Tools & Soldering

Introduction to the Online Project

This project accompanies part 1 of Servo magazine series Beginner's Robotics on $50 a Month.  In the magazine article, you learned about the basic tools that every robot hobbyist should have.  You also learned some basic tips on how to find the best prices for tools and parts both online and locally.  Now, you will learn one of the fundamental skills that every robotic hobbyist should know, soldering

Soldering

There are many sites on the net that tell you how to solder.  One of the best sets of instructions is the N0SS solder notes at Elecraft: http://www.elecraft.com/TechNotes/N0SS_SolderNotes/N0SS_SolderNotesV6.pdf .  There are also short how to solder training videos on the web at: http://radiojove.gsfc.nasa.gov/elab/soldering.htm.

The N0SS instructions and the radiojove videos are very good. Here are few extra tips to make things easier for the beginner:

A well cleaned soldering iron tip is important.  On cheap irons, the tips are coated with oil for shipping.  This oil needs to be burned off before the first time the iron is used.  In addition, the plating used on the cheap irons does not "wet" easily.  That means solder will not easily spread over the tip.  When you first get your iron, plug it in and set it in a place where it will not be bumped.  Allow the iron to heat up for several minutes.  You will probably see a little bit of smoke come off the tip.  This is the oil burning off.  Once the smoke stops, unplug the iron and let it cool.  Once it is cool, take fine sandpaper and sand the plating off of the tip.  Don't worry, it is just a $5 iron.  The copper below the plating will wet better. 

Now the tip needs to be tinned.  There are tinning compounds available, but you can also use regular solder.  Get your sponge wet and squeeze most of the water out of it.  Heat up your iron and touch the tip of the solder wire to the tip of the iron.  When a small amount of the solder liquefies and begins to bead on the tip, pull the solder wire away and wipe the tip of the iron on the wet sponge.  Most, but not all of the solder will come off the tip.  Repeat this several times until there is a fairly even silver color to the tip.  Unplug the iron and let it cool.  When it is cool, lightly sand the tip again.  This time, do not sand it so much that you remove the coating you just put on it.

Now you are ready to learn the biggest secret in soldering: rosin paste. This stuff is available from Radio Shack, All Electronics, DigiKey, and Wright Hobbies. Apply the paste only to the areas where you want the solder to flow - use the point of a toothpick - and only apply enough to give a slightly oily look to the area - no lumps. Rosin paste is the biggest aid to beginner's I have seen. I wish some one had told me about this when I first started.

Each time you use the soldering iron, the tip of the iron needs to be re-wetted with a tiny amount of solder. I guarantee you will use too much solder at first when wetting the tip. This is ok since it will allow you to do the one hand soldering method that my wife, Kate, came up with: Apply rosin paste to the joint to be soldered. Clean the hot iron tip with the wet sponge. Apply a small bit of solder to the tip. When there is a small bead of solder on the tip, touch the tip to the joint. Hold the tip to the joint until the bead of solder spreads across all of the area where you applied rosin paste (1-2 sec). Repeat all steps for the next joint. When you are finished for the day, use a Q-tip and alcohol to clean any excess rosin (there should be almost none).  NEVER use alcohol near a hot iron! Let everything cool first, or clean the board well away from where you solder.

Keep trying to get the amount of solder in the bead smaller and smaller each time you wet the tip. When you get to the point where there is not enough bead on the tip to make a solder joint, start touching the solder wire to the joint as described in the N0SS instructions. Keep trying to work yourself toward the N0SS method from the one handed method. It will give you better long term joints.

Even knowing the secret of rosin paste, soldering takes practice.  That is why the PC board and components were included in the kit.  You will be making a very simple circuit several times on the board to get the feel of soldering.

This is a good place to point out some safety related items.  The first thing is always work in a well-ventilated area.  A little bit of airflow through your work area will keep you from inhaling most of the fumes.  This is important because most solder contains lead as well as some organic compounds in the rosin that you do not want to take into your lungs.  The amount of lead exposure when soldering is small and should not cause problems for the hobbyist.  Lead can also be absorbed slowly through the skin so always wash your hands when you finish soldering. 

The next thing is eye protection.  Solder occasionally splatters or bubbles and pops.  You really don't want hot solder to hit your eyes.  In addition, you will be clipping leads off of components when soldering.  Cupping one had around the component when clipping will help contain the lead, but there is still a chance that the lead could fly out.  It's worth spending the $5-$15 for that extra protection provided by a pair of safety glasses.  Safety glasses were not included in the kit because you really need to find a pair that fits well to your particular face.  Purchase your safety glasses at a local hardware or home improvement store where you can hold the glasses up to your face before you buy them.  The glasses should fit with few gaps around the edges.  They also need to hold well around the ears since you will be leaning forward while soldering.  Glasses that keep slipping are worse than having no glasses at all.

