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Replacing QSPI Flash On ItsyBitsy M4
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Replacing QSPI Flash On ItsyBitsy M4

by bcochran1 on Sat Mar 30, 2019 7:48 am

Hi!

What is the easiest technique for replacing the QSPI flash on the ItsyBitsy M4? Is there a technique that can be used successfully in the home, with just a Hakko soldering iron? It looks doubtful to me. Even if hot air were used, it looks to me as if an inexperienced technician will likely remove all the neighboring parts, too.

Thanks so much

Bob

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Re: Replacing QSPI Flash On ItsyBitsy M4

by adafruit_support_mike on Sun Mar 31, 2019 1:00 am

It would be a touchy operation.

The chip is a DFQN package, meaning the pins are flat pads that come out the bottom of the package. A soldering iron won't do any good, since there's no place to physically touch the traces that will transfer heat where it needs to go.. and certainly not for all the pads at the same time, which would be necessary to remove the chip or place a new one.

Hot air will work, but you'd need a small nozzle and low air speed.

Melting the solder on parts you don't want to modify is really no problem in SMT work. The dominant force is the surface tension of the molten solder, so even if the joints do melt, the solder solder will hold the parts where they need to go. You can actually be a bit sloppy about placing components on solder paste because the solder will pull the parts into alignment with the PCB pads when it melts.

The big concern is that you don't want a hot air gun blowing on the parts hard enough to overpower the solder's surface tension. If components lift off and get blown away, you have have a mess.

For removing and replacing DFQN packages, I'd probably want a combination of an IR preheating tray and a hot air gun. The IR tray is more or less a high-tech hot plate that will bring the whole PCB up to a certain temperature. For rework, you want that temperature to be maybe 20 degrees below the melting point of the solder. Then you use the low-volume hot air gun to heat the DFQN package that last few degrees necessary to make the solder melt.

After all the prep, actually removing a chip is kind of anticlimactic. You grab it with tweezers, lift gently, bring in the hot air, and it lifts right off the board when the temperature is right.

To place a new chip on the same pads, you'd start by cleaning the existing solder off the pads as thoroughly as possible with solder wick, then cleaning away any flux residue. Once the pads and footprint are clean, you can apply solder paste, set the new component on the paste, and use standard reflow heating to melt the paste: bring the temperature up gradually to about 20 degrees less than the melting point of the solder, let things soak for a couple of minutes to make sure the temperature is even across the PCB, then rasie the temperature again until the solder melts.

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Re: Replacing QSPI Flash On ItsyBitsy M4

by bcochran1 on Sun Mar 31, 2019 1:50 pm

Hi!

Thank you for the really excellent explanation of the chip replacement process. I'll give thought to my ability to do that. Let me ask about a different option going in the same direction.

I'm interested in purchasing an ItsyBitsy M0 Express and/or M4 Express board, but I would like it to have quite a lot more QSPI flash memory on board than just 2 MiB. Would it be possible for me to purchase a board from Adafruit which can be optioned up with a larger-density QSPI flash part? Say 16 MiB instead of the standard 2? If that is possible, then how can I get an estimate of the additional cost? If Adafruit does not offer a service that can do this, would there be a kit builder able to replace the flash memory for me?

This all has to do with a budding interest in CircuitPython. I am going to try replacing the QSPI flash on my Metro M4 Express board -- I have the memory on order from Digi-Key, see my thread about this in the CircuitPython forum. I think I can succeed with just a soldering iron, solder wick and patience.

Thanks so much

Bob

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Re: Replacing QSPI Flash On ItsyBitsy M4

by adafruit_support_mike on Mon Apr 01, 2019 1:42 am

I'm afraid we don't do custom orders at all.. our fab teams would never have time to do anything else, and we'd have to charge shift-the-decimal-point more per board to cover the costs.

That's one of the unexpected surprises of working with job shops.. a first run of PCBs will cost around $500, whether you get one or a hundred. For assembled boards, tweak the price up to about $1500. Most of the cost is front-loaded setup expense, and gets divided by the number of boards in the run. A 1-board run will have an amortized setup cost of $1500-ish, while a 100-board run will have an amortized cost of $15-ish per board. A production run of 1000 boards will amortize to $1.50-ish for setup cost, and at that point you'll start to see the costs of the PCB, components, and per-board fabrication cost.


The QSPI Flash chip on the Metro M4 is an SOT package, and those are easy to solder by hand. The pin pitch is 0.050", which is only half of the 0.1" pitch of a DIP.

Removing them can be a nuisance if you want to preserve the chip. The trick is to put a thin probe under the end of the chip and run a fairly large blob of molten solder back and forth across the pins on one side. It isn't hard to melt all four joints on the same side of an SOT-8 package, and to lift the pins slightly above the pads once they're free to move. Then you can do the same thing on the other side of the chip, lifting it all the way off the board when the pins come free.

If you aren't worried above saving the chip, it's faster and easier to clip all the pins with a flush cutter, remove the package, then desolder the remains of each pin one by one.

To place a new SOT-8, clean the pads well with solder wick, then melt a small blob of solder onto the pad for one of the corner pins. Then flux the pad and solder, place the chip onto the footprint, and melt the solder with the tip of your iron. You shouldn't even need to touch the pin with your iron.. the molten solder will transfer enough heat to make a good joint.

Once you have the chip tacked down, double-check the pins to make sure they're aligned to the rest of the pads. If you need to move the chip, reheat the tack joint and nudge the chip into a better position. It's a good idea to use a toothpick standing vertically as a lever, nudging the chip a few tenths of a millimeter by leaning the top of the toothpick in the direction you want the chip to go. Trying to push a chip directly is a good way to send it halfway across the room.

Once the pins are more or less aligned to the pads, heat the pad for the corner opposite the pin that's already tacked down, and solder that one. Once you have two joints, you can make finer adjustments in its position by melting one joint, pressing the chip to move it slightly, letting that joint cool, then melting and cooling the other joint to release the torque stored in it.

When the chip is in place and the two corner joints have been de-stressed, making the rest of the joints will take a few seconds.

Another quirk of SMT soldering is that you can forget the age-old advice, "heat the joint, not the solder." It's easier to make SMT joints if you tap the tip of your iron against the solder to pick up a small blob, dip that in flux, then touch the blob to the pin/pad you want to solder. The pin and pad are so small that the molten solder will heat them almost instantly, and capillary action will pull the molten solder off the tip and into the joint.

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Re: Replacing QSPI Flash On ItsyBitsy M4

by bcochran1 on Tue Apr 02, 2019 8:24 pm

Hi!

Thank you so much for the excellent advice. My new flash memory is on the way to me, and should be here in a few days. I'm going to follow your desoldering and soldering suggestions.

Thanks so much

Bob


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Posts: 359
Joined: Mon Jan 21, 2013 10:46 pm

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