SMT Tutorial

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The initiative to investigate how to do surface mount electronics was started by Ed Bennett (research specialist at the School of the Art Institute) five years ago.  Bringing the capablity to the school was very important for the future of electronics in our ciriculum. About  seven years ago, we found that more modern chips were not available in dip packages. As the years gone by this became more common. In the process of the investigation, we found that doing SMT was a fairly easy process, even for DYI or hobbist.  Below is a step by step tutorial on how to do SMT.

Items you would need are a flux pen, solderpaste, syringe, a good pair of tweezer, pair of magnifing visor or magnifier, and isopropyl alcohol.

flux pen and solder paste can be purchase from digikey. http:// www.digikey.com

solder paste  part# KE-1507-ND

flux pen   part#KE1804-ND

You can purchase a syringe from a hobby shop or an acrylic supplier. Lastly you will need  good pair of tweezer, we suggest purchasing a diamond tweezer. They seem to be the best quality tweezer you can get.

Mignifier or magnifier visorhelps in reworking boards

The alcohol is used as a solvent for clean up.

Here is a prefab circuit board, which happens to be an ArtBus board. The process works the same for surf boards or your own pre made boards.  You need to flux the solder pads for all the SMT parts that you'll be placing on the board.

SOLDER PASTE:

Load your syringe with your tube of solder paste. We found that if you are doing one or two boards, it saves from wasting the solder paste. Once the syringe is loaded, you need to squeeze out some to make sure that the paste isn't runny and the tip is not plugged.

Use the syringe to apply the solder paste to the pad. If you are doing multiples, you can make a stencil from your cad file.  This works best if you have access to a laser cutter.

FOR MULTIPLES:

If you're using a stencil you would need to make a fixture to register your board  to the stencil.  You will be using the solder paste straight out of the tube instead of using the syringe. (If anyone have done screen printing before, it is a very similiar process.) You put down a 1/4" bead of solder paste above the whole length of the stencil, using a old credit card or any rigid plastic you find. Drag the credit card , pushing the solder paste as you go over the stencil. At the end you get a perfectly registered board every time. The solder paste takes a while to dry so you have plenty of working time.

You don't necessarily have to be too careful about placing your component on the solder paste. For instance: for chips, instead of dotting every pad with solder, you can draw a line of solder paste across all the pads on the same side. It may look like the pins will  bridge each other, but they won't. The surface tension will cause the solder to pull to the the metal pads pulling the chip along with it.

PLACING PARTS:

Alot of the parts are fairly small, so investing in a good set of tweezers will less agravating in the end.

Carefully place the part on the your solder paste. It doesn't have to be exact. Again, the surface tension will help out with the positioning.

SMT compone come in different packages. The packages are the sizes of the component, size and pitch of the leads. This chip here is a SOIC chip, it is one of the coarser chip package available. We have successfully worked with a variety of packages including SOIC,TSSOP, and QTFP.

BAKING THE BOARD:

We initiatly used a toaster oven in one of our first experiments. We found that a regular toaster worked better. This is a pic of toaster  2.0. We have a  another toaster  that we modified to looks  like an easy bake oven, where nothing is exposed. (Hopefully when time is permitted I will post a pic of 3.0) The equipment that the oven is sitting on is a variac, which allowed us to control the voltage in our intial experiments. It also allows us to turn the toaster oven on and off easily. You do not need this. You just need to devise an on/off system. Lastly, you will need to purchase a temperature probe. We got ours from Digikey. An oven thermometer will probably work too.

Carefully place the board(s) in the middle of the toaster. You can see where the sensor portion of the probe is located. Be sure when you position the probe sensor, position it where the board most likely be when set up your toaster.

All chips comes with a oven profile.You can find them in the manufacturer's data sheet. The profile tells you the heating time, temperature and cooling time. These are the ideal conditions. For our applications we found most of the ICs we worked with worked with the same profile we've been using. We heated the board for roughly a minute  to 230C, then the oven is turned off.  The boards are left in the oven for cooling for a couple of seconds.

Carefully remove the board from the toaster and place it on a surface to cooled down. Once the board is  completely cooled down, you can reworked the board with a very fine tip soldering iron. If there are any bridging on the ICs, you can drag the tip of the iron in between the leads to remove it. Another option would be using solder braid to remove the excess solder.

Once you're done, the board should look like this. The joints should all be shiny and the solder should be making good contact with the pads.

For the most part we found that it was easier to do surface mount electronics than it is to do hand soldering. The solder joints in SMT are nearly perfect everytime. In comparison hand soldering does require some amount of skills. We are constantly looking at student work and often find cold or bad solder joints.

To do simple SMT you don't even need to follow this process. You can nearly every time, use a very fine tip soldering iron and pair of tweezers on coarser SMT packages up to TSSOP.

As in Margarita Benitez wearables class. http://kineticsandelectronics.com/SMT

 

These are the process we've been using for a least three years now. If you like to know more about surface mount electronics here is a good URL: http://www.curiousinventor.com/guides/Surface_Mount_Soldering/