SMT Board Fabrication Project

 

The object is to be able to make SMT boards in a easier and more reliable way than hand soldering.

The following pictures are my first attempt that failed and later successes to bake a SMT board. I miscalculated the final temperature, due to the thermometer lagging way behind the actual temperature. You should not exceed 425 degrees Fahrenheit for you peak temperature. After correcting for this oversight I have made over 10 boards and every one of them came out perfect. Iím quite happy with the technique since it allows me to do SMT boards in a fraction of the time it use to take me and the results look quite professional.

My second attempt and all other attempts using the visual monitoring worked great, see the picture at the bottom of the page.

The board consist of SMT components on both sides of a board making it more difficult for the first attempt, still it worked fine, the paste holds the components even on the under side.

My equipment consist of a Black and Decker T0700T convention oven with two heating elements on the bottom and two heating elements on the top, there is a small fan to recycle the air inside the oven and help keep the temperature more even. Also an oven thermometer that reads from 100 degrees to 600 degrees, a fine pair of tweezers, soldering paste, and good lighting. Please note that you do not need as fancy an oven as I purchased, that was a personal choice. I would rather spend a little extra money on a oven that would make the process more reliable because of the expensive nature of some of the boards that Iím planning on doing. I want all the odds in my favor. I did see some ovens in the $20 range that will do fine.

Soldering paste is only sold in large quantities, Cash Olsen will sell you a small hypodermic needle of paste for $5 that will do quite a few of boards, I have done 10 boards with 40 to 50 components each and have used only 1/3 of the supplied paste.

A note about soldering paste, soldering paste is advertised as a very short lived component, this is due to itís use in automatic machines where itís consistency must be very precise, but for manual application the actual life of paste is over a year if keep in a refrigerator. If you live near houses that use soldering paste, they would get rid of paste that is perfectly acceptable for manual use, so you might be able to get for free supplies for several lifetimes, if you do feel free to share it with others.

 An oven is not the only way to accomplish this task, Cash Olsen and others prefers to use a Embossing heat gun which cost $20 but can often be found at Hobby Lobby on sale for $10, and a small heating plate to warm up the board for $2. A link to his site with a tutorial on how to use an Embossing gun, and a link on how to order paste from him is found at the following link. Even if you use the oven method the Embossing gun is a nice tool to have, itís very useful for removing SMT components, and shrinking heat shrink tubing, and off course soldering components on a SMT board.

Another simple method is the use of an electric skillet, a link to a tutorial by Paul Alexander is shown below. The folks at SparkFun Electronics also use the skillet method for making large quantities of single sided boards for their company.

Link to Cash Olsenís Embossing gun tutorial.

Hobby Lobby Embossing Gun

Paul Alexander tutorial on use of skillet for SMT boards

These are the basic steps to soldering SMTís with an oven.

1.       Apply small amounts of soldering paste to the solder pads, but first ascertain were to put the paste by seeing where on the pads the component soldering pad will line up. Be very sparing with the paste, you need a lot less paste than you would think, too much will cause problems and shorts.

2.     Place the components on the board with a fine pair of tweezers, place them flush against the board, the paste will hold on to the components, even when upside down.

3.     Pre-heat the board in the oven at a temperature of 150 degrees to 180 degrees for at least three minutes, to dry out the soldering paste, and relieve temperature related stress from the board and components.

4.     Turn the temperature on high bake, and watch the board very carefully, the temperature will rise very quickly(qood). Within 60 to 120 seconds the solder will turn from a dull gray to a shinny surface, allow 5 to 10 seconds extra to make sure all is OK.

5.     Carefully open the oven and slide the tray out so the board stops heating and cools down.

6.       You are done.

 

See the pictures below showing the whole process.

On the pictures below, you can click on them to see the full size version.

 

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Blank Board

 

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Paste applied in one side, different amounts to test how much paste is needed.

 

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Components added.

 

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Components on side 2, as a test I added larger amount of paste, the smaller quantity on the top side did just fine.

 

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Tools used; fine tweezers, soldering paste, two heat sinks to keep the board off the table, and really good lighting. The tweezers are from Harbor Freight a set of six different shapes for about less than $4

 

Due to the thermometer lagging behind in temperature my first try failed.

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Oven at 180 degrees F, board ready to go in.

 

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Let it warm up at 180 degrees for two to three minutes. Notice that the soldering paste is a very dull grey.

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Crank up the heat, I was not realizing that the thermometer was about 100 degrees behind the real temperature once I turned on full heat

 

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At 330 degrees (more like 430 degrees real temperature) the board looks great, joints nice and shiny, I should have popped the oven door open and pulled out the tray, the results would have been really good.

 

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Houston, we have a meltdown, temperature is way too high, the solder is OK but the Epoxy on the board started melting, at 435(about 550 real temperature) degrees it caught fire.

 

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Fire has been put out, as soon as I find the other identical board I will try again using visual inspection. The process worked the only problems was cause by me trusting the thermometer to read the real temperature while the temperature was climbing quickly. The picture at 330 degrees shows the board was soldered correctly without heat damage.

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I tried it again with the correct temperature using visual inspection, as you can see, no fire, board looks great.

The soldering paste was from Cash Olsen, see the link at the top of the page. A little soldering paste goes a long way.

Also notice the perfect alingment of the parts, that is due to the surface tension of the melted solder making the part move to the center of the pad.

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I turned off the oven at 300 degrees Fahrenheit on the thermometer, it kept going up to about 340 degrees F, perfect.

My conclusion, donít trust the thermometer except to measure a steady state temperature, it doesnít react quickly enough, visual inspection is more reliable.

Later I will post pictures of a board with high density pins, with before soldering and after soldering pictures.