PCB Blog - Causes of cold welding problems in PCB reflow soldering

What is cold welding?PCB cold welding is a phenomenon that occurs when the required minimum wetting temperature between the electronic component and the PCB is not reached during the soldering process.Even if there is localized wetting, if the metallurgical reaction is incomplete, it will also lead to the occurrence of cold soldering. Cold soldering is characterized by a darker, rougher appearance on the surface of the solder joint and a failure to fully fuse with the soldered material.

PCB cold soldering occurs during the soldering process, when the required minimum wetting temperature is not achieved between the electronic component and the PCB, the solder joint will form a poor connection.Even if there is a certain amount of local wetting, such as the failure to achieve a complete metallurgical reaction, will also lead to the emergence of cold welding. Cold soldering is  characterized by a darker, rougher surface appearance of the solder joint, which cannot be fully fused with the solder material.

Reasons for Cold Welding:

1.Insufficient heating

One of the main reasons for cold welding is that during the soldering process,the electronic components and PCB between the minimum wetting temperature is not reached. If the flow soldering temperature is too low,the solder can not be completely melted, which leads to the solder joints failed to form a strong metal bond.

2.Short soldering time

Insufficient soldering time is also an important factor leading to cold soldering.If the flow soldering time is not enough, the solder can not be in the appropriate time to fully flow and heat dissipation, the formation of the solder joint will not be strong enough.In this case,the surface of the solder joint may appear dull and rough, affecting the performance of the circuit.

3.Surface contamination

The presence of contaminants on the soldering surface, such as oil, dust, etc., can affect the wetting properties of the solder, making the joint unable to form a good metal connection. Even if the correct temperature and time are applied, contamination can still lead to cold soldering.

4.Improper temperature control

In thereflow soldering process, the furnace temperature curve is not set properly,resulting in a rapid rise or fall in temperature, can also trigger cold soldering. Uneven tension of solder paste in the heating process can cause incoherent solder joints. Cold soldering can be effectively reduced by precise temperature control.

5.Tin-eating Problems

Cold soldering can also occur due to the lack of tin-eating properties of the solder joint itself. Failure of the solder to form a good bond on the interface will result in a weakened solder joint, which will lead to cold soldering. Therefore, choosing the right solder and ensuring the good condition of the soldering surface are important measures to prevent cold soldering.

In the FR4-PCB welding of electronic industry, gold has become one of the most commonly used surface coating metals due to its excellent stability and reliability. However, as an impurity in the solder, gold is very harmful to the ductility of the solder, because brittle Sn-Au (Sn-Au) intermetallic compounds (mainly AuSn4) will be formed in the solder. Although low concentration of AuSn4 can improve the mechanical properties of many South Korean tin-containing solders, when the content of gold in the solder exceeds 4%, the tensile strength and elongation at failure will decline rapidly. The 1.5um thick pure gold and alloy layer on the pad can be completely dissolved into the fusion solder during wave soldering, and the AuSn4 formed is not enough to damage the mechanical properties of the plate. However, for the surface assembly process, the acceptable thickness of the gold coating is very low and requires accurate calculation. Glazer et al. reported that the reliability of the solder joint between the Cu-Ni-Au metal coating on the plastic quadrilateral flat package (PQFP) and FR-4 PCB will not be damaged when the gold concentration does not exceed 3.0 W/O.

Too much IMC endangers the mechanical strength of the solder joint due to its brittleness, and affects the formation of the bell hole in the solder joint. For example, the solder joint formed on the 1.63um gold layer of the Cu-Ni-Au pad can be reflow welded after printing 7mil (175um) 91% metal content Sn63Pb37 solder paste on the pad. Sn-Au metal compounds become particles and are widely dispersed in solder joints.

In PCB board processing, in addition to selecting appropriate solder alloy and controlling the thickness of gold layer, changing the composition of base metal containing gold can also reduce the formation of intermetallic compounds.For example,if Sn60Pb40 solder is welded onto Au85Ni15,there will be no gold brittleness.

In the circuit board reflow soldering process, the cold soldering inspection method usually used:

Visual Inspection: Specialized inspection criteria are designed to observe the appearance of the solder joints and check for phenomena such as granular solder joints or non-smooth surfaces, which are common features of cold soldering.

Automated Optical Inspection (AOI): Utilizing automated equipment to image solder joints with high precision, it can quickly and accurately identify potential cold solder joints and provide quantitative data support.

X-ray Inspection: For complex circuits, especially BGA packages, utilizing X-rays for the internal structure of solder joints is a very effective means of detecting hidden cold soldering problems.

Effective methods to prevent PCB cold welding:

1.Ensure proper soldering temperature

In the reflow soldering process of PCB, the appropriate soldering temperature is an important factor in preventing cold soldering. Should be based on the size and thickness of the PCBA board, as well as the thermal coefficient of the components, set a reasonable reflow soldering temperature curve to ensure that the solder can fully melt and wet the pad. The accuracy of temperature control directly affects the quality of welding, so the design should pay special attention to this point.

2.Control of soldering time

Soldering time control is equally important, too short a soldering time may lead to the solder can not fully flow and adhesion,the formation of cold soldering. Should ensure that the time setting of the soldering process to meet the melting and wetting requirements of the solder,usually by adjusting the operating parameters of the reflow soldering equipment to achieve. Accurate time control helps to ensure the integrity of the solder joint.

3.Material selection and use

The use of high activity solder pastes and appropriate fluxes can also help prevent cold soldering. Low-quality solder paste tends to lead to poor wetting, so it is important to select high-quality solder paste and strictly control the conditions of its use and storage. In addition, the right amount of flux will promote the flow of solder and enhance the adhesion of the solder.

4.Clean soldering surface

Before soldering, it is vital to ensure that the soldering surface is clean. Any oil, dust or oxides will prevent good contact between the solder and the pad, which can easily lead to cold soldering. Therefore, PCBs and component pins should be cleaned before soldering to provide good soldering conditions.

5.Maintain a stable component position

Components should be kept in a stable position during the soldering process to avoid jiggling or lifting. During reflow soldering, lifting and positional movement of components can lead to a decrease in the uniformity of the solder joint, which can cause cold soldering. This can be accomplished by ensuring the rigidity of the PCB and proper soldering equipment.

Cold welding of PCBs is a critical problem that affects circuit performance and is mainly caused by factors such as insufficient heating, short soldering time and surface contamination and it can be effectively prevented by ensuring proper soldering temperature, time control, material selection, and cleanliness of the soldering surface. Focusing on these preventive measures will significantly improve solder joint quality, thereby increasing the overall reliability and performance of the board.