Pocobor.

Hand Soldering – Part 1

Context

I originally set out this week to put together a tutorial on hand soldering surface mount components. Then, I realized that there are some excellent resources already out there and decided instead to write a post supplementing one of the existing tutorials with some things I have found based on our experience at Pocobor. Accordingly, this post builds on some excellent tutorials from the Curious Inventor website:

  • Soldering Surface Mount Passives
  • Soldering a QFP and a Fine Pitch QFP
  • Soldering a QFN Package by Hand (to be covered in Hand Soldering -  Part 2)
  • Why Should You Care?

    If you aren’t familiar with how to hand solder surface mount components and you are reading this blog (which potentially indicates some level of interest in electronics), you should definitely think about trying it out. It is a very valuable skill for several reasons: (1) assembling your own board can be cheaper than outsourcing assembly (although it can also be fairly time-consuming), (2) being able to modify or rework a board is integral to the development process. Prototyping inherently involves some trial and error / experimentation and the ability to perform a little circuit board surgery can save a ton of time and money on new board iterations.

    Passives

    I don’t have much to add to the Curious Inventor’s take on this – the two things I would just emphasize are that (1) I generally find that flux is unnecessary for components that are 0805 or larger, and (2) very little solder is needed on the pad to tack down the first side of the component (too much solder actually makes it harder to align the component and get a good joint). In addition, I would point out that I generally find that a very fine-tipped soldering iron is more effective for most operations than a blunt or dull tip.

    QFP Chips

    qfp

    Quad Flat Pack (QFP) chips are the little chips with legs coming off the sides that look like little spiders or insects. I endorse most of the points made in the tutorial and have a few of my own to add:

  • Drag soldering is great in theory and can be effective in practice. However, it is very important to be gentle with your iron; otherwise, you can bend pins and destroy either the chip or the pads on the board. The leads are not strong in the transverse direction. For drag soldering, a fine-tipped iron that is small enough to get between adjacent leads is actually a detriment and a larger iron should be used that can be lightly dragged over top of the pins.
  • The adage about machining holds for soldering as well – spend 80% of your time fixturing and 20% of your time soldering (machining). Making sure that your component is correctly aligned and robustly fixed in place before you start soldering will save a ton of time in the long run.
  • One alternative method is to use a hot air / rework station with solder paste instead of a soldering iron. Lay down the solder paste using a syringe then align and fixture the chip. After this, heat the paste until molten and then clean up the joints with a soldering iron and flux as described in the tutorial. Note that the danger with this method is that the chip may be overheated and fried by the hot air (on the flip side of the equation is the fact that you are less likely to break any of the leads or pads).
  • Any time you are reworking or trying to reheat or re-flow solder, flux is invaluable. Use it fairly liberally but be careful with clean-up, especially if you are not using no-clean flux.
  • Personally, I do not recommend the flood and wick method – in fact, be careful anytime you use a solder wick. There is a tendency for the wick to remove more solder than you want, which can result in a bad connection (or, even worse, an intermittently bad connection, which is a nightmare to debug). A solder-sucker or vacuum based solder remover can be a useful alternative to a wick that tends to leave a more ideal amount of solder.
  • When you think you are done, make sure to perform a thorough visual inspection of the component with a loupe or ideally a microscope. Again, a little extra time verifying things early on can save a lot of debugging headaches later.
  • Happy Soldering!

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