What Is a Cold Solder Joint? Signs, Tests, and Fixes

A cold solder joint is the name given to a suspect or defective joint that didn’t receive or transfer sufficient heat to allow the solder to flow and wet the metals it was actually intended to attach to. The term cold denotes how the joint was produced. It does not signify that the solder joint, when completed, is actually cold to the touch and is not a catch-all term for every dull, cracked, sparse, or otherwise intermittent joint.

Although a suspect solder joint may conduct electricity when tested quickly, it could ultimately fail when subjected to heat, increased load, or other types of mechanical stress or changing conditions. When this happens, the behavior is useful when it comes to troubleshooting; however, it does not provide the technician with enough information to identify the specific failure mechanism.

• A cold solder joint is produced when solder fails to adequately flow and wet the joint surface area.
• Dull appearance is not definitive proof of a cold joint failure; lead-free solder can appear matte in finish.
• It is important to distinguish formation defects from cracks, low solder volume, disturbed joint conditions, vague “dry joint” labels, and normal alloy-dependent surface texturing before selecting the appropriate repair method.
• After a controlled repair is performed, the technician must verify workmanship and product function independently; therefore, the fact that the original product can now operate successfully does not validate the original defect mechanism for the product.

What Is a Cold Solder Joint?

Defective formation of cold solder joints occurs due to insufficient actual heat transfer and inadequate surface conditions to allow for proper solder flow and wetting. To clarify, the term wetting refers to the way in which molten solder flows across and bonds to the desired metal surfaces. During the soldering process, it is important to apply sufficient heat to both the component lead and PCB pad and feed solder from the opposite side, as instructed by HAKKO’s soldering training tutorial. The most common causes of cold solder joint failure include contamination, oxidation, uneven heating, and inadequate heat transfer.

The eutectic temperature for Sn63/Pb37 is 183°C, while for SAC305 the solidus is 217°C and liquidus is 220°C. These temperatures apply to alloy transitions and not to universally defined soldering-iron temperatures or reflow temperatures.

A solder joint may have formed correctly but developed cracks through mechanical forces applied after formation. A solder joint may have formed with too little solder applied to it but have acceptable levels of wetting on the surface area that is covered. Disturbance during solidification (i.e., moving the part during cooling) may lead to a disturbed solder joint. Each of these conditions can exist concurrently, but they do not share one underlying cause or solution.

Cold Solder Joint vs Dry Joint

There is inconsistency in the use of the term “dry joint” within the soldering community. Some references indicate that “dry joint” is used when there is an insufficient amount of solder on the joint, while others indicate it is used when there is poor wetting due to oxidation or contamination. The reason this inconsistency matters is that the two conditions require different repair processes. A person using a reference that defines “dry joint” as having insufficient solder volume would be instructed to add solder, which would correct the situation in terms of volume but would not correct the lack of wetting due to oxidation or contamination. A person using a reference that defines “dry joint” as a wetting failure would first be instructed to clean and reflux prior to adding solder, which would be incorrect if the original problem was caused by insufficient solder volume. Determine the specific type of failure of a solder joint—insufficient solder, nonwetting or dewetting, disturbed solder, or cracking—rather than treating “dry joint” as a specific failure mechanism.

What Problems Can a Cold Solder Joint Cause?

An improperly created solder joint can introduce intermittent connectivity, raise contact resistance, or have different characteristics when the device is subjected to thermal cycling, vibration, and mechanical forces. In some cases, intermittent connectivity may produce reset conditions, blinking indicators, dropped signals or sensors, audio noise, or hot spots from the I²R effect. Equivalent symptoms may also be produced by other factors such as cracks, connectors, conductors, or components.

A defective connection may pass a momentary continuity check but fail when temperature, load, or mechanical conditions change. It is possible that test current, probe pressure, temperature, and mechanical loading will differ from service conditions; therefore, you cannot identify the specific failure mechanism based on a single intermittent result, since this could also be attributed to a crack, an issue with the connector, component failure, or a damaged conductor.

IPC J-STD-001J addresses the materials, methods, process controls, and acceptance requirements for soldered assemblies, whereas IPC-A-610J contains assembly acceptability criteria. When assessing acceptability or determining if a repaired product meets expectations, the applicable standards will dictate what requirements should be utilized.

What Does a Cold Solder Joint Look Like?

