Drag soldering for surface mount chips

This video by Steve of Big Mess o’ Wires demonstrates how to solder a 44-pin chip in a typical TQFP package using the drag soldering technique:

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!

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3 Comments

      1. Alan, you can avoid the risk to move th chip when you drag across pins. When you solder first corner to maintain the chip in place (like on the video), just solder the diagonally opposite corner pins of you chip. This way, when you drag across pins, you won’t move the chip. Of course, you have to start drag soldering on the two sides that have no pin soldered first ;)

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