Cold solder joint
A cold solder joint is a poorly formed solder joint that has a frosted, grainy, and dull appearance. Cold solder joints are also called “cold joints” or “gray joints”.
They are caused by insufficient heat, dirt, or grease preventing the solder from melting onto metal surfaces properly. The cause of cold solder joints can also be the movement of the component’s lead in the solder joint before the solder has cool down and solidified.
One of the most common mistakes beginners in soldering make is not heating both surfaces simultaneously, or not heating them enough which results in a cold solder joint. This is the case when soldering iron is placed on the component pin for too short a time or when the soldering iron is set at too low temperature.
Another case is when the soldering iron is touching and heating a pad on the circuit board but, at the same time, is not heating the pin of the component. This makes the solder bond to the pad on the circuit board but not to the pin of the component.
The same happens vice versa – when soldering iron is heating the pin of the component but not pad on circuit board solder. This makes the solder bond to the pin of the component but not to the pad of the circuit board.
Sometimes technician wants to move too fast through the soldering project, which results in a few cold solder joints.
It is usually noticeable when the solder does not want to bond to dirty surfaces because it forms beads or globules. Another sign that may lead to cold solder joint is when it takes a very long time for the solder to wet the joint.
The cold solder joint is physically weaker and higher in electrical resistance than a properly soldered connection.
When the solder cools down far too quickly and doesn’t flow properly because of inadequate heating, solder joints get a matt appearance instead of a shiny look.
Cold solder joints can be detected by visual inspection or using a microscope or magnifying glass.
In addition, cold solder joint can be detected by using a digital multimeter since one of the effects of a cold solder joint is an increase in electrical resistance.
Repair of cold solder joint
When you want to repair cold solder joint, it is usually necessary to remove the troublesome solder and then re-solder it afresh. Logically, this time you should apply proper soldering techniques and use proper soldering tools.
It will be very bad practice to simply re-melt solder in cold solder joint hoping that the molten solder will eventually bond to the surfaces of the solder joint.
Neither won’t help to add more solder. The conductivity and strength of the solder are degraded once the solder is overheated, so the solder should be replaced.
First, remove the solder from the cold solder joint with a de-soldering pump or solder wick (also known as solder braid).
To start removing solder, place solder wick on top of solder in the cold solder joint. Apply heat to solder wick with a soldering iron.
The temperature of soldering iron should bit a little bit higher than the normal temperature to compensate for some heat that solder wick takes away from the joint.
The solder wick will get very hot so if you hold it with fingers hold it at least 4 inches (10 cm) from the end. You can use small pliers to hold the wick as well.
Always apply light pressure to soldering iron - solder wick will absorb solder once the solder and solder wick are hot enough. Very often beginners in soldering make mistake and strongly push soldering iron to solder joints hoping that harder they push iron the faster will melt solder.
Usually, one portion of solder wick will not absorb all solder from the cold solder joint so move the solder wick along the cold solder joint.
Once all solder is removed from the cold solder joint you should brush off any loose particles and clean surfaces of the joint with isopropyl alcohol.
Apply a small quantity of rosin flux to the joint and then re-solder the component following the appropriate soldering technique.
The tip of the soldering iron should be clean and tinned.
Apply solder to the hot iron tip and wipe it on the moist sponge.
The solder should cover all surfaces of the iron tip.
After tinning iron tip should be shiny and have a bright silver color.
Most of the cold solder joints could be avoided by using a proper soldering tool.
Use one of temperature controlled soldering irons.
A temperature-controlled soldering iron maintains a set temperature all time during soldering.
In addition, use soldering iron which wattage is 50W-70W since it recovers faster. If you use a cheap soldering pencil which wattage is around 20W you will have more chance to make a badly soldered joint.
Keep in mind that the selection of the proper size and shape of the iron tip is a very important step.
If you use a too small conical iron tip you won’t be able to heat surfaces to be soldered at the proper temperature.
If you use a too big and wide iron tip you may damage the circuit board or temperature-sensitive electronic component.
The correct iron tip should be no wider than the solder joint on which it will be used.
In addition, use the correct size and type of solder.
Most of the solder wires available in the USA come as flux-cored solder wires.
That means that it contains a rosin flux in its core.
Select the diameter of the soldering wire that is the best size for your soldering operation.
If you use solder wire with a diameter of 0.062” (1.5 mm) for soldering circuit boards, solder joints will be buried in excess solder.
However, this size of solder wire will be perfect when you tinning medium-size chisel iron tip, soldering thick wires or soldering alligator clips, banana connectors, etc.
Solder wire with a diameter of 0.025” (0.6 mm) can be used for most soldering projects in electronics.
Cold solder joints happen more often when soldering terminals of potentiometers and toggle switches than when soldering electronic components to circuit boards. The main reason for this is that terminals are not heated enough before the technician applies solder.
