Working with surface mount components (SMT.)

Introduction

Size and weight constraints are familliar problems with all rocketeers.  As you build electronic payload packages for your rockets you will try to keep both the size and the weight of these packages to a minimum.  Surface mount technology is a valuable tool in accomplishing this.  Nearly all electronic components and integrated circuits are available in surface mount packages today.  In fact, it is becoming difficult in some cases to find components that are not surface mount.

Surface mount technology differs from "through hole" technology in that the components are soldered to the surface of the PCB instead of soldered into holes in the PCB.  Additionally SMT components are generally smaller and lighter than their through hole cousins.  If a surface mount IC is half as long, and half as wide as a standard through hole mount, it means you can mount nearly 4 of them in the same space as the standard part.  And that is if you are using a single sided board.  If you use both sides of the PCB then you can mount nearly 8 of them in the space that one through hole part occupied.  This means that your electronics package can be considerably smaller.   In rocketry, smaller usually means lighter as well.

You will almost certainly need to use a printed circuit board (PCB) to utilize SMT devices.  While you can solder them to wires dead-bug style, it really misses the point.  It won't be as clean, neat, or light as if you use a PCB.  PCBs will have to be custom designed.  See my comments on the electronics index page regarding software packages to design PCBs.  Get one of these.  They are free, and will make life much easier.

There are a number of circuit board houses which offer cheap PCB's.  There are places that do 24 hour quick turn circuit boards in practically every large city.  There are some in eastern Europe that have web access, will do a PCB run in a couple days, and are so inexpensive that some have claimed it is worth the money to have custom PCBs made instead of breadboarding circuits on perf board.  This is old technology.  They've been making PCB's now for more years than many of you have been alive.  Even in the third world it's possible to get decent quality PCBs for very little.

Tools and Equipment

In order to work with surface mount components there are a few things you simply must have:

• A good, fine point soldering iron.  These are expensive.  Deal with it.  (Having said that, here is a nifty kludge for using a $15 radio shack desoldering iron for surface mount work.)
• Fine gauge wire solder.  28 to 32 gauge is about right.  The finer the better.
• A pair of sharp, tweezers for handling parts.  These should be made of stainless steel or aluminum so that solder won't stick to them.
• A wooden stick like popsicle stick or an orange stick (ask your wife or girlfriend - if you are a true, real man, rocket geek and don't have a girlfriend, go find the prettiest girl you can who has beautiful finger nails and ask her what an orange stick is.  What a great opening line!  <Snicker, snicker>  Or just ask a manicurist.)
• Solder wick - this is braided, fine copper wire.  On contact with molten solder it will suck it up like a sponge.  Used for removing excess solder, or for desoldering components you need to remove for some reason.

This equipment is optional, but strongly recommended:

• A good magnifying lamp that fastens to your desk or work bench so you can look through it and work with both hands free.
• A good, low power, jewlers loupe or small magnifying glass.  3X is about right.  10X is way too much.
• An assortment of small needle nose pliers, wire cutters, and flush cutters.
• An assortment of small flat lids about 5 mm deep and at least 40 mm in diameter to hold components while you work.
• A "third hand" device, while not essential, can be helpful.
• If you have access to a stereomicroscope, like a geology microscope, it can be helpful for examining the joints of very fine pitch ICs.

Your work area should be flat, smooth, uncrowded, uncluttered, well lighted and well ventilated.  Looking for lost SMT components less than 1 mm long on a crowded, cluttered, dimly lit work surface that is pitted and/or tilted so the parts can slide off would be a nightmare.  Trust me.  Don't try it.  Ventilation is crucial.  You will be working directly over where you are soldering and the fumes from the solder are rising right into your face.  Get a small fan or something if nothing else.

How to solder surface mount components (Finally!)

At last we get to the good stuff.  Once you have received your PCB and your parts have arrived from Digikey or wherever, you are ready to begin.  I like to begin with the 2 lead components: resistors, capacitors, diodes, etc.  Lets say you have a PCB that has places for ten 5K-ohm resistors.  Put the ten resistors in one of the lids or other type of container that is easy to get them out of, and turn your PCB so the majority of the bonding pads for the resistors are oriented horizontally in front of you.  Some PCB's will come already coated with solder.  Some will have bare copper bonding pads.  In either case, you want to put a small amount of solder on one of the bonding pads for each resistor.  I'm right handed, and I hold the soldering iron in my right hand and work with the tweezers in my left.  Consequently, I put the dot of solder on the rightmost bonding pad.  It should just be enough solder to form a small hump on the pad.  Not a mound, or a ball.  Just a slight rise of the pad.  Put one dot of solder on each of the pads you are going to solder your resistors to.

