Pocobor.

Quad Flat No-Lead Chip

This post builds on my previous post, which discussed hand soldering surface mount passives and QFP chips. As I mentioned last time, these are vital skills for any circuit board designer – the ability to modify or rework a board is integral to a time-effective development process.

Today I’m going to talk a little about hand soldering QFN chips – these chips are extremely small, which can be very useful for tiny, densely packed boards but makes them a little difficult to work with by hand. As a reminder, these posts are building on some excellent tutorials created by Curious Inventor. The relevant information for QFN chips can be found here.

Quad Flat No-lead (QFN) chips are like QFP chips but don’t have leads extending beyond the sides of the chip; instead, they have pads on the bottom of the chip. For these types of chips, however, my experience diverges from the Curious Inventor tutorial a bit. Their method certainly works but I have found it easier and more effective to use solder paste with the hot-air station as opposed to tinning the pads with actual solder. In terms of equipment, you will need good tweezers, a hot air source (either hot air gun or dedicated soldering rework station), solder paste and solder flux.

The technique that we have found works best involves the following steps:

1. Clean the chip and circuit board pads and make sure that the chip can lie flush on the board.
2. Apply solder paste on all of the circuit board pads using an applicator syringe (below). Make sure there is paste on all pads but use sparingly – too much could lead to shorts.



Solder Paste

3. Place the chip in the correct position on the board using tweezers. The solder paste may make it difficult to see if the chip is correctly aligned with the pads but use silkscreen markings on the PCB to index the chip’s location. Fortunately, surface tension effects with the large center pad will help the chip self-align as long as it is close to being in the correct position. Make sure that the chip is firmly pressed down on the board and is flush or as close as possible to it.
4. Preheat the chip and board using the hot air gun to about 200 degrees Fahrenheit. Depending on your setup, this can be accomplished by varying the heat and flow rate of the gun or changing the distance between the chip and the hot air nozzle. The chip should remain in place without fixturing as long as the board is horizontal and the hot air is coming orthogonally from above.
5. Turn up the heat to about 350 or 400 degrees. Once you see the solder melt, continue applying heat for up to 15 seconds more. You want to be sure that the solder has fully melted and reflowed to all of the pads but too long and you can overheat and break the chip and / or board.
6. There will probably be excess solder along the sides of the chip. Apply flux and use a soldering iron and wick to remove this solder. Try to remove all accessible solder, not just visible accumulations.
7. Verify that no shorts are visible with a loupe or microscope. Professional shops sometimes use x-rays to check underneath the chip and if this kind of equipment is available, it can be very helpful. However, in our experience it is not necessary.

QFN chip mounted on a PCB

I’d love to hear what works (or doesn’t) for others so feel free to add your two cents in the comments.

Happy Soldering!

DISCLAIMER: This experiment was conducted in a controlled laboratory environment with the appropriate safety equipment and considerations. DON’T TRY THIS AT HOME!

It is often said that failure is the fastest way to success.  If you’ve ever mistakenly hooked up a capacitor backwards or exceeded the operational voltage, you’ll do your best to never do it again! With this in mind, I thought it would be interesting to intentionally trigger and document the failure of capacitors, an electronic component that we use every day at Pocobor.  I have chosen to focus on one very dramatic failure mode: explosion.  Capacitors don’t often explode, but when they do it is a remarkable event.  Capacitors all have a voltage rating that should not be exceeded (unless you want them to explode), and some capacitors, known as electrolytic capacitors, also have a preferred polarity  (one side is “+” and one side is “-“).  Reversing the polarity of these caps is an easy mistake to make and an easy way to produce an explosion. In this case it’s no mistake, but the result is the same.

In the video, I have intentionally connected two electrolytic capacitors in reverse polarity to induce an explosion.  As I slowly turn up the voltage (off camera), current starts to flow through the capacitor and heat up the interior.  DC current is not supposed to flow through capacitors, a sign of reverse polarity and looming failure.  As the heating causes pressure to build inside the capacitor, the capacitor’s end plug is pushed out of the housing, and it’s only a matter of time before the entire capacitor explodes.

Big takeaways:

In the electronics world, capacitors are referred to as “passives”  (along with resistors, inductors, and the up-and-coming memristor), but there is nothing passive about these explosions.  Some very passive aggressive, aggressive passives?

10uF Electrolytic Capacitor (Reverse Polarity)

47uF Electrolytic Capacitor (Reverse Polarity)

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:

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

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:

Happy Soldering!