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It worked for me 8 times :)

Thankfully the bottom pad on DFN packages reach the edge on 2 sides and I would think this technique wouldn't work with QFN.

QFN is (very, very slowly) opening up to the idea of "wettable flanks", which is increasing the reliability of QFN soldering anyway... which is increasing the opportunity of "side-soldering" QFN packages.

https://www.ti.com/document-viewer/lit/html/SSZTBN6

Here's to hoping for more manual soldering techniques to work into the future! We all know that these parts are getting extremely small and harder to work with, but its still clearly possible to make devices.

But yeah, QFNs, if this wettable-flank trend continues, could be quite reliably soldered and inspected from the sides soon. The needs of the high-reliability automotive industry to create an inspection methodology outside of X-Rays is driving this change.

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Glad to hear that your technique worked in any case. I wouldn't have tried that, but maybe I'll try it once next time I have a hot-air gun out and a DFN part to try it on.

EDIT: Note that Microchip apparently has some DFNs that also have wettable-flanks: https://www.microchip.com/en-us/about/media-center/blog/2022...

That's very interesting, I've always found QFN to be annoying to work with

> The needs of the high-reliability automotive industry to create an inspection methodology outside of X-Rays is driving this change.

Why is that a particular requirement? Or is it just nobody wants/rather avoid x-ray, and automotive has greater need for inspection at all?

It's probably cost? X-ray is an expensive technique.

Yeah but that's not specific to automotive right, that's what I meant. I was trying to understand whether they meant the need for inspection or the non-X-ray was the particular concern in the automotive industry.

Automotive has much higher reliability requirements than consumer devices, and since solder failures are probably the leading cause of failures using *FN parts, they want to have effective inspection methods to ensure devices are assembled properly.

When you use parts with leads, you can easily perform all of your QA checks using machine vision (all all the components in the right place, are they correctly aligned, are the connections properly soldered, etc), but if you use parts with no leads and the solder connections are hidden, you need a separate X-ray check which takes additional time, assembly-line space, and money. Depending on the components in question, it may also be tricky to get enough contrast in the X-ray image to identify defects.

To save others from looking up DFN / QFN:

DFN -> Dual-flat no-leads package

QFN -> Quad-flat no-leads package

https://en.wikipedia.org/wiki/Flat_no-leads_package

The important note is:

1. Longer leads of older packaging have higher parasitic inductance, parasitic capacitance, and parasitic resistance. Which hurts many electronic designs.

2. The thermal pad on DFN / QFNs allows for direct-heat transfer off of the chip-package (while DIP, SOIC, and other older designs do NOT allow for this direct-heat transfer off). This improves thermals of hot parts, as you can use the whole PCB as a heatsink now, instead of having to add a heatsink elsewhere.

3. DFN and QFN are physically much smaller than traditional DIP, SOIC or TQFP packages. So people who use these smaller chip packages can make even smaller circuit boards, or alternatively, a more complex design fit in smaller spaces.

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QFN and DFN are very popular parts for modern professional level electronics. But do note that their small size and tiny leads make them harder to deal with at the hobbyist level. But its possible with enough flux, solder paste, and hot-air rework to work with these parts.

I would recommend that beginner electronics who are just getting into custom PCB design work with larger parts (like SOIC packages) instead. Bigger is often easier. Move to these tiny DFN or QFN parts only after you've gained confidence in your surface-mount skills.

Any tips for handling BGA packages? Is it possible for hobbyists?

I've never used a BGA unfortunately.

My understanding is that BGA solder-balls come pre-attached to your chips in most cases. As long as you reflow-solder it on correctly on the 1st try, you're good.

If you need to "fix" BGA, that's where things become very difficult for hobbyists. To do this, you need to unsolder the BGA with hot-air, lift it off, then "Reball" the BGA, which is equipment that likely hobbyists don't have.

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With regards to QFN or DFN vs BGA, I would expect BGA to be easier, as long as you've precisely made your soldermask to accept the correct sized solder-balls (which should be part of your chip's documentation).

That is: BGA's hardest part is getting the board design just right. Especially because most BGAs require 4 or more layers before you can route all of the lines to them. In fact, the massive BGAs are physically impossible on anything less than 8 layer boards.

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Hobbyists won't have XRay equipment to see if the BGAs successfully worked either. So rework and quality-control are basically impossible. Your only hope is to "do it blind" and hope for the best.

Which... is fine? Even BGA parts are relatively cheap. So build 3+ of your design at once. Have appropriate test-probes to take measurements during the "smoke test", and bam, you're good. One of those three tries is probably going to be successful.

I wonder how you'd align them. The silkscreen is often not very precisely aligned to the copper layers, so you can't use markings in the silkscreen for alignment. Maybe use the top copper layer then instead?