I want to say a word here about soldering technique.  It is often taught in courses on soldering that you apply solder to the component, not the iron.  In general, this is true.  However remember that soldering is not so much about where you put the solder, as it is about heat transfer.  A small amount of molten solder on the tip of your iron will transfer heat into the component lead faster than a dry iron touching the edge of the lead at some weird angle.  It may help to keep your iron "wet."  Now that doesn't mean a big glob of solder hanging off the tip of your iron.  It means a nice, shiny, silver surface on your iron, with maybe just a tiny, tiny dot of liquid solder at the tip.  If you can't see the exact shape of your iron for all the solder, you've got too much.  'Nuff said.

Using your tweezers, grasp (in my case) the left end of one resistor.  Using the soldering iron, heat the solder dot on the bonding pad until it flows.  (Don't get your iron too hot or you will lift the bonding pad off the PCB.  I use solder which melts at about 650 degrees F and I set my iron to 700 to 720 degrees F.)  Slide the resistor into the puddle of molten solder keeping it molten with the iron until it is in position.  Try to keep the resistor flat and oriented straight between the two bonding pads.  When it is in the right position, pull the iron away, but hold the resistor still with the tweezers.  If you move it, you will get what is called a "cold joint," which is a high resistance connection.  A good solder joint is shiny and glossy.  A cold joint has a matte finish to it, looks white or frosted.  If that happens, just hold the resistor steady and re-heat the joint till it flows again.

Now that the resistor is "tacked down" on one end, take your tweezers, or your orange stick, and push down firmly on the top of the resistor.  Heat the joint again to make sure the resistor is bottomed out on the PCB and not "flying" above it.  Once it is bonded down on one end, heat the opposite end with just the point of your iron and put just enough solder to flow from the bonding pad on the resistor to the bonding pad on the PCB.  It will take some practice before you get so you can make a nice looking, shiny, joint without a big glob of solder.  Practice, practice, practice.

It is important to remember that one of the reasons for SMT devices is to save weight.  Don't loose that advantage by slathering solder all over.  Keep your joints lean.  Don't use more solder than is necessary to form an electrical connection and hold the part in place.  The parts don't weigh much, and electrons are pretty small, so you won't need solder balls the size of BB's to do the job.

The technique above can be used to solder any two lead device.

Transistors, FETs and other 3 lead devices take a little different approach.  But once you have the mechanics of soldering down 2 lead devices, it's pretty easy to slide into doing 3, 4 and more leads.  The concept is basically the same.  Tack down one lead while you hold the device as close to the proper position as you can.  Then, once the first lead is tacked down, you can wiggle the device around slightly to get the other leads centered over their bonding pads.  (Provided you haven't tacked it down with enough solder to ballast an aircraft carrier.)  Hold the part firmly with the tweezers and solder down a second lead, preferably one opposite the first, or as near to opposite as you can.  Once both sides of the device are soldered down so the part can't move, go back.  Press the part down firmly and re-heat both joints one at a time, allowing the joints to cool between each one.  This makes sure the part is firmly down on the board.  You may need to go back and forth between the two leads several times to get the part fully seated.  Now go around the part and solder down the rest of the leads.

Small outline IC's are handled in much the same way.  Solder down one corner in a coarse position, wiggle the part into final alignment and solder down the opposite corner.  Seat the part, and solder down the leads one at a time.  Now it is important to note that this works well with packages with "gull wing" leads.  Parts with "J" leads, where the lead curls under the package are vastly more difficult.  If you can, stick with gull wing parts only.  Don't use PLCC packages if you can avoid it.  Soldering down their sockets are horribly frustrating.  Ball Grid Array (BGA) parts are a completely different story altogether.  The Seattle Robotics Society has a web page devoted to soldering these difficult SMT parts that is well worth reading.  I haven't tried it yet, as my toaster oven is, well, toasted.

The first time I tried to solder surface mount parts like this I was amazed at how easy it was.  I had been afraid to try it for some time.  I had a friend who showed me how, and it was so easy.  I hope this little tutorial has been helpful to you.
 

Last updated 19 November 2002