MMA7455L soldering explained

In our previous Workshop Video we mentioned we asked Pietja to solder up those pesky MMA7455L accelerometers. Here is how he got around to doing it. Check out the pics and his instructions below.

Ian asked me to solder some MMA7455L on the USB POV Toy boards. So when I got them the first thing i did was clean them all with some acetone. After that I pre tinning the pad s of the chip so it had some solder with flux on them.

I did the same thing on the boars and used solder wick to get the solder of but leaving a layer of flux.

Then i placed the chip on the board (no extra flux used, all the flux was from the solder) together with 2 bits of solder so i got an indication of the board temperature.

I used 2 extra boards so only the board i’m reflowing heats up and not the thick A4 sized FR4 board I use as a soldering surface.

After cleaning the flux of with some acetone and cotton swabs

Testing the communication to the chip with my Bus Pirate v4 and using some tape to keep the cable in place because the connector isn’t soldered but kept in the board by bending the pins so there is no need to unsolder it when I’m done

When testing the last board i measured a short between the chip-select and ground

So with the chip removed i charged 4 18000uF capacitors to 15V and pressed the probes to test connector.

After connecting the other end of the test lead to the caps the short just blow away with a nice flash and a little bit of smoke

And with all the boards soldered and tested it was time to ship the back to Ian

He let me keep the board that had the sort in it, when putting some more parts on it I found some more shorts but those had to go with a scalpel.

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  1. Seriously guys, you SHOULD get some quality soldering paste. In fact I’m thinking about adding a syringe to my next digikey order and sending it to Ian or something, so you finally start soldering this thingies the proper way. I won’t get tired of promoting its usage.

    Soldering leadless chips is actually MUCH easier than soldering leaded chips (as long as they’re not BGA’s which play in a different league).

    You just need some paste!

    1. The problem is getting the right amount of paste if doing it by hand. If using a stencil then obviously it’s easier, but just applying with a syringe of a toothpick it’s a lot harder than it seems to get a consistent amount and not over-do it or under-do it. A proper metering pump setup would make it more consistent, but for most it is just not worth the $. This method does work and it’s not difficult.

      1. I spread the paste with the tip of my sharp point tweezers. Much better idea than using a toothpik. Once you get it right (takes a bit of practise, but it’s not hard at all) you’ll rarely over or under do it. If the paste is good and if you follow the right temperature profile (most common mistake among newbies is to just directly heat the paste to 350ÂșC) then surface tension will do magic. I solder leadless chips in a regular basis and if you do things the right way they are easier to solder than leaded packages. I get fewer shorts.

      2. What I’ve found works is using a small putty knife to spread a thin even layer over a whole area (thin enough to see the pads/silkscreen through). The surface tension will pull all the solder from nearby each pad. Bonus points to wiping the excess off where there’s no pads, but it just leaves a polka-dotted solder-ball effect. There’s enough solder to make the joint, but not so much there’s bridges.

      3. Seems like the middle ground between having a real stencil and Zach’s “buttered toast” method would be a hand-cut stencil (say with an Xacto knife) from some really thin material (1-2mils) that spans multiple pads rather than cutting-out each tiny pad individually.

        I would think that removing the bulk of the past out from under the center of the chip would be a necessity (rather than a bonus opportunity). I’d worry that all that solder would float the package up high enough to prevent the real connections from being made around the circumference.

        At least with a handmade-stencil you’d have a more repeatable (albeit ghetto) process. Hand-cutting something that’s only 1-2 mil thick might be a challenge without tearing it could be a problem unless it’s tough like acetate or Kapton.

    2. erdabyz: +1 on the leadless chips.

      If you have patience you can apply right amount of paste just using some toothpick. Takes some practice. Stencil makes things simpler, but it is not necessary for single prototype.

  2. I really like the idea of using nearby pads as solder temp indicators. I was in this same situation today and didn’t think of that.

    However, I get terrible solderpaste release from wooden toothpicks. I inevitably find it to be a two-handed/toothpicked job. I’ll have to keep my eyes out at the $1 store for a 0.5mm-wide butter knife…

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