Through Hole Soldering

Soldering a through-hole or axial lead component to a circuit board is not as hard as you might think.  The first thing to ask yourself is this, "do I know what I'm doing?"

If you answered yes, then you should proceed without reading any further.  If you answered no, then you might need to read this article.  Let's start with the assumption that you have very little or no skill at all in soldering a through-hole component to a circuit board.

Take the board below as an example.  When soldering a through-hole component to a circuit board you should ensure that the eyelets, or holes, should be free and clear of any solder or debris.  Use some isopropyl alcohol and clean the surfaces thoroughly.  If the eyelets have not been tinned then now is the time to do that.  Solder will flow better if the eyelets are tinned prior to soldering the component into the holes.  Ensure you don't use too much solder when you perform this task.  The eyelets should look similar to the ones on the right of the illustration below.  if they look like the ones on the left you have not used enough solder.  The eyelets in the middle need tinned.

Examples of tinned eyelets

 

Now that you have cleaned the holes, it's time to place your component into the appropriate spot.  If it's a multi-leaded component or similar, be sure that all of the leads are placed in their holes.  Once placed, turn the circuit board over and slightly bend the leads so the component will remain in place.  Some people prefer to leave them straight and use tape.  

 Axial or Through-Hole leads bent on PCB

While this method will work, I prefer not to use tape as it leaves residue on the components.  The industry standard is to bend the leads slightly on the backside and then solder them in.  Above you will see in the illustration the leads are slightly bent on the backside.  You can use either method.  The industry standard is generally to measure the leads on the backside, tack them in place, and then cut them to the appropriate height.  Once cut and tacked they can then be soldered.

Now that the leads are in place it's time to solder.  Add a little flux, "I prefer a little tacky flux", and then apply your hot iron.  Place the tip of you iron to one side of the lead and the solder to the other.  Once the lead heats up the solder should flow around the lead and leave a nice concave fillet like the one shown below. 

Through Hole Soldering Technique

Although, I don't recommend using more solder than is necessary as shown in the video.  You should not have to use a desoldering tool to achieve a nice fillet if you flow the solder right the first time.  If you follow this method below there should be no need of two tools to achieve a nice fillet around the lead.

Proper Way to Solder Axial Leads

The video shown above is the correct way to solder in axial lead components.  Inspect your board and compare it to the end product in the video.

If you've used too much solder it might create a cold solder joint.  If you've used too little solder the joint may be weak and could crack over time.  This can also cause a cold solder joint from lack of solder.  The ideal solder joint for a through hole component should be concave and shiny all the way around the lead.  The eyelet should be completely filled, but not filled to the point of overflowing onto the component side.  

Once you have completed the soldering process it's time to clean the fillet.  Use some isopropyl alcohol and then inspect the fillets for signs of pits, depressions, and too much or too little solder.  Practice a while and you will become an expert in no time.