Terminals of potentiometers and switches are metal leads that are bigger than leads and pins of other electronic components such as resistors, diodes, transistors, etc. That means it will take more time to heat them until they reach the proper temperature than it requires to heat pad and conductors on circuit boards.
One way to apply more heat to terminals is by setting soldering iron to a higher temperature – set iron to 750 F (400 C) or higher. Another way to apply more heat to the terminal is by holding soldering iron on the terminal for a longer time.
And now comes a tricky part when soldering toggle switches. If you hold soldering iron on the terminal of the toggle switch for too long, the plastic body that surrounds the terminal will start to melt. This results in permanent damage to the toggle switch since the terminal will easily fall out of the plastic body of the switch. When this happens, there is no way to repair this switch - you need to buy a new one.
So, the proper amount of heat applied to the terminal it is crucial when soldering terminals of the toggle switch.
Too short heating time results in cold solder joints and attached wire to terminal peels easily. On the other hand, too long heating time results in permanent damage. With experience, you’ll get a feel for how long you should hold the iron on the terminal before you can apply solder.
A little trick that always helps to make a proper solder joint is to apply a small quantity of rosin flux to the connection before soldering.
Rosin flux (also known as colophony) is distilled from pine resin.
Rosin flux melts at a lower temperature than solder, flows around solder joint, and prevents the formation of oxides during soldering. That makes a soldering operation much easier and smoother.
Insufficient heating of solder joint can lead to flux pitting – unburned flux remains on the surface of the solder joint.
Once the flux has been activated, apply enough solder to the solder joint to achieve good wetting.
Even though overheating of solder joint usually results in a good joint it may damage heat-sensitive electronic components such as semiconductors. In addition, overheating can cause the solder joint to oxidize. Underheating of solder joint results in a poor bond between solder and surfaces.
When removing soldering iron from the solder joint you should do that by wiping up along the lead. Removing the iron away from the side can cause a bulge.
Thoroughly clean the solder joint with alcohol when finished. This will remove all residues left on the solder joint caused by the soldering operation.
Perfect soldered joints should be smooth, shiny, slightly concave, volcano-shaped, and without cracks, pores and holes. Dull or rough surfaces can be caused by either overheating or underheating.
We have to inspect the quality of the solder joint to ensure that it provides a good electrical connection.
A microscope or a good magnifying lamp is required for a proper and thorough inspection.
Use an ohm-meter on a portable digital multimeter to test for continuity.
A solder joint that is badly soldered is likely to be electrically unreliable and may worsen over time.
Sometimes the solder joint may look good and pass visual inspection but underneath it may have a poor electrical connection or could work at the beginning and then cause the electronic device to fail at a later date. These intermittent problems of solder joints are the most difficult to troubleshoot. They are usually caused by the expansion and contraction of the joint due to heating with a soldering iron.
Good solder joints are:
Bad solder joint
Bad solder joints reduce the reliability of SMT or through-hole PCB assembly and may lead to failure. They are a result of the improper soldering process.
Besides cold solder joints, there are a few more types of bad solder joints:
If you apply too much solder to the joint it can bridge two adjacent pads on the circuit board.
Bridging also occurs when soldering iron accidentally touches two adjacent leads which result in forming a span of solder between them.
A solder bridge can not be tolerated on an electronic device because it forms an electrically conductive connection.
The probability of forming solder bridges increases as the spacing between solder joints decreases.
Concern for solder bridges is present more in surface mount assemblies than in through-hole assemblies, since surface mount assemblies use fine-pitch surface-mount components. Recent developments in technology allow the design of high-density circuit boards
To repair solder bridge you should remove excess solder with solder wicks and soldering iron.
The solder wick will absorb most of the solder in the solder bridge. Usually, after removing the solder bridge there is too little solder left in the solder joint to provide a reliable connection. Re-solder solder joint with fresh solder.
The cause of solder balls may be impurities in the solder or improper solder cooling.
Solder balls should be removed with a small brush.
Insufficient amount of solder
Another very common mistake is not feeding enough solder in the solder joint.
In this case the joint is not completely covered with solder. This joint is weak.
This may happen when beginners in soldering are concern they will burn the circuit board and they rush to finish soldering as soon as possible. They apply just a small quantity of solder and quickly remove iron.
This can be easily repaired by adding more solder to the solder joint.
Damaged or lifted pads
Too much heat applied to the solder joint can damage, brake or lift pads and conductors on printed circuit boards.
It is not easy to fix this since pads are separated and peeled off the board.
You can only partly repair this damage of pads or conductors - scrape the damaged copper foil and run a piece of thin wire instead.
The only way to deal with this problem is to prevent it from happening – avoid applying excess heat to solder joint, use a proper temperature-controlled soldering iron, don't set soldering iron to a maximum temperature, and follow proper soldering guide.
Remains of flux
All flux should be removed with isopropyl alcohol or another solvent once when the solder has cooled and solidified.
Updated: June 15, 2023