Introduction: How to Assemble an SMD GPS Device - OHARARP LLC
SD GPS Data Logger - www.ohararp.com/products
I am a small developer of embedded soultions for all types of applications. One my personal interests is with GPS positioning products. To satisfy some previous patent work I was doing I needed a device that would record large amounts of gps data (lots of memory) over a long period of time (big battery) and was compatible with mac's and pc's. Unfortunately, nothing of this sort existed and I had to create a device to do this.
This product is the culmination of implementing a number of new designs for myself. Mainly, using surface mount components, lithium ion/polymer batteries, and Sandisk SD memory cards.
Since so many electronics parts are surface mount these days I needed to use these components but was a little bit fearful in making the leap from using through hole/traditional soldered parts. Sparkfun Electronics helped my learning with their tutorials and I thought I would continue the tradition through an Instructable.
Step 1: APPLY SOLDER PASTE WITH ACRYLIC STENCIL
Picture 1. Place circuit board where it can be held securely
Picture 2. Tape Top of acrylic stencil to board; make sure it is properly aligned over the pads of your pcb.
Picture 3. Using your preferred solder paste apply a good sized glob to a simple plastic paint spreader
Picture 4. Spread the solder past liberally over your acrylic stencil. Make sure to cover all the holes!
Picture 5. In the next 2-3 passes try and remove as much solder paste from the stencil as possible.
Picture 6. You should have almost the same amount of solder paste your "spatula" as you began with except for the holes in the stencil.
Picture 7. After carefully pulling up your stencil and removing the pcb you would repeat this process for the next board. When complete be sure to remove all solder paste from the stencil. I prefer windex and a toothbrush to clear out soem of the holes for really tight pitch components.
Step 2: PROGRAM PIC 18LF4620
This design has its firmware locked into the ic and is only programmed at the time of manufacture.
Picture 1. We are assembling 15 boards today. Here you cansee 12 of the Pic 18LF4620 uProcessors that are used.
Picture 2 & 3. We place each part in a special adaptor that is tied to an www.melabs.com serial programmer.
Picture 4. The firmware is downloaded via a pc using the melabs meProg software.
Step 3: PLACE IC'S AND PASSIVE COMPONENTS
Picture 1. You can clearly see that there is a nice bit of solder paste on all of our pads. Using a nice set of tweezers (www.zeph.com) I place all of the components by hand.
Picture 2. Here I have placed the uPic and you can see how nicely things are lined up. This is where a good set of dental needle tools come in handy (www.zeph.com). It looks like the solder paste might bridge between the ic legs. However, there is just the right amound of solder paste that when we heat the board up these bridges should not appear. If they do a quick application of heat from a soldering iron and copper solderwick will fix this.
Picture 3. Step and repeat until you place all of your components.
Step 4: IT'S GRIDDLE TIME!
So my wife is pretty pissed that she can't make pancakes anymore, but now I can solder all my boards quickly and easily!
Picture 1. This griddle tops out at 400+F for temperature. I set it to max and let it heat up. After 5 mins or so I apply as many boards as possible on the griddle.
Picture 2. More of the same, just a little closer. Make sure you have plenty of ventilation as the fumes from the solder paste are toxic. It can get stinky pretty quick! I usually place a box fan in front of a window and draw the fumes outside.
Picture 3. After a few minutes you can clearly tell when the solder has melted. Some cold spots may appear on your griddle or larger parts may not always reflow. I have a hot air rework gun handly to help out the usb connectors.
Step 5: ADDING LI-ION POWER!!!
This step should be helpful to alot of people trying to add li-ion or li-polymer power to their design. It is of EXTREME importance that a rechargeable lithium battery have protection circuitry. Your cell comes with this built into the package of the battery by the manufacturer. We purchase bare cells from www.batteryspace.com and highly recommend their Protection Circuit Modules (PCM). They basically work to isolate the dround of the battery from the rest of the circuit when a fault condition is encountered. This is especially handy in case you short something on your board. The power in the battery will quickly "vaporize" your copper power traces!
Picture 1. Battery and PCM
Picture 2. Battery and PCM assembled
Picture 3. Add a nice heat shrink wrapping for protection
Picture 4. Place foam tape on pcb to help hold battery in place
Picture 5. Foam tape peel removed.
Step 6: FINAL ASSEMBLY
We are nearly complete...just need to add the final casing. Because the 18650 battery is slightly larger than the www.circuitshell.com case depth we have to add some .125" acrylic spacers that are manufactured by www.pololu.com.
Picture 1,2,3. Remove the protective paper from the acrylic pieces.
Picture 4,5. The www.circuitshell.com case must be slightly machined to account for the additional height of the usb connector and power switch. This is done using a wood router table.
Pciture 6,7. Add 4 4-40 screws, acrylic pieces and circuit shell case and VOILA!
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