Appearance may influence inspection decisions regarding cold solder joints; however, it does not provide tangible evidence that confirms the internal interface or identifies the root cause of the cold solder joint. Cleaning the area will provide a better basis for further inspections. Controlled illumination, multiple viewing angles, and enough magnification to inspect the lead, land, solder fillet, and visible portion of any plated hole are key to inspecting solder connections.

Some characteristics to look for indicating an improperly connected joint include poor wetting boundaries, solder balled against a surface, visible separation, ring cracks, movement, and incomplete solder joints. A good place to begin comparisons is with neighboring solder joints that share the same board, package, alloy, and manufacturing process. A solder joint that appears bright due to the surface finish of the product does not necessarily validate a good solder joint on another product. The Kester FAQ states that SAC solder joints will generally feature a duller finish than Sn63 solder joints. This appearance does not by itself indicate the integrity of the solder joint; it simply indicates that SAC solder joints have a different surface texture when compared to tin-lead solder joints. The material type and other characteristics such as geometry, wetting, and cracking are still critical to the quality of the solder joint.

Although IPC-A-610J provides visual acceptability criteria, visual inspection cannot determine all solder joint interface conditions.

Cold Solder Joint vs Other Solder Defects

There are multiple conditions impacting solder joints that exhibit dullness, roughness, open circuits, or intermittent connectivity; however, they are created by distinct events and require different checks.

The formation timing of a solder or rework joint is the most useful way of defining the boundary of a solder-joint problem. A formation problem is associated with the process of either creating or repairing a solder joint. As opposed to a formation problem, a service crack can develop once a well-formed solder joint has undergone strain or thermal cycling. For instance, if several adjacent solder joints on the same board and package, using an identical PCB processing method, exhibit a consistent appearance, but only one solder joint fails, the defective solder joint should be considered an isolated formation or disturbance issue and the associated solder joint’s formation history should be reviewed first, rather than reworking all solder joints in the same manufacturing batch. Conversely, if many solder joints in the same manufacturing batch exhibit a duller or textured appearance but pass continuity testing and maintain function, these solder joints may reflect a normal alloy-dependent appearance and the material and process records should be reviewed. There is no benefit in resoldering solder joints that were never truly defective.

For example, a solder joint with ample solder volume may not wet the surfaces effectively, whereas a solder joint with limited solder volume may demonstrate acceptable wetting characteristics over a smaller area of the solder joint’s surface. Thus, before determining whether the identified solder defect can be fixed through cleaning and reflowing or by simply adding solder, the true solder characteristic in question must be established.

How Do You Test a Suspected Cold Solder Joint?

There is no single visual inspection, continuity beep, thermal response, or reflow outcome that can accurately classify or identify all types of solder-interface conditions. Employ the most useful means of screening when needed and in accordance with available tools, equipment, and procedures. Other methods should be utilized according to risk and with controlled energy before conducting thermal or mechanical stimulation.

1. Control Energy First. When performing any work on a product, first control the power source and ensure none of the remaining stored energy presents a safety hazard to personnel when de-energized. Under normal circumstances, OSHA 1910.333 requires de-energization before work. De-energized products should have all hazardous stored energy released so the personnel performing the work are not exposed to potential hazards.
2. Preserve Evidence of the Untouched State. When the technician begins to identify a failed or defective joint, he or she should make note of the specific joint location, symptom (failure indication), and network characteristics, as well as document the specific joint’s geometric orientation compared to adjacent joints prior to any cleaning or heating of the joint; photographs can preserve evidence that rework may destroy.
3. Inspect Under Controlled Conditions. If an assembly has been cleaned, the technician should approach the inspection of the assembly with a combination of different light sources at various angles in conjunction with magnification when necessary. When performing an inspection, the technician should make sure he or she has a reference for the same geometry to support like-for-like comparison.
4. Test the Unpowered Network. Testing of the network while it is unpowered can assist the technician in determining if the network has any open or unstable connections. When the circuit permits meaningful measurement, continuity or resistance measurements can indicate that an electrically conductive connection exists at that time. However, these methods do not indicate mechanical strength, current capacity, long-term stability, or the original formation mechanism.
5. Carefully Correlate a Symptom to the Suspected Cause. The technician can identify possible electrical and functional behavior results through controlled troubleshooting, taking into consideration any anticipated product state. The technician must not simply twist (flex), bang, or heat the suspect joint with an uncontrolled heat source to troubleshoot. Cracked conductors and connectors, as well as other faults, may also cause thermal or mechanical sensitivity.
6. Use Appropriate Professional Tools/Methods for Each Hypothesis. Inspection methods or tools may differ based on the specific requirements of the product’s fault. For example, AOI can assess visible geometry on a completed assembly, while X-ray inspection may assist with locating selected hidden characteristics within the product. Cross-section or material analysis may be performed when stronger interface evidence is required and the cost and destructive nature are justified.
7. Document All Work on the Completed Assembly, Including Inspection and Measurements. Once the technician has performed the appropriate post-rework inspection, unpowered measurements, and product-level testing relevant to the original symptom, he or she should document the results of all three processes separately rather than documenting them in one pass/fail report. The technician should enter the original symptom, the specific corrective action taken, and the post-repair results as three separate records in the appropriate record-keeping process.