EDIT: I found this quite interesting video: https://www.youtube.com/watch?v=0KbINAq0hcU

I'm not sure why "too much" solder paste is needed to make this work. I've done something similar on quite a few occasions, but using regular solder. Just put a slightly generous amount on each pad, but not enough to create any shorts. Add flux (ideally, something rosin-based that will last longer in the heat of the next step). Heat up the whole footprint with the hot air gun, and gently place the IC on top.

This method saves blasting the IC with hot air while heating up the whole lot.

"Way too much solder paste" is a codeword for "I didn't buy a solder-stencil", which is reasonably common on cheaper projects in my experience.

Instead of using a solder-stencil to deposit a carefully calibrated amount of solder onto any location, you can buy a solder-paste syringe and squeeze the solder-paste into place.

This almost *always* results in too much solder, because its a hand application process, and its often better to have more-solder + cleanup than too little solder in my experience.

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https://www.amazon.com/Solder-Bi57-6-No-Clean-Lead-Free-Temp...

Obviously if you had perfect hand-control (ex: a solder paste robot... or a solder stencil, or other methodology), that's preferred.

But if you're doing these cheap designs you'll need to plan for the "too much solder" situation. In fact, "too much solder" is the default choice and quite fine, albeit with a simple cleanup step. (Ex: swish around a small piece of solder wick in my experience while the solder is still hot, or while the solder-wick is touching a soldering iron, etc. etc. Plenty of cleanup methods available).

Do it dirty then clean-up is just the default mode of operation in hobby-level manufacturing / solder paste / soldering. Its surprisingly effective.

I don't know if it's equipment or a skill issue but I had trouble with putting an even amount of solder on the pads this way. I have seen this approach on YouTube.

Assuming you have the thinnest tips, solder, and solder braid you can find (they shold be comparable to the pad sizes you are soldering, for this kind of work), honestly it's probably a skill issue. There's a "feel" for how much solder the pad needs, and it's a bit less than the most it can take, as it'll get displaced outwards when the IC is applied.

If we're talking solder paste here, I doubt its a skill issue.

Its super easy to grab a toothpick and pull out excess solder paste on a per-pad basis. Just do it before you heat the board up (solder paste is applied cold, so there's plenty of time to "correct" things if they look bad).

The "problem" is that this whole exercise is an unnecessary waste of time in my experience. Just live with the excess solder and solder-wick it up at a later point. Swishing around a solder-wick is way easier than tooth-picking the solder-paste on each pad to the right size.

Perfectly valid method, IMO. I always put some vias in the thermal pad and expose the copper on the other side. That way the vias don't get plugged with solder mask and wick away excess solder. It also lets you heat the thermal pad with an iron from the other side for real low-budget SMD work.

Squishing the chip down is fine, you just have to be careful to align it with the pads properly. I don't think I've ever seen a solder bridge on a QFN other than a big ball on the outside edge. Modern solder alloys and solder masks are really good at preventing bridges. The solder really doesn't want to stay between the package body and the solder mask, it will try very hard to only stick to metal.

Perhaps I should better explain why I dislike it.

A lot of parts (ex: Crystals) are extremely heat-sensitive. So even just a few dozen seconds extra of soldering could damage your crystals (ex: internal solder will reflow and break the vacuum seal).

So *ANY* action which requires you to "carefully squeeze down" the solder paste out means you're going in there with tweezers, and possibly leaving the board in 200C+ soldering temperatures a bit longer than usual.

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IE: Its not that this technique "won't work" in isolation, its that you won't get "consistent and reliable results", especially when we consider the nearby parts (ex: a possible XTAL) that could be damaged while you're messing around with the hot-air gun.

The standard technique of reflow soldering is to use a solder-paste stencil to deposit the "right" amount of solder paste onto your board, then use a reflow oven to carefully heat the board (and hold the heat) at the right temperatures, so that nothing gets damaged.

Once we start relying upon "Well, keep the board heated a bit longer so that my tweezers can do the work" is when you start overheating chips and otherwise creating failed parts.

> Modern solder alloys and solder masks are really good at preventing bridges. The solder really doesn't want to stay between the package body and the solder mask, it will try very hard to only stick to metal.

This also matches my experience. The surface tension of solder (be it lead based, or lead-free, or low-temperature solder paste) is far greater than most people expect.

You can strongly rely upon "surface tension" to magically do the right thing on your boards, as long as you've got good quality soldermasks.

In fact, a big problem with reflow soldering is "tombstoning" (ex: surface tension of lead is so strong, it pulls your components OUT of position). There's a LOT of surface tension at play here. Its great in most cases, but keep an eye out for those tombstones!

Everything that could go wrong in my experience is fixed with rework hot-air + generation applications of flux + solder wick + soldering iron + tweezers. And rather easily mind you, I know that didn't sound very easy but... the right tools make any situation fixable. Its just knowing how to use all those tools in the right cases.

I extend the pads out from underneath the chip. Then it's easily hand-solderable if needed.