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A project log for NoteOn Smartpen

Slim, wireless, self-contained. No special paper, no base station.

Nick AmesNick Ames 09/07/2014 at 22:152 Comments

(Apologies for the lack of updates; some family stuff has prevented me from working on NoteOn for the last few weeks.)

As of 2014-09-07, the IMU driver (which fetches data from the IMU and aux. accelerometer) has been completed, and prototype 2's defective aux. accelerometer has been replaced.

Auxiliary Accelerometer Replacement Process

I started out by removing the passive components surrounding the defective aux. accelerometer using a soldering iron, in order to give myself more room to work. (402-sized components can be easily removed by wiping them away with a large blob of solder.) I applied Kapton tape to the adjacent microcontroller to shield it from the hot air, and bundled the SPI debugging wires underneath the board to keep them from melting.

I used an Aoyue 850++ hot-air rework station to remove the defective device, then used an iron and solder wick to remove excess solder from the pads.

I decided to pre-tin the pads on the new device with solder balls, rather than using solder paste. The only solder paste I have is lead-free, which requires higher temperatures. I stuck the device upside-down on a piece of Kapton tape, applied copious liquid flux, and ran a molten solder blob over the pads.

Before attaching the new device, I pre-heated the PCB for about ten seconds with a medium air flow. I then applied flux, aligned the device with tweezers, and heated it for 25 seconds with a low air flow. (A higher air flow is much more effective at heating, but it can also blow the chip right off the board.)

After replacing the passive components I removed earlier, the new aux. accelerometer responded over I2C and produced valid data.

Discussions

Nate wrote 04/26/2016 at 17:38 point

It makes a huge mess, but I've had the most success with the following process: 

1. Remove old package via hot air,

2. Apply a small amount of flux and re-tin the pads. Apply new solder if necessary if pads look deficient in solder. 

3. Apply a large blob of flux on top of the tined pads, 

4. Place un-tined QFN package on flux blob. 

5. Heat up with hot air rework gun until you see package start to float on the pads. 

6. Poke (with tweezers) the package a few times and the package should self align with the pads if all pads are reflowing. At this point, provided the package aligned itself, you can remove the heat and let the package cool down. 

7. Use flux remover to clean up excess flux.

8. Continuity check all pads for shorts.

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Sean Hodgins wrote 09/17/2014 at 04:56 point
Gah, I hate those QFN packages without the side pads to see if they are actually connected. I have done this technique before but used solder paste and a stencil, I stuck the component to tape like you did and applied the stencil to the component upside down, then heated it to melt the paste. I found if I used solder balls if even one was a less amount of solder on the pin(pre tinned) then there was a higher chance that it would not connect due to the gap that would be produced. So there is another option as I mentioned before. Glad you got it working, this is an awesome project!

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