What Causes a Cold Solder Joint?

Cold solder joint failures can be traced back to inadequate joint formation due to the type of heat delivered, the condition of the surface, the compatibility of the materials used, the movement of parts, or joint geometry.

Tip selection guidance from HAKKO highlights that effective heat delivery depends on the tip’s thermal capacity and contact area. Tip maintenance guidance emphasizes keeping the tip “wet” and preventing unnecessary temperature increases.

The fact that a large copper plane can dissipate heat more effectively than a small tip supplies it suggests that all the components that contribute to providing that heat should be assessed together as a complete heat path instead of only focusing on the temperature at the tip or the setting on the soldering iron.

When a pad connects directly to a large copper zone, the copper can act as a heat sink during hand soldering. The KiCad documentation states that thermal-relief spokes can increase thermal resistance between a pad and a zone and thus improve the hand-solderability of components, but this will depend on the electrical, current, thermal, and assembly requirements.

The temperature profile, or “process window,” for a given production assembly will be dictated by the actual alloy used, the type of solder paste used, the finish, the component, the PCB construction, and the equipment used. For example, Indium6.6HF recommends peak temperatures of 235–250°C, with 230–262°C as the wider acceptable window, and time above liquidus of 45–60 seconds for SAC305. However, the PDS notes that the temperature profile parameters for the process and design must be reviewed and updated as required.

How Do You Fix a Cold Solder Joint Safely?

Rework is reasonable, provided that control of the heat source and applicator are managed, that the solder joint can be inspected and verified, that the substrate remains intact after rework, and that the rework operator has the tools needed to apply the proper heat and time for the specific application.

Controlled Rework Steps

1. Disconnect all energy sources and address any stored energy hazards prior to touching the board.
2. Maintain the integrity of the original evidence. Verify the exact joint, side, network, and symptom prior to the application of heat.
3. Evaluate the lead, pad, plated barrel, solder mask, and adjacent laminate. A lifted land, a defective barrel, delamination, or carbonization has an impact on the rework path.
4. If the connection and assembly meet the criteria, apply compatible fresh flux with controlled heat, allowing the connection surfaces to achieve the requisite condition for solder flow and wetting. Avoid using a single setpoint or dwell time for all alloys and boards.
5. Keep the connection stationary during solder solidification and inspect the connection for wetting, bridges, residue, displaced components, lifted copper, and adjacent heat damage.
6. Repeat the measurements and functional tests that were required to establish the original symptom. Document workmanship acceptance and functional recovery independently.

Control fume emissions during work. The UK Health and Safety Executive’s Electronics Soldering guidance requires suitable fume control and keeping the operator’s breathing zone clear of the fume plume. Avoid excess heat as it adds to both process damage and fume exposure.

When Should You Stop Rework?

• High voltage, stored energy, or battery damage.
• The suspect connection is located beneath a BGA, QFN, LGA, shield, or another inaccessible structure.
• A pad has lifted, the barrel has the potential for damage, the laminate has discolored, or repeated heating has provided no additional information.
• The product is safety-critical, controlled for traceability purposes, under warranty, or cannot be adequately inspected and functionally verified after repair.
• The operator does not have the necessary tools, product procedures, access to inspection, or qualifications to provide a controlled outcome.

The HAKKO desoldering manual cautions against the potential for peeling a land by direct nozzle contact. Therefore, do not apply any mechanical force until the solder has completely melted. If it is necessary to remove the component, follow a controlled desoldering process and, if required, utilize the appropriate IPC-7711/21 rework or repair procedure.

If operation returns to normal after rework but there is no documentation of the prior condition, then only report what the evidence shows: that the intervention affected the fault path, but that the original defect mechanism has yet to be confirmed.