Finally, the most obvious warning: The tip of a soldering iron is very hot.  Even momentary contact with the tip or other metal parts of the iron will cause burns.  Any material that has a lower flash point than wood will catch fire instantly when touched by the iron.  Wood will smolder, char, and eventually catch fire if the iron is held to it.  Always arrange your work area so that the iron's electric cord cannot catch on anything.  Have an open box of baking soda nearby whenever you are working.  Baking soda makes a great fire extinguisher.  Just dump the soda on any 'thermal event' (i.e. fire) that occurs.  The soda can be brushed off and vacuumed up easily without hurting electronic components.  Do not leave your iron plugged in if you are leaving your work area.

Now, for the practice circuit: Figure 2 shows the components and tools you will be using.  Soldering iron, multipurpose holder, solder, rosin paste and toothpicks, multipurpose PC board, header pins, resistors, LED's, and fine sandpaper.  The multipurpose holder holds your iron when you are not using it and has a wet sponge to wipe off the tip of your iron.  It also holds your PC board or components while soldering.  You can also use any odd little pieces of metal, ceramic, solid wood, paper clips, or oven safe plastic to help hold your components in the correct position while you work on them. 

Condition your iron as previously described.  Adjust the work holder so that the grips are just above your worktable.  Place the PC board into the clips with the copper side up.  Snap off a set of ten header pins from the header pin strip.  Put the short end of the pins through the PC board from the painted side to the copper side as shown in Figure 3.  Note that the pins are in the outer set of holes.  Adjust the work holder to get the PC board to the right height, you can get the pins to stand straight as shown in the figure.  You may need to find a book or block of wood to place under the pins to get the right height.

Apply a very thin layer of rosin paste to the part of the pin exposed above the PC board and to the copper pads that surround the holes the pins are in.  Do not leave any lumps of rosin on the board and do not smear rosin from one pad to the next.  Remember that the solder is going to flow wherever the rosin is spread.  Hold the iron and apply a small amount of solder to the tip.  Touch the tip to the intersection of the first pin and its PC board pad.  Some of the solder will flow off the tip and onto the pad and the pin.  Touch the solder wire to the point where the iron is touching the pad and pin if there does not appear to be enough solder on the joint.  Make sure not to cover the second hole in the pad with solder.  The tip should not be in contact with the pin and pad for more than about two seconds at a time.  Wipe the tip of the iron on the sponge to remove any excess solder.  The plastic that holds the pins together is not very heat resistant so once you have finished the first pin, go to pin six in the strip and solder it.  Continue to alternate between pins (1,6,2,7,3,8,4,9,5,10) until you have all the pins soldered as shown in Figure 4.  In the figure, pin six has an ideal solder joint.  The other joints shown are sloppy, but serviceable.

Turn the board over and insert a resistor as shown in Figure 5.  Since the resistor has to stay in place when the board is turned back over, bend the leads slightly outward so that they can not slide back through the hole.  Clip the leads to about the same length as the pins you just soldered.  Keep one of the clipped leads to make the jumper.  Put rosin paste on the resistor leads and the pads around the holes that they go through and solder them like you did the pins.

Turn the board over again.  Take one of the leads you clipped from the resistor.  Bend it and insert it in the row next to the resistor as shown in Figure 6 to make a jumper.  Turn the board back over, apply rosin paste, and solder the jumper.

Turn the board over once again.  Insert an LED as shown.  Note that one of the leads on the LED is longer than the other and that the lip around the plastic part of the LED has a flat spot on the side with the long lead.  This is to indicate the negative side of the LED.  Put the negative lead of the LED into the third hole of the pad where the resistor is soldered and the positive end into the third hole of the pad where the jumper is soldered as shown in Figure 6.  You know what happens next.  Turn the board over and solder the LED in place.  Make sure that there is a gap between the board and the bottom of the plastic case of the LED large enough to allow you to insert a multimeter probe.  This will make troubleshooting easier if you need to do it later.  When finished the bottom of the board will look something like Figure 7.

Congratulations, you have just soldered your first electronic circuit.  Now you need to test it.  Snap a 9V battery onto the battery holder.  Make a small loop in the exposed part of the black insulated wire and hook it around the pin next to the resistor.  Touch the exposed part of the red insulated wire to the pin next to the jumper.  If the LED lights up, all of your solder joints in that circuit are serviceable.

