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Simon / 2021-06-15
Whereas soldering is one of the most critical steps in assembling an electronic circuit, it can be where the unreliability of an electronic circuit begins. The most common issue with soldering is the formation of a cold solder joint.
As cold weld can adversely affect the quality of a board and is a particularly hard-to-avoid occurrence if one doesn't have the right skills and tools. By skills, I mean experience at soldering, while regarding tools, there is a soldering station that has controlled temperature, unlike the soldering iron. Without controlled temperatures, a colder solder joint is much more likely to form.
We will discuss what a cold solder joint is, how it forms, and how to test and correct it, but before then, let's familiarize ourselves with the common solder joint issues.
The ideal solder joint should appear shiny and concave-shaped against the surface and the soldered pin. Ideally, the solder should slant by between 40 to 70 degrees with the horizontal.
A perfect solder does not always occur. In most cases, some common mistakes do occur, leading to the formation of a cold solder joint. Such mistakes include:
Disturbed joint- forms when a disturbance occurs at the joint before the solder paste solidifies
Cold joint- happens whenever the solder doesn't completely melt when soldering.
Overheated joint- occurs when the flux on the board is overheated, primarily due to the solder wire failing to melt as it should. This often results in a complicated soldering experience.
Insufficient wetting- happens when either the pin or the board is burned more than the other. Insufficient wetting of the pin indicates an excessively burned pin, whereas, on the board, it could indicate insufficient application of the wetting material.
From the above definitions, it now relatively easy to deduce what a cold solder joint is. A cold solder joint happens when the solder wire is insufficiently heated during soldering, forming an improper joint as a result.
A cold solder joint can also arise when either the board or the soldered wire is disturbed before the molten solder solidifies.
If undetected or repaired, a cold solder joint causes increased electrical resistance at that joint. In the worst-case scenario, the joint may completely fail to conduct, resulting in the failure of the board.
Cold solder joints are classified into several types, each with specific causes and behavior. We look at the common types of cold solder joints.
The main types of cold solder joints are:
Cracked solder joint
Dry solder joint
Cold solder joint
Just as the name suggests, a cracked solder joint results from a stressed solder joint that yields to form a crack. Once the crack sets in, the pin is partially detached from the board and is free to move. This movement can close or break the electrical connecting depending on the profile of the crack.
The uncontrolled electrical connection start and stop will cause the appliance in question to work or fail on seemingly random occasions.
The most susceptible components are the heavy vertically mounted components as a result of gravity. If the boards are in constant motion or flipping, the problem is exaggerated as the crack widens.
Cracked solder joints are often a problem with cathode ray tube (CRT) monitors on transit. Manufacturers place these monitors in padded boxes to reduce vibrations and thus the chances of causing cracked solder joints.
A dry solder joint is simply a joint soldered with an insufficient amount of solder. When the molten solder flows into the joint, the entire connection wets so that when it solidifies back, a perfect electrical connection is established.
A dry solder lacks a sufficient amount of metal, leading to a completely open circuit in some cases. In other instances, the joint may have intermittent electrical conductivity and high resistance, resulting in noise when in operation.
Dry solder joints have an easily identifiable dull or matt finish.
A cold solder joint is generally either a cracked joint or a dry solder joint. The identifying characteristic is that it doesn't work. Generally, if you aren't sure if it is a cracked or a dry solder joint, but it doesn't seem to work, it is a cold solder joint.
Sometimes the cold solder joint may be both dry and cracked. Nevertheless, all cracked solder joints are not cold joints. But this doesn't take away the fact that a cracked solder joint will be problematic at some point if not attended to.
Apart from what we've generally discussed above, there are several specific predisposing factors to a cold solder joint. These factors are:
Mismatched components geometry
Too low soldering temperature leading to incomplete wetting
Too high soldering temperature causing a premature breakdown of the flux
Relative disturbance(s) between the board and the component before the solder has completely solidified
Uncomely melting, wetting, and/or reflow at the joint
Solder alloy contamination
Selective soldering frame
Presently most electrical projects are assigned to freelancers who are usually on tight deadlines. The freelancer has to be really fast to avoid messing up the client's timelines. More often than not, this fast pace results in cold solder joints that may be hard to detect when soldering.
