What is pseudo soldering? What is his harm and what causes it?

What is pseudo soldering? What is his harm and what causes it?

Among the failures of electronic products, the failure of pseudo soldering joints accounts for a large proportion. According to statistics, nearly half of the failures of electronic products are caused by poor welding, which almost exceeds the probability of failure of electronic components. As a result, the reliability of electronic products is reduced, ranging from increased noise and deterioration of technical indicators, and in severe cases, the circuit board cannot complete the design function, and more seriously, the entire system suddenly collapses without any precursor, resulting in significant economic losses and loss of reputation. 

Reasons for pseudo soldering:

The solderable surface and the solderable surface of electronic components are oxidized or contaminated; poor solder performance, poor flux performance, poor metal plating of substrate pads; improper setting of welding parameters (temperature, time).

The effects of pseudo soldering:

Pseudo soldering makes the solder joints connect or has contact resistance, which causes the circuit to work abnormally, or the unstable phenomenon that the electrical connection is not connected when it is connected. As a result, the noise in the circuit (especially in the communication circuit) increases without regularity. It brings significant hidden dangers to the circuit's debugging, use, and maintenance. In addition, some pseudo soldering joints maintain electrical contact well for a long time when the circuit starts to work, so it is not easy to find. However, under environmental conditions such as temperature change, humidity change, and vibration, the contact surface is gradually oxidized, and the contact gradually becomes incomplete, causing the circuit to "strike." In addition, the contact resistance of the pseudo solder joints will cause local heating, and the local temperature increase will further deteriorate the situation of the incompletely contacted solder joints and eventually even make the solder joints fall off. The circuit cannot normally work at all.

The causes of pseudo soldering can be roughly divided into several aspects:

1. Pseudo soldering caused by component factors and its prevention

Insufficient metal coating thickness, oxidation, pollution, and deformation of the solderable parts of the components can cause pseudo soldering.

1) The metal coating thickness of the weldable part is not enough

Usually, the solderable surface of the components is plated with a certain thickness, silver-white, uniform and easy-to-solder layer. However, if the coating is too thin or uneven, and the copper-based tin-plated or steel-based copper-plated copper is then tin-plated, the difference between copper and tin At the copper-tin interface formed by the mutual contact between the two metals, the two metals will penetrate each other for a long time to form an alloy layer diffusion, which will make the tin layer thinner, resulting in a decrease in the solderability of the soldering surface.

2) The solderable surface of components is oxidized

Due to the long storage time or improper storage conditions of electronic components, the surface of the pins or welding ends of electronic components can be oxidized, resulting in the generation of pseudo soldering. The oxidized solder surface turns gray and black, and severe oxidation can be detected visually. For the oxidized electronic components, either discard or use them after the deoxidation treatment is qualified. Generally, the oxide layer is treated by external force rubbing and scraping, slightly acid cleaning, smearing flux, and then tinning.

In addition to visual inspection of whether components are oxidized, there are more complex and detailed test standards for solderability testing. If the conditions are not met, manual, wave soldering, or reflow soldering methods can be used to conduct batch sampling test soldering of components.

3) Contamination of the solderable surface of components

In the production of electronic products, components have to go through processes or operations such as incoming material receiving and counting, storage, material delivery, molding and inserting (THT process), SMC and SMD loading and unloading, placement and manual repair welding, etc., which will inevitably generate dust, oil and sweat pollution, resulting in a decrease in the solderability of the soldering surface of electronic components.

In the electronic assembly production site, maintaining a clean production environment, wearing protective equipment, and operating in strict accordance with operating procedures are effective measures to prevent component contamination.

4) Deformation of component pins

SMD devices, especially QFP and SOP packaged devices with fine inter-pin spacing, are easily damaged and deformed, and the coplanarity of the pins deteriorates. In addition, after mounting, some pins are not close to the pads, resulting in pseudo soldering.

2. Pseudo soldering caused by PCB and its prevention

In electronic assembly, PCB oxidation, pollution, deformation, etc., can cause pseudo soldering.

1) PCB oxidation caused by pseudo soldering and prevention

Due to the long storage time or improper storage conditions, the PCB can cause oxidation of the pads and the wall of the insertion hole, resulting in the generation of pseudo soldering.

The oxidized pads lose their metallic luster, turn gray or black, and there are cases where there is no abnormality in visual inspection. For PCBs suspected to be oxidized, the solderability test should be carried out according to the standard, and the results are good before use.

2) PCB pollution caused by pseudo soldering and prevention:

In the production process of the PCB board, the PCB receiving, storage, SMT printing, patch, THT plug-in, wave soldering, and other processes, the operator must contact the PCB; dust, oil, and sweat will contaminate the pad so that the PCB can be weldability decreased, resulting in pseudo soldering.

Maintaining a clean production environment and operating by the production process procedures is a good habit for avoiding PCB pollution. If a contaminated PCB is found, it should be cