Kionix Accelerometers & 2x2 LGA parts - Page 1
Forum section: electronics / projects

author: phil from seattle
author_rank: Super Contributor
timestamp: December 12, 2017, 07:02:41 pm
content: I'm designing a device that needs a 3-axis accelerometer. I've prototyped it with a module but want something smaller. Basically I want low cost - around $1 in small quantities. It looks like the Kionix KX-122-1037 meets my needs but have not found many examples or reports of people using it. Anyone used that or similar Kionix Accs? Any other favorites that costs about a buck?One thing I am concerned about is home assembly with a 2x2 LGA (looks like all the cheap Accs use that package now). I'll use a reflow oven and stencil so I think it will go OK but hope someone with experience can give me words of wisdom, things to watch out for, etc. Kionix does have a nice appnote on this that give me some confidencehttp://kionixfs.kionix.com/en/document/TN008%20PCB%20Design%20Guidelines%20for%202x2%20LGA%20Accelerometers.pdfI'm going to try to cut a stencil on my CNC mill. It's going to be a learning experience. I can always bail and have it made for me at the worst case.

author: phil from seattle
author_rank: Super Contributor
timestamp: December 16, 2017, 02:13:31 am
content: Nothing, huh?Oh, well. Boards and chips arrive next week. We shall see.

author: mairo
author_rank: Unknown
timestamp: December 16, 2017, 02:20:06 am
content: Quote from: phil from seattle on December 12, 2017, 07:02:41 pmHow/where the board will be made? If you use PCBWay (or other similar alternatives), a unframed stainless steel stencil will cost ~$10 and you can have it delivered with the boards. If the board does not have many smd solder pads you may try depositing the solder with syringe.Watch some of Louis Rossmann repair videos where he manually solders (with a hot air) small ICs.

author: andyturk
author_rank: Frequent Contributor
timestamp: December 16, 2017, 05:55:12 am
content: Sure, you can solder those things at home with hot air, if you have sufficient patience. I've soldered 0.4mm QFN/LGA packages on OSHPark boards, no problem.If it's just one board you're making, I'd "wet" the pads with wire solder and an iron, making sure to get the same amount of solder on each pad. Then, clean it well with IPA, and hose down the entire area with liquid flux. Place the chip and then heat it from below with hot air until it reflows and you see the chip move into place. Buy a few extra chips so you can pull a bad one off and try again until it's right.If you're building several boards, definitely go with a stainless stencil--the thinner the better. Almost always, your enemy is too much solder on the pads rather than too little. It's definitely doable even with el cheapo $50 hot air guns, but it may take you a few tries to get the chip soldered with no shorts or opens.How will you know it works? Well, it won't be a dead short on the power rails, and it it won't short out the clock/data signals you send via firmware. Bringup is always fun, and it helps if you "know" your firmware is doing the right thing *before* you try to talk to a board you built at home. If there's a eval board from the OEM, spend some time with that first before building your own.

author: phil from seattle
author_rank: Super Contributor
timestamp: December 16, 2017, 08:00:01 am
content: Thanks guys. That gives me a bit more confidence. I have an oven, hot air station, paste syringe and 3D microscope - I feel like it's just a question of practice. Usually my problem is not enough solder paste though my last couple of boards worked fairly well. Yeah, using PCBWay these days. My biggest worry is solder mask slivers from between the LGA pads.I would have bought an eval kit but they seem to be out of stock everywhere until April of next year. Too bad, they're quite cheap.

author: phil from seattle
author_rank: Super Contributor
timestamp: December 20, 2017, 11:40:49 pm
content: A followup.Got the boards from AllPCB (whew, 4 days from order to delivery, $5.49 - I'm still stunned by this) and ICs from Mouser.Used a syringe to put the solder paste on. Well, actually, the needle was way too big and the solder paste (Kessler) not that sticky. So, I put a very light coating of flux down and a blob of paste off the the side and used a needle to get the solder balls onto the 12 pads. Next time I'll wipe most of the flux off before hand. Used a stereo microscope at about 20X. It wasn't pretty but gave it a try in the toaster oven.Happy to say it worked just fine. You can see on the microscope picture that there is a little flux left (after 3 washing in isopropyl) and a few stray solder blobs. Those capacitors are 1206s, that accelerometer is tiny! The LA trace shows reading the X, Y and Z axis 16 bit values with SPI.I haven't tested the interrupt pins so there could still be some errors but the important ones all seem to be working. Also, I need to try and make a stencil.What's really great about this is now I can add an accelerometer to my projects for $1 and a tiny bit of PCB real estate.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 21, 2017, 09:23:44 am
content: I have hand-soldered a few similar component (LSM303) with the same footprint, as well as some 0.5mm pitch WLCSPs. Here's a few tips:1) Any random solder paste won't necessarily do. I have happily used some "Mechanic" Ebay paste for most SMD work before, but it's too coarse for such small stencil openings (around 0.25mmx0.25mm)2) Order the stencil with the PCBs - it's fairly cheap.3) One thing I do with larger, complex boards with one difficult small component is:- I make a separate copy of my design files, for the stencil- I delete the difficult component so that I get no paste there on the first pass- I make a few copies of the component, on the outside of the main PCB, and adjust each one to have slightly different paste openings- Now I order the PCBs with the (unframed) stencil, and cut out the small mini stencils- I can reflow all the "easy" stuff and leave the difficult thing later - this way I can better control the paste release, look the paste with a microscope, remove the paste and reapply if unsatisfactory, as many times as needed, try out different openings. Since everything else is already reflown in place, this is easy and unstressful.- Preheat the board from the bottom side so that the board is at least at about 100 deg C, then use hot air with a small nozzle to work locally.I have also succeeded soldering LSM303 by applying standard solder with an iron to the pads - it's easy to control the amount - then some flux to both the board and the component, preheat from the bottom, and reflow with hot air, just using tweezers to press the component in the right place - redo as many times as needed to get it align right, and all pads to connect. Not an optimum/reliable way to do this, but when I didn't have a proper stencil (the openings were too small in the first revision), this worked way better than trying to use a syringe to apply the paste which was too coarse-grained.

author: phil from seattle
author_rank: Super Contributor
timestamp: December 21, 2017, 07:18:08 pm
content: I don't understand how you stencil on a PCB that already has components mounted. Or perhaps I should ask, how do you hold the mini stencils in place? They must be very small.

author: texaspyro
author_rank: Super Contributor
timestamp: December 22, 2017, 05:09:45 am
content: Quote from: phil from seattle on December 12, 2017, 07:02:41 pmI built a couple of laser diode heads for my mill (a small MaxNC with the spindle replaced by the laser head). One laser is a 3W, 445 nm, multi-mode head.The other is a 0.5W, 400 nm, single mode head. The "kerf" is less than 10 microns. I cut stencils with it using red "vellum" from Michaels / Hobby Lobby. Lately, I've just been using OSHSTENCIL's stainless stencils.I wrote a little (OK, 100,000 lines of C) program that has the ability to read Gerber filed and convert it to Mach3 gcode... it can also read/write/convert gcode formats for several dozen type of machines, dxf, pdf, ps, ai, hpgl, etc.

author: phil from seattle
author_rank: Super Contributor
timestamp: December 22, 2017, 11:04:06 pm
content: Interesting about using vellum. How thick is it?

author: texaspyro
author_rank: Super Contributor
timestamp: December 23, 2017, 12:51:44 am
content: Quote from: phil from seattle on December 22, 2017, 11:04:06 pm0.003" thick.It's probably not true vellum. It's like a translucent plasticy paper. Sold in 12x12" sheets. It holds up to use surprisingly well. Red cuts best with a blue laser.I've also used black children's construction paper, but it is rather thick. Also used stick-on vinyl sheet, but laser cutting vinyl is a bad idea... releases nasty/corrosive vapors. Also used red copier paper.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 23, 2017, 12:24:00 pm
content: Quote from: phil from seattle on December 21, 2017, 07:18:08 pmCut the stencil as small as needed. I hold it in the place from one edge with some tape. With stencil small enough, non-planarity issues are minimized, so I get good results even without a frame, or stencil printer/holder. What's best, applying paste takes only a few seconds, with minimum amount of paste, so if the result looks unsatisfactory just clean and reapply new paste - no problem since all the other components are already in place, you can't accidentally wipe them off.

author: technix
author_rank: Super Contributor
timestamp: December 23, 2017, 03:54:42 pm
content: Seeing this reminded me to make a breakout for MPU-9250 (should it be a simple breakout, or should I attach it using the Raspberry Pi or Arduino pinout?) 2x2 LGA - how do you solder this?

author: pieman103021
author_rank: Contributor
timestamp: December 23, 2017, 04:38:47 pm
content: Quote from: texaspyro on December 22, 2017, 05:09:45 amThat sounds a bit overkill, but awesome. Do you have it on GitHub by any chance?Sent from my BLN-L24 using Tapatalk

author: phil from seattle
author_rank: Super Contributor
timestamp: December 23, 2017, 06:14:24 pm
content: Quote from: technix on December 23, 2017, 03:54:42 pmWell, a reflow oven is the best (says Captain Obvious). I used a toaster oven with thermocouple to track the temperature and it works fine for Pb/Sn solder. Just make sure the temp doesn't get much over 235 or so. My method is to turn the heat off when I see that all the visible paste has melted (temp will continue to rise), wait 10-15 seconds and then pop the door open. I am still tweaking the process but so far it seems to work well. At some point I'll move to lead-free solder but there is a lot less margin for error on the max temperature.Several folks above use hot air - I've done other chips with it but I've never been very happy with it. And the 2x2 LGA is so tiny and sticks up fairly high so it has some "sail area". I'd be afraid of blowing it right off the PCB.

author: phil from seattle
author_rank: Super Contributor
timestamp: December 23, 2017, 06:19:54 pm
content: Quote from: pieman103021 on December 23, 2017, 04:38:47 pmIt does sound like a useful program. Though, I use Eagle and there is an add-in that produces gcode for a stencil. It needs a little work (only does smd pads, any drawing in the paste layer is ignored) but the source is available and doesn't look that hard to fix.

author: texaspyro
author_rank: Super Contributor
timestamp: December 23, 2017, 06:39:17 pm
content: Quote from: pieman103021 on December 23, 2017, 04:38:47 pmNot yet... it needs some documentation.If you feed it gerber and output PDF, you can do toner transfer. The gerber processor is quite good... it handles my torture suite of wonky gerbers. It handles my complete torture suite of wonky gerbers. It also supports the new X2 gerber extensions.It also handles the full Excellon drill/mill language (and a couple of others). It can convert them to gcode.I need to add the ability to convert gerber to gerber so you can scale a gerber file... I know a guy that scales his Eagle libraries to 1/10 scale and upscales the gerbers x10 to let him do any size board with the free Eagle. If you convert gerber to gcode, it would mill the outlines of the traces... I was wanting it to do pcb to gcode conversion but never did that properly... it needs to handle intersections of traces.The program does do DF/HPGL to gcode, gcode to dxf/hpgl, etc. It has a cutting path optimizer. I originally wrote the program to optimize the cutting time of Eagle's PCB2GCODE plugin... I could get a 70% reduction in cutting time.

author: texaspyro
author_rank: Super Contributor
timestamp: December 23, 2017, 08:06:33 pm
content: Quote from: phil from seattle on December 21, 2017, 07:18:08 pmI have used stencils I laser cut out of adhesive backed vinyl sheet.

author: phil from seattle
author_rank: Super Contributor
timestamp: December 23, 2017, 08:34:07 pm
content: That makes sense. You must have a tiny little squeegee. Hope you cut the vinyl in a well ventilated location.

author: texaspyro
author_rank: Super Contributor
timestamp: December 23, 2017, 08:51:58 pm
content: Quote from: phil from seattle on December 23, 2017, 08:34:07 pmCut a strip off a credit card, etc.Cutting a tiny stencil does not release much gas... still, I use a fan to keep the vapors off the mill.Positioning a tiny pad, sticky backed stencil can be a pain, but you don't do it very often. I consult my well thumbed copy of "The Drunken Russian Sailors Guide to Curses and Swears."

author: phil from seattle
author_rank: Super Contributor
timestamp: December 23, 2017, 09:31:04 pm
content: Quote from: texaspyro on December 23, 2017, 08:51:58 pmEssential book for any EE Hobbyist's library. I've had to get a new one - wore it out when first learning Eagle.

author: phil from seattle
author_rank: Super Contributor
timestamp: December 26, 2017, 11:37:33 pm
content: I used AllPCB for the boards. See the attached image of the layout and the image of the board. You can see that they didn't do the worlds greatest job on the mask. Though I didn't give them any special instructions.I applied paste by hand. My needle was too big (19 ga.) so I just used a fine sewing needle to push the paste around under a stereo microscope. Toaster oven reflow. Way crude I know but it worked OK.

author: phil from seattle
author_rank: Super Contributor
timestamp: December 26, 2017, 11:44:34 pm
content: By the way, a totally gratuitous shot with a grain of rice for scale.