core flux - you will get bridges all over the place. Those who use this technique use external flux and anything else is not advised.
  • Solder hot air + paste. This is obviously a different story entirely, but here too you'll probably want external flux and applied in advance. (And a solder hot air station with temperature and air flow control, not some generic heat gun that you use for shrink tubes!) This is about the only way to hand solder heat pads, QFN and BGA by hand too, though a reflow oven is obviously what's recommended for such parts.

No matter method, external flux is highly recommended for cleaning up bridges or when removing small solder blobs.

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    \$\begingroup\$ strongly advice again against additional flux for the paste+hot air approach. Parts already tend to "swim about" and drift when the flux in the paste melts if the nozzle is set to a high air flow. I've never heard about a process where additional flux is added to paste before reflow, and I use this method constantly. \$\endgroup\$
    – tobalt
    Commented Feb 20 at 17:29
  • \$\begingroup\$ unfortunately, my local hobby store doesn't sell any solder smaller than 0.71mm sadly. the only other solder I have is the 1mm type, which I use for DIPs and other through hole components. I would use a hot air gun if I could, but I don't have the funds to buy one as of yet. \$\endgroup\$
    – lemon
    Commented Feb 20 at 20:27
  • \$\begingroup\$ @tobalt Not to the paste but to the surfaces. Maybe I'm biased since when I use hot air it is most often for replacing an existing part and not for first time soldering. You can never have too much flux though, I think. \$\endgroup\$
    – Lundin
    Commented Feb 21 at 7:26
  • \$\begingroup\$ @lemon So order some online? I wouldn't expect a hobby store to sell much in the way of useful equipment, soldering is too specialized. \$\endgroup\$
    – Lundin
    Commented Feb 21 at 7:27
  • \$\begingroup\$ <$60 for a 200gm roll of solder is a bit overpriced, I don't solder that often so 15g tubes are what I usually use(will last me a few years). \$\endgroup\$
    – lemon
    Commented Feb 22 at 5:51
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Youi absolutely need lots of flux for any kind of fine soldering with any kind of solder core. The rosin inside the core is not enough. I'd recommend liquid or gel rosin-based flux.

For drag soldering you also need a wide soldering tip which can hold enough solder on the surface to create a blob. Applying too much solder is less of a problem than applying too little, which can lead to unreliable, hard-to diagnose faults where the contact sometimes works and sometimes doesn't. If there is enough flux and you drag it right, it will draw away just enough solder to make a good joint. Otherwise, you can remove excess solder with a soldering wick.

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    \$\begingroup\$ Umm. No. You do not absolutely need lots of flux. You need an appropriate amount of flux. For most tasks, the flux core of the solder is appropriate. \$\endgroup\$
    – JRE
    Commented Feb 20 at 16:52

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