The facts you should know about Cold Solder Joint

The facts you should know about Cold Solder Joint

Cold soldering can adversely affect the quality of a board and is a particularly difficult situation to avoid without the right skills and tools. By skills, I mean soldering experience; and when it comes to tools, unlike a soldering iron, you have to have a soldering station that can control the temperature. If the temperature is not controlled, a cooler solder joint is more likely to form.

What is a cold solder joint, how it forms, and how to test and correct it?

Common Solder Joint Problems

As we know, an ideal solder joint should appear shiny and concave on the surface and solder pins. Ideally, the solder should be at an angle of 40 to 70 degrees from the horizontal.

Perfect solder doesn't always come about. In most cases, some common mistakes do occur, resulting in cold solder joints. Such errors include:

Disturbed Joints - Formed when disturbances occur at the joint before the solder paste solidifies

Cold Soldering - Occurs whenever the solder is not completely melted when soldering.

Joint Overheating - Occurs when the flux on the board overheats, mainly due to the solder wire not melting properly. This often results in a complicated welding experience.

Insufficient wetting - Occurs when the pins or board burn more than the other. Insufficient wetting of the pins indicates excessive pin burn, and on a circuit board, may indicate insufficient application of wetting material.

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What is a Cold Solder Joint?

From the above definition, it is now relatively easy to deduce what is a cold solder joint. Cold solder joints occur when the solder wire is not heated enough during the soldering process, creating an incorrect joint.

Cold solder joints can also occur when the board or wires are disturbed before the molten solder solidifies.

If not detected or repaired, cold solder joints can cause increased resistance at that joint. In the worst case, the connector may fail to conduct at all, causing the board to fail.

Types of cold solder joints

There are several types of cold solder joints, each with specific causes and behaviors. Let's take a look at the common types of cold solder joints.

The main types of cold solder joints are:

l?cracked solder joints

l?dry solder joint

l?cold solder joint

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What factors can cause cold solder joints?

In addition to what we have discussed in general above, there are several specific cold solder joint triggers. These factors are:

l?Mismatched component geometry

l?Incomplete wetting due to low soldering temperature

l?Excessive soldering temperature leads to premature flux failure

l?Relative interference between the board and the component before the solder has completely solidified

l?Unusual melting, wetting, and/or reflow at seams

l?Solder Alloy Contamination

l?Selectively Welded Frame

How to detect and test cold solder joints

Currently, most electrical projects are assigned to freelancers, often on tight deadlines. Freelancers have to be very fast so as not to disrupt clients' schedules. Often, this fast pace results in cold solder joints that are hard to detect when soldering.

With the board complete, the electrical engineer must now deal with the fact that the board is not working. The first thing to do is usually a test. So how does this work? Let's find out.

Visual inspection

PCB solder joints are small (as the board size decreases); therefore, a magnifying glass and spotlight are often required.

The first thing to notice should be the color of the joints. As we mentioned before, dry solder joints are dull.

The next thing to check should be the shape of the solder joints. If it is not concave or deformed, it may be due to insufficient heating and the solder alloy has not melted sufficiently. When the resistance heats up the joint, this can lead to cracking and eventual separation from the board.

Next, use a flashlight and magnifying glass to see if the light passes through any joints. If it does pass, it's an insufficient joint.

Finally, tilt the board to see if any of the connector sections come off the bottom of the board. You can also check for overflows to make sure there are no shorts as they can completely ruin the board.

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Check with a multimeter

A multimeter will help to check resistance and continuity at the joints. That way, you can tell if there is a problem around that joint.

l?resistance test

Set the multimeter to a value of 1000 in resistance mode. Interconnect test probes to determine if the device is functioning properly. You should get a zero reading. Connect one test terminal on one connector and another terminal through a different component. If that component is not a resistor, you should always get a zero reading. If the reading is above zero, the joint is most likely cold.

l?continuity test

Switch the multimeter to continuity mode and test the functionality of the device by connecting the test terminals. You should hear a beep.

Connect the two test terminals to either end of the solder joints. If it doesn't beep, the joint needs to be reworked.

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How to Repair Cold Solder Joints

Disturbed cold solder joints or just cold solder joints are the most common forms of cold solder joints.

l?Disturbed cold solder joint

These are the most common types of cold solder joint problems. This happens when the board or soldered component moves before the solder has fully solidified.

The most notable features here are the concave and angled pins inside the solder spool.

To avoid this problem, you'll need to solder the board on a firmly secured table or a wall-mounted soldering vise in case space is a constraint.

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l?Ordinary cold solder joint

Insufficient heating of the soldering iron and contaminated tip are other common causes of cold solder joints.

Make sure the tip of the soldering iron is clean before use. After use, store the tool in the case to prevent dust particles from settling on the tip.

Make sure the tip of the soldering iron is fully heated before use. If possible, you can use a soldering station whose operating temperature can be easily adjusted.

Once the desired heat is reached, reheat the solder, and mount. If your soldering iron seems to cool down quickly, use a lead-free SN96 solder alloy. The alloy melts at lower temperatures and exhibits a plastic transition time.

Cold solder joints usually have high electrical resistance due to incomplete melting of the solder alloy. High resistance can cause overheating, and immediate or long-term damage to components or boards. Not to mention the associated high power consumption.

Incomplete circuits also increase the power consumption of the circuit, which is undesirable, especially in tiny devices.

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Best Practices for Soldering

Cold solder joints are a real pain for electrical engineers. While these situations cannot be completely eliminated, especially if time is limited, there are a few practices that can help make the soldering process go more smoothly. they are:

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l?Have the right tools for the job. You should have a good soldering iron and alloy.

l?Use as little solder alloy as possible.

l?Store your soldering iron and alloy away from dust and moisture to prevent contamination and its consequences. If in doubt about contamination, perform an alloy analysis.

l?Always allow the soldering iron to heat sufficiently so that the alloy is fully melted. For best results, set the peak temperature at least 15 degrees above the melting point of the alloy for at least 45 seconds.

l?Use lead-free alloys for a smooth soldering experience. As mentioned earlier, SN96 alloy is one of the best alloys.

l?Always work on a sturdy bench or vise to avoid moving boards or components. Eliminate all sources of vibration as much as possible.

l?Allow enough time for the solder joint to fully solidify.

l?Always follow the manufacturer's reflow profile specifications.

l?Stay calm and collected and avoid shaking, which may cause a spill.

Conclusion

Cold solder joints are one of the most frustrating things for electrical engineers. While not completely avoidable, you can reduce the occurrence of cold solder joints by employing the practices described above. For the few cases that may still occur, you can use the test and fix method described.