Soldering instructions

This exercises goal is to get you familiar with some basic soldering.

To finish the assignment you need to solder the same circuit that you designed in the earlier exercise, and show that you have a working sensor.

All of the components that you need are available in the makerspace on the table next to the soldering stations. If any component runs out please tell me directly or shoot me an e-mail (remahl@kth.se).

The placement of the components are provided from a reference sheet that you can find along with the components.

Normally we have two stations set up for soldering, but there are a lot of people wanting to solder at the same time you can ask me and I'll set up an extra temporary station.

Normally it is recommended to solder the smallest components first, to make sure that you can reach with the soldering iron, but if you are new to soldering you can start with the through hole resistors before attempting to solder the SMD components, the board is designed so that the resistors won’t block the rest of the components much.

I would however save soldering the LED and the header pins until last since they get in the way of the SMD components otherwise.

Through hole soldering

Start by taking one of the components and put it in its corresponding position. First bend the components legs and insert it so that the components lies flush with the board. To secure the component you can bend the legs outward slightly on the back after having inserted it. Now put the board upside down in a stable position and solder from the underside.

The resistors do not have a polarity so they can be placed either direction, but the following goes for the other THT components:

• The diode in the front has a black band which should line up with the white band marked on the board.
• The LED has a flat side on it’s base, which normally would line up with the marking on the board, but due to an error in the design you should have it the other way around.

When soldering use the soldering iron to heat up both the through hole and the component lead by resting the tip against both surfaces.

When both surfaces has had some time to heat up (~1-3 seconds) you can apply some solder. The solder wire should not touch the iron itself, rather try to come in with the solder wire from the opposite side of the iron tip. If the joint is hot enough the solder should flow into the joint and form a smooth surface.

After both joints have been soldered snip off the back of the leads flush to the board.

You can do multiple components at once or do them one by one.

Surface mount

Once you have finished soldering the resistors and the diode you can move on to soldering the SMD components.

First the OP-amp. Pin one is the bottom left pin if you are looking at the chip with the text facing you, and the IC should be placed with pin one towards R7.

Before placing the IC on the board put a small amount of solder on the pad for pin one. Now take a tweezer and place the IC while heating the pad you just put some solder on. Try to align the IC to the pads as well as you can and remove the iron while holding the IC. If you miss alignment by just a bit you can just rotate the component a little (bending the soldered leg a bit) or if you missed by a lot you can heat up the joint and try again.

Once you have good alignment you can solder the opposite pin (on the diagonal) using the same technique as with the through hole components.

This will lock the IC in place, and it should not be able to wiggle any more.

Now you can solder the rest of the pins. If you get a bridge between pins just clean the tip of the iron and then drag it along the pins to try to remove some of the solder. This will usually resolve most solder bridges. If you get way too much solder so that all of the legs bridge you can remove some solder using a solder wick. (If you don’t know how to use one just grab me and I will show you how)

The capacitors are soldered using the same general technique, add some solder to one of the pads, solder the component to that pad. Here alignment needs to be correct since there is no 'leg' to bend on the capacitors. Then solder the other side. When soldering the second pad try to apply heat quite fast, if you linger too long the opposite pad can heat up enough to come loose.

If you run into problems don’t be afraid to come ask for help. This part can be pretty hard if you have never soldered before.

Testing the board

Once you have soldered all of the components the board should look something like this:

Make sure that you don’t have any solder bridges and that all of your connections are solid. There is a microscope in the makerspace that you can use to inspect your board.

If everything looks good you can connect the board to a power supply, a 5V supply works best.

It can take a bit of time for the sensor to stabilize (10-20 seconds) after applying power the first time but after a while the LED come on and the light should pulse if you breathe on the diode. (You need to get pretty close for it to work properly though)

If you have a working sensor you can show me (Linus) in the makerspace and I will record that you finished the assignment.