Surface Component Soldering

Surface mounted device or (SMD) soldering, consist of placing a component on a circuit board and soldering the device on to pads provided for the item.  Each component is sized and usually silk screened, for the appropriate placement on the circuit board.  

Example circuit board with surface mounted components

SMD components come in many sizes.  Some are smaller than a speck of dust.  Other's are as large as your hand.  Below is an illustration showing the different sizes as they relate to your hand.

Example of SMD component sizes

Now, when you decide to solder one of these devices on to your own circuit board you must first ask yourself, “what is the proper size and type of soldering tip I need?”  That’s a good question and we’ll go over that.

For example, if you are soldering a 0603 resistor you would definitely not need an 800 degree tip to perform this process.  I generally use a 600 to 650 degree chisel tip or bent conical for this action, depending on where the component needs to be installed.  If the resistor is easily accessible I will use the chisel tip.  If it is in a tight spot I might change to the slim bent conical to gain easy access. 

You will have to make your own decision on what type to use for your job as I can only make a suggestion based on my own experience as an IPC Specialist. 

Example of a 0603 SMD resistor

So, now that you have selected a tip it is time to solder that resistor to the board.  First off, clean the pads well with some isopropyl alcohol and an acid brush.  Dry the area thoroughly.  If you are using rosin core solder make sure to use rosin flux if you decide that you need more wetting action. 

Note: (you should never mix fluxes with the incorrect type of solder) i.e. do not use no-clean flux with a rosin core solder.  Consult your user’s manual before proceeding if unsure.

Place the component on the pad and position into place.  Ensure the resistor’s leads cover at least 75% of the pad area on each side.  Add some flux to the component.  Add a dab of solder to your tip and tack the component on one side to hold it in place.  Once tacked you can add more flux and then proceed to solder both sides. Don’t be afraid to use flux liberally if needed.

Once soldered the fillet should be shiny and concave on each side, (unless you are using lead-free solder).  If you used rosin core solder and rosin flux you must clean the area of residue.  If you have used no-clean flux and non-rosin core solder you may forego cleaning the area if desired.  I prefer to clean all of my solder jobs upon completion just to ensure that all debris and residue are removed.

Most SMD’s are soldered to a circuit board in this manner when a technician is hand soldering.   

Example video of proper way to solder an SMD resistor

There are machines that can do the work much quicker and are generally used for mass production of circuit boards.  I will cover those in future articles.  We will just cover the basics in this article to help get you started soldering SMD components to your own circuit boards.

Now that you have practiced a bit on a simple SMD resistor let’s move up to a multi-lead component such as an SOIC. 

Place the component on the appropriate pads and line up pin one of the component, usually designated with a dot or arrow, with the silk screen designation for pin one on the board.  It is usually a white dot at one corner.  Line up the leads and once again ensure that all of the leads cover at least 75% of their pads. 

Next, flux the leads and place a dab of solder on your tip.  I prefer to use a hoof tip for multi-leaded SOIC's.  Place a dab of solder on the bottom of the hoof tip and tack 2 corners of the component to hold it in place.  Now, take you hoof tip and drag it across 2 or 3 leads at a time.  You will see that the solder will flow up the leads and pads in a uniform manner.  If you get too much on the leads a solder bridge may be formed.  To remove the bridge just take your empty hoof tip and drag it down and away from the 2 leads.  If this doesn’t work use some wicking braid and then re-solder the affected leads.

Ensure that you don’t use too much solder.  The solder should not flow up the leads of the component past the halfway point.  Rather, the solder should flow over the pad and cover just enough of the component leads to adhere the component to the pad.  You should be able to make out the lead under the solder fillet.

In the video below the technician has chosen to use a bent conical tip for soldering his component into place.  This is perfectly acceptable.  There is no "one rule" for each specific job.  You must chose what works for your job in order to attain the best outcome.

Example video of the proper way to solder an SMD SOIC

Inspect the fillet for correct concavity and the lead for proper placement.  The fillet should be shiny and free of any pitting, cavities, or debris. Clean the area you just soldered well with isopropyl alcohol and a good stiff bristle brush, such as an esd safe acid brush. Dry everything thoroughly before placing back in a bag or equipment cavity.

 

There, now you’re almost an expert.  Keep practicing and try different tips.  I generally select a tip for my own jobs based on what works best for me.  You will learn how to select the right tips for each of your own job as you progress on your journey through the exciting world of soldering.