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Discussion Starter #1
Hi Guys

I thought I would start a thread on using the very small SMD (Surface Mount Device) LED chips for modeling.
LEDs often scare people and these teeny tiny ones can be especially daunting, but they are not too difficult to use and can help create amazing models.

I don't claim to be an expert in this area so I encourage people to add to this thread with their hints and tips. And ask questions of course.

Lets go.
Here is an example of just some SMD LEDs with a standard 5mm LED as a reference.

SMD LEDs are packaged in 'tape' form. This tape has containers for the LEDs and has a pull off retaining strip to stop them all falling out. (Sort of like a yogurt pottle, but a long line of connected pottles.) There are also sprocket holes down the length of the tape to enable automated machinery to accurately feed it.
SMD LED tape is designed to be loaded into 'Pick and Place' machines that automatically feed the tape through to a vacuum head that will remove the LED, orient it, and place it in the correct position on a printed circuit board that has solder paste applied. The printed circuit board will then head off to a 'reflow' oven where the solder paste is melted to solder the component down to the board.
The LEDs, as well as all other SMD components, are never touched by human hand.
Not so for us in the modeling world!

When you buy SMD LEDs from ebay, or other suppliers, you will typically receive a small length of tape containing your LEDs.

Size matters.
The size of an SMD LED is typically stated as a four digit number.
eg. 1206, 3216, 0805, 0603, 4040 etc etc
These numbers relate to the length and width of the LED.
And you must first identify if these measurements are in imperial or metric.
1206 = 0.12” x 0.06” (Each pair of numbers is 100th of an inch)
3216 = 3.2mm x 1.6mm (Each pair of numbers is millimetres times 10)

Eagle eyed readers will note that 1206 and 3216 is actually the same size.

Because we are not placing these LEDs on a board, we need to solder wires onto them.

Let the fun begin.
Really small LED's can be blown off your work bench with a sneeze, and like that small styrene part that was sitting beside it, you will never find it again.

My best tip is to use spring loaded pointy tweezers (Reverse action.) and pick up one of the LEDs so that its connection points are easily accessible.


Polarity
Your LEDs data sheet will tell you how to find which end of the LED is the anode (Positive) and cathode (Negative)
  • Some SMD LEDs have what looks like a squashed capital 'T' printed on their underside. The top of the 'T' is the anode and the bottom is the cathode.
  • Others may have a line or a dot close to one of the connections. This identifies the cathode.
Of course you can just connect it up to your power supply and see which way it lights up. (That's cheating.:p)

Wire me up
Thin 'magnet wire' or 'wire wrap' wire is good to use. Any thin wire will do, but it has got to be thin enough to thread through your particular model.
Tin the end of the wire with solder. (Obviously strip any insulation off first.)
I like to apply a very small amount of flux paste onto the LED. This helps the solder stick and allows for a quick solder job so not too much heat is applied to the LED.

Put a small amount of solder on to your soldering iron tip. Hold your tinned wire in place on the LED (Secured in your tweezers.) with one hand. Touch the soldering iron on to the connection of where the wire meets the LED with your other hand.
Hopefully the flux will sizzle and the joint will be made.
Turn over your tweezers and repeat the process for the other side of the LED.
It does take a bit of practice but it is not that hard to achieve a good result.

Now you have a teeny tiny LED with wires attached...
Time for mounting in your model?? Not quite!

Time to test it and make sure you can see the light.

Connect a power source to the wires you have soldered on. Don't forget an appropriate dropping resistor!!!!
If you have not done so already, you may also like to identify the positive and negative wires.


Mount up!

The LEDs can just be glued directly into your model. I tend to use a clear epoxy glue to do this.
Sometimes placement of the LEDs can be critical so I often drill a hole in some styrene sheet, or a cut off piece of rod, and epoxy the LED into it. The hole needs to be just big enough for the raised head of the LED to fit into. Slop some epoxy over the LED and wire connections to secure it all in place.
Once dried, you can use your favorite plastic cement to attach it to your model. Easy!

Here is a LED that has been glued into a styrene donut made from a cut piece of rod with hole drilled through. It will become one of the flood lights in the small Moebius Flying Sub model.


And just a word of advice. This applies to any bright LED. Don't stare into a brightly lit LED at close range. These suckers are a very bright, pin point, light source and can damage your eyes.
Its hard to make models when blind!.:eek:

Alien
 

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I like to place tape sticky side up and attach 5 or 6 SMD's to it and flux them all and solder away.
I prep all the magnet wires first and can solder them all at one time.
I hate using tweezers because my parts always pop out of them to places unknown.:tongue:
 

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Discussion Starter #3
Thanks JimKirk

I like your 'production line' approach with the wires.

A good pair of tweezers does help with the popping out issue.
I have never had that problem with mine. (No idea what brand they are.)
Putting some masking tape on the tips can help give them a bit more grip too.

But I will certainly give your masking tape method a try on my next project.

Alien
 

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A few more tips.
These things will very easily "fly" out of their little containers if you flex the strip to much.
The clear plastic film that keeps them in will come off very easily and then the LEDS can fall out.
I would recommened using a flux to help solder the wire to the led contacts. There are times I can get these to take with one touch and other times I've ruined the LED.
You can use a small drop of Tamiya clear (red, blue, green, etc) paint to change the color.
When ordering these from China via ebay many of the sellers will post all sizes on one page. You then have to use a drop down box to select the size you need. It can be a bit confusing and very easy to get the wrong size. The price may also be a bit more for larger sizes.
 

