How-to: Sjaak hand-solders a DFN chip

in DIY, how-to by Sjaak | 20 comments

A couple of days ago we discussed some cute little PIC 12F chips. As promised, here is a post about hand-soldering one.

Tools we used:

How-to continues below.

When we added the DFN package to our library, we forgot to remove the soldermask under the middle pad. This pad isn’t connected and has no electrical function. We suspect it cools the chip down and  helps position the chip during the reflow soldering to snap the package into place.

We used the scalpel to remove the silkscreen.

We tinned the paths both on the PCB and the package. Be sure to use flux as this will improve the flow of the solder. Don’t use an excessive amount of solder, just wet the pads. After tinning apply extra flux. We used kapton tape to protect the PCB and a trace we accidentally scraped.

We used the hot-air tool to melt the solder on the pads, then carefully put the package in the right spot.

When the airspeed is set high (it always is too high with light objects!!) the chances are the little chip will fly away. We had the best luck with dropping the chip and removing the hot air, wait till the solder has cooled down, and use the hot-air tool again to flow the solder underneath it (aim the nozzle straight on the package to avoid  it from flying away).

After using the hot air, use flux to wet the sides of the chip and slide with the soldering iron along the sides.

After soldering the DFN, the remaining parts are easy. We can’t say the satisfaction is as great as your first blinking LEDs project, but it came very close! Ow, just a note: check the DFN before soldering the remaining components to the board, this will save lots of time! ;)

The result is on the top of this post. If someone has a good use for these little PICs leave a comment

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Comments

  1. Menno says:

    Would this work with an LFN type component too?

    • Sjaak says:

      It prolly does (My guess is based on the name!!). This technique can also be used for QFN.

      Later on we will alter our footprints in the library for easy handsoldering.

  2. Using solder paste instead of tinning usually works better.

    a) Because the paste is tacky enough to resist the airflow (so you don’t have to press on the component with tweezers, which draws away some of the heat, causing the package to heat up unevenly and stressing it further), and

    b) Because you can see when it’s done clearly :)

    There’s one downside to it – you have to apply the paste very carefully and evenly. Solder bridges under the chip are quite possible. If it doesn’t work, however, you can always reflow, remove, wick it up, put the component back and reflow again (this time using tweezers).

    • (PS: no need for a stencil, you can use a toothpick, plus, you don’t have to apply the paste to each pad perfectly… even having the paste smeared across multiple pads usually works, as surface tension does its thing)

      • erdabyz says:

        I use the tip of my electronics tweezers. Works far better than a toothpick and it’s reusable.

  3. Sjaak says:

    There was a post about ghetto stencilling some time ago. It uses a transparant sheet with the chip cut out. This will prolly help to evenly spread the paste.

    See this blogpost for more info: http://dangerousprototypes.com/2011/07/21/mma7455l-accelerometer-free-pcb-build/

    We are going to alter the footprints to allow easier handsoldering. However I did 5 simular chips using this technique in about 15 mins and they were all soldered ok. The key thing is flux I guess.

  4. Only when generously applied.

  5. megabug says:

    I like this pcb… dunno why, but it looks cool. :D

  6. megabug says:

    Oh yeah, project idea: I have a (liquid-)soap dispenser hanging on my wall and I want some LED effects in it when someone is taking soap! :D Just for shocking people… And maybe a simple rf transmitter (433MHz) for notifying me when the battery is running low.

  7. Rubu says:

    I’ve soldered QFN with just an iron. What I did was extend the pad outwards, stick the part to the PCB with tape, put some paste on the pads outside of the part, and ‘push’ it under the package with the iron. If you use enough flux, the solder will ‘suck’ itself under the package and you can make a decent connection, but personally I wouldn’t use it for anything other than prototyping.
    Note that this is only possible if the part doesn’t have a middle pad.
    I posted a picture in this http://dangerousprototypes.com/forum/viewtopic.php?f=19&t=2518&p=24398&hilit=mma8453Q#p24398 thread.

  8. Drone says:

    I was scratching my head on this one. Overkill I kept saying to myself. Like Rubu, I’ve soldered QFN parts on board with just a hand iron. If you are a bit careful, use fine gauge solder, and are a bit generous with the flux, the solder will wick itself under the part. As for the center pad (when it is required), make a large via and solder it from the other side being generous with the solder if the center pad is intended to act as a heat sink..

    • rsdio says:

      Several vias on the center pad are a good idea because they add additional mass to the heat sink. Some companies give examples in their data sheets, but you might have to look beyond the specific data sheet for the part number and instead find a data sheet for the package itself.

      • Sjaak says:

        We are going to add a through-hole to the centerpad and make the pads for the pin rounded. This should help handsoldering a lot (we hope :))

  9. ewertz says:

    “we forgot to remove the silkscreen under the middle pad”

    silkscreen -> soldermask

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