Once the board is complete, the electrical engineer has to now deal with the fact that the board isn't working. The first thing to do is usually testing. So, how is this carried out? Let's find out.
PCB solder joints are tiny (with the reducing board sizes); thus, a magnifying lens and a spotlight are often necessary.
The first thing to watch out for should be the color of the joints. As we mentioned earlier, a dry solder joint is dull..
The next thing to check should be the shape of the solder joint. If it lacks the concave shape or is deformed, likely, the solder alloy didn't melt sufficiently due to inadequate heating. This could lead to cracking and the eventual detachment from the board when resistance heats up the joint.
Next, use the torch and the magnifying lens to see if light passes through any joint. If it indeed passes, then that's an inadequately bonded joint.
Finally, tilt the board to see if any joint partially detaches from the base of the board. You can also check for overspills to ensure no short circuits are created, as they can totally ruin the board.
A multimeter will help check the resistance and the continuity across a joint. This way, you can tell if something is off around that joint.
Testing for resistance
Set the multimeter to a value of 1000 on the resistance mode. Interconnect the testing probe to ascertain the device is working. You should get a zero reading. Connect one testing terminal on one joint and the other terminal through a different component. If the component is not a resistor, you should always get a zero reading. If the reading is above zero, then you most probably have a cold joint.
Testing for continuity
Switch the multimeter to continuity mode and test the device's functionality by connecting the test terminals. You should hear a beep sound.
Connect the two test terminals to either end of a solder joint. If it doesn't beep, that joint needs rework.
Disturbed cold solder joints or just cold solder joints are the most common forms of cold solder joints.
Disturbed cold solder joints
These are the most common types of cold solder joint problems. They occur when the board or the soldered component is move before the solder has sufficiently solidified.
The most distinguishing characteristic here is the concave shape and a tilted pin within the solder spool.
To avoid this problem, you need to solder your board on a securely fixed table or a wall-mounted soldering vice in case space is a constrain.
Normal cold solder joints
Inadequately heated soldering iron and a contaminated soldering tip are other common causes of a cold solder joint.
Ensure the soldering iron tip is clean before use. After use, store the tool in a box to prevent dust particles from settling on the tip.
Ensure the soldering iron tip is sufficiently heated before using it. If possible, you can use a soldering station whose working temperature you can easily regulate.
After the ideal heat is achieved, reheat the solder and then mount it. If your soldering iron seems to cool fast, use the lead-free SN96 solder alloy. This alloy melts at a lower temperature and exhibits plastic transition time.
Cold solder joints usually have high resistances resulting from incomplete melting of the solder alloy. The high resistance can cause excessive heating, ruining the component or the board, either immediately or in the long run. This is not to mention the associated high power consumption.
Incomplete circuits will also increase the circuit's power consumption, which is undesirable, especially in miniature devices.
Cold solder joints can be a real pain for electrical engineers. Although these occurrences cannot be totally eliminated, especially where time is limited, there are practices that help make the process of soldering smoother. They are:
Have the right tools for the job. You should have a good soldering iron and alloy.
Use as minimal solder alloy as possible.
Store the soldering iron and alloy free from dust and moisture to prevent contamination and the consequences thereof. If unsure of contamination, perform an alloy analysis.
Always allow the soldering iron time to heat up sufficiently so that the alloy melts completely. For best results, set the peak temperature not less than 15 degrees above the alloy's melting point for at least 45 seconds.
Use the lead-free alloy for a smooth soldering experience. As mentioned earlier, the SN96 alloy is one of the best alloys to work with.
Always work on a sturdy workbench or vice to avoid moving the board or the components. Eliminate all sources of vibrations where possible
Allow the soldered joint sufficient time to fully solidify
Always follow the manufacturer's reflow profile specifications
Be calm and composed to avoid shaking, which could cause spillovers
Cold solder joints are one of the most frustrating things for electrical engineers. Although not totally avoidable, you can employ the practices described above to reduces the instances of cold solder joints. For the few that may still happen, you can employ the testing and repair methods described.