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I like to place tape sticky side up and attach 5 or 6 SMD's to it and flux them all and solder away.
I prep all the magnet wires first and can solder them all at one time.
I hate using tweezers because my parts always pop out of them to places unknown.:tongue:
Ditto on all of that - the inverted tape isn't perfect, but it works.

Also I use .022 diameter solder when doing SMDs - no way a larger diameter is going to not make a mess.

When soldering, it is best to place your palms onto the steady surface and only move your fingertips to solder the connection.
 

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Discussion Starter #6
Just thought that I would add a photo of what I am currently working on as it inspired me to start this thread.
This is the tiny Moebius Flying Sub model from Voyage to the Bottom of the Sea. It is about 80mm (3.25”) across and I wanted to illuminate it, so tiny SMD LEDs are ideal.
I thought I would take this photo before cementing the two halves together for ever and ever.




I purchased a Paragrafix etched brass interior for the sub, but I wanted to illuminate the three hexagons on the floor, as on the studio set, so I ended up just using the photo etched girders. Sorry Paul.
I made a new floor from the tapered plastic from the back of an old cell phone screen. I epoxied a couple of SMD LEDs to one edge and the tapered plastic distributes the light over the surface. A bit of masking and painting created the lighted floor effect. I built the rest of the interior from sheet styrene adding the photo etched girders.
My two previously shown LEDs, epoxied into styrene donuts, were glued onto the rear of the front clear molding behind the headlights.
I also added a single SMD LED right at the very tip of the top to illuminate the front internal area of the sub and the pilot figure. (Otherwise the interior was going to be a bit gloomy.)
The big bulbous (5mm) LED actually contains a red, green and blue LED with a flashing chip.
This lights up the 'reactor wall' with changing light patterns.
You can see the dropping resistors soldered on. Not very pretty but no one will ever see them.
Power comes in through two tiny holes drilled into the under side (Below that big white styrene block)
into two little sockets cemented into the styrene. This allows the model to be easily removed from its mount. (Yet to be built.)

On with the gluing together.. Scary :(

Alien
 

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Ditto on all of that - the inverted tape isn't perfect, but it works.

Also I use .022 diameter solder when doing SMDs - no way a larger diameter is going to not make a mess.

When soldering, it is best to place your palms onto the steady surface and only move your fingertips to solder the connection.
It takes a bit of practice using the tape because once heated the led's will move a bit as the tape get warm.
I like the tape because I can do a few at a time and can use both hands to solder.
You need to hit it quick and be done with it.
I flux then place the tinned wire on the led pad and hit it with the iron with a bit of solder on it.
 

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I like to place tape sticky side up and attach 5 or 6 SMD's to it and flux them all and solder away.
I prep all the magnet wires first and can solder them all at one time.
I hate using tweezers because my parts always pop out of them to places unknown.:tongue:
I use Z-axis tape between the wire and small SMDs.

It works like double sided tape but conducts current from top to bottom without conducting side to side.

I take a strip of regular tape and place the wires on it. then place the z-axis tape over that, press the led on top of the tape and squeeze the tape and wire sandwich and led together and it should work.
Digikey sells it bu the roll (96 yards) for around $100 or if you shop around you can find 3 yard rolls for about $20.00... but not at SparkFun as they have stopped carrying it.

As the demo below shows you can use it to attach almost anything.

Here is SparkFun's video:
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=video&cd=1&cad=rja&uact=8&ved=0CCUQtwIwAGoVChMIr_y0vcOayAIVAkiICh35jABs&url=http://www.youtube.com/watch?v=K3SPijvXtew&usg=AFQjCNGTniuiMyN4adtcaTj56BgINLnUCg&bvm=bv.103388427,d.cGU

Demo starts at about 1:07
 

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Discussion Starter #9
Wow, I had never heard of that Z-axis tape.
It looks pretty cool.

Do you know how good the glue is?? i.e. Will it last for a number of years and not release your LEDs sealed in a model.

Alien
 

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Ditto - cool stuff that Z-Axis tape,...

The only need I've had to solder my own SMD's has been some 0603 RGB's that have 4 tiny solder pads (one in each corner). I doubt there is enough surface area to make anything like tape stick to it, so I've always added a bit of epoxy backing onto them afterward to hold things in place.
 

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Wow, I had never heard of that Z-axis tape.
It looks pretty cool.

Do you know how good the glue is?? i.e. Will it last for a number of years and not release your LEDs sealed in a model.

Alien
It's the same tape they use to attach flex circuit boards together in cellphones and cars so it should hold up well.

I live in Arizona and have used it to stick leds to the bottom of a work bench in the garage and so far nothing has fallen off in the heat.

Ditto - cool stuff that Z-Axis tape,...

The only need I've had to solder my own SMD's has been some 0603 RGB's that have 4 tiny solder pads (one in each corner). I doubt there is enough surface area to make anything like tape stick to it, so I've always added a bit of epoxy backing onto them afterward to hold things in place.
It should work without a problem as the pad size and the tape conductive area are the same.
 

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Discussion Starter #12
Just a quick update with a tip...

If you have mounted your LEDs (Don't have to be SMD ones.) into your model, and you find that they are a little too bright, and you don't want to, or can't, change the dropping resistor for a bigger value then try this....

Grab some Tamiya Smoke. This is like their clear red, green, orange paints but it is a translucent smokey gray. It doesn't sound too exciting but slopping some on an LED will reduce the light output without affecting the color.
The more coats you apply, the dimmer the LED will become.

Alien
 

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Very helpful indeed! Thanks everyone! :smile2:
 
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