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Drag Soldering for Surface Mount Chips

I’ve seen many people look at surface mount chips, with their tiny sub-millimeter pin spacing, and assume it’s impossible to hand solder them without special tools and equipment. I used to believe it myself, but fortunately for us hobbyists, it’s actually quite easy to hand solder most SMT chips with nothing but a standard soldering iron!

The video above shows how I use the drag soldering technique to solder a 44-pin chip in a typical TQFP package. The chip is only about 1 square centimeter, so those pins are tiny. Attempting to solder them one at a time in through-hole style will never work. Instead, the trick is to use the magic of flux and the surface tension of solder to do the hard work for you. Once you get the hang of it, it’s as easy or easier than soldering a through-hole component.

The process begins by applying a liberal amount of flux to the pads, then positioning the chip on top. I use a pencil eraser to hold the chip steady while I tack down a couple of pins with a blob of solder from my iron. If some pins accidentally get bridged together while tacking them down, it’s OK. Next, I apply more flux to the sides of the chip, wetting both the pins and the pads underneath. The final step is to lay a few millimeters of solder onto the pins at the edge of a row, then use the iron to melt it and drag the molten solder blob horizontally across all the pins in the row. It seems as if that should bridge every single pin together into a giant mess, but with enough flux the solder will magically stick only to the pins and pads, without creating any bridges between them. It’s fun to watch!

Most of the time, I’m able to solder all 44 pins with this technique without creating any bridges. If I do create a bridge, I can often fix it by applying more flux and then briefly heating the bridged pins with the iron. The video shows how to recover when that trick doesn’t work: a piece of solder wick (a braid of thin copper wire) can be laid on top of the bridge, with the iron laid on top of the wick, and the excess solder will be sucked up into the wick and leave a clean joint behind.

Read 6 comments and join the conversation 

6 Comments so far

  1. Tom Davies - February 1st, 2014 3:06 pm

    What sort of flux do you use?

  2. Steve Chamberlin - February 2nd, 2014 8:04 pm

    It’s a no-name brand of “no clean” flux in a dispensing pen. I’ve used it so much that the label is worn off, and I can’t even tell what brand it is!

  3. David - February 6th, 2014 4:06 am

    Hi Steve, are you using lead free solder in this video. Is drag soldering even doable with lead free?

  4. Steve Chamberlin - February 6th, 2014 7:01 am

    It’s leaded solder, but I don’t know of any reason it wouldn’t also work with lead-free solder.

  5. Mark Leuhorn - May 30th, 2016 2:18 pm

    Informative video! What size soldering tip and solder are you using?

  6. chrisoutwright - September 26th, 2017 9:22 am

    This is a great post and video!
    And I thought a human could not solder such things!
    Now I can resize my PCB better!

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