Troubleshooting

If the LED does not light up, you will need to troubleshoot the circuit.  Grab your multimeter.  If this is the first time that you have used the meter, very lightly sand the surface of the probes. The probes on low cost meters get a small amount of oxidation on them during shipping which will affect the meter's accuracy.  Set the meter to read voltage.  Remove the battery pack from the circuit and touch the probes from the multimeter to the leads from the battery.  Your meter should read around 9 Volts.  If it is reading less than 8 Volts, your battery is dead and needs to be replaced.

If the batteries are fine, check that you have the correct polarity, i.e. that the negative battery lead is connected to the negative side of the circuit.  Test this by switching around where the two battery leads are connected.  Note that while this is OK to do with this circuit, there are some cases where reversing the polarity can harm sensitive components such as microcontrollers in the circuit.

If your polarity was correct, next you need to check the circuit path.  Switch the meter to read resistance.  Touch the one probe from the multimeter to the pin by the resistor and the other probe to the pin by the jumper wire.  Most meters will read overload since most multimeters are not strong enough current source to cause the LED to conduct electricity.  Surprisingly, a few cheap meters do produce enough current and actually light the LED if the circuit is complete, however, if your circuit was complete, the LED would have lit when the battery was connected to it.  Depending on the meter you have the overload indicator may be different.  On some meters, OL is displayed.  On others, a 1 is displayed on the far-left side of the screen.  Still others flash a screen full of 9's.  Check the instruction that came with the meter to see which overload indicator it utilizes.  For this article, we will use OL to indicate an over load reading. 

If the meter reads anything other than OL, there is a short circuit.  One of the solder joints on one side of your LED is touching something in the circuit on the other side of the LED.  This allows the electricity to complete the circuit without going through the LED.  Turn the board over and carefully look at each solder joint until you find one where the solder connects two pads.  Remove some of the solder that bridges the pads by scraping it and then try to power up the circuit again.  If it still doesn't light up, repeat the previous steps.

If you get OL for the complete circuit, but no light you have an open connection, probably a bad solder joint, somewhere in the circuit.  To isolate which solder joint is bad, you need to look at the resistance through half of the circuit at a time. Set the multimeter to resistance and put one probe of the multimeter on the pin closest to the resistor.  Put the other probe on to the little bit of LED lead that you left between the plastic case of the LED and the board.  You should be touching only the lead on the side of the LED connected to the resistor.  The meter should read around 360 Ohms.  If it reads OL, the bad joint is somewhere between the first pin and the LED.  In that case, move the probe that was on the LED to the resistor lead closest to the LED while keeping the other probe on the pin.  If you still see OL, the bad joint is neither the connection between the LED and the pad nor the connection between the resistor and the pad.  In that case, move the meter probe to the other side of the resistor.  If you still get an OL reading, the bad joint is not on the pin side of the resistor and either the solder joint for the pin or the joint for the resistor lead on the pin side is bad.  Keeping one probe touching the pin, flip the board over and touch the other to the copper pad on the board between the solder joints for the pin and the resistor.  If the reading is still OL, the pin joint is bad. 

What you just did was eliminate one joint at a time from the "suspect list".  If the reading had started at OL and then in one of the later steps had gone to the correct reading (360 Ohms if the resistor was still in the part of the circuit being tested or less than1 Ohm if the resistor was not in the part being tested), the bad joint would have been the one that had just been taken out the part of the circuit being tested.  If the meter does read around 360 ohms for the resistor side of the circuit, repeat the process on the jumper side of the LED.  This time, you should get a reading of less than 1 ohm.  If instead, you get an OL reading, the bad joint is in that half of the circuit.  Isolate the joint using the same steps described above.  If you identify a bad joint, put a very small amount of rosin paste on the solder joint and reheat it with your iron until the solder once again liquefies.  Then remove the iron, let the joint cool, and try the circuit again.

If you have a good battery and get good readings on both halves of the circuit, but still no light, the problem is in the LED.  While rare, you will at times get a pad part.  Go on to the next iteration of the circuit.

Conclusion

By the time you have made five iterations of the circuit, you should be fairly comfortable with soldering and bad joints should be rare.  You now have the basic tools and the basic skill needed to build a robot! 

Next month, we will build a complete robot.  This robot will be more than just a platform.  It will be mobile, sense its environment, change how it moves based on what it senses, and be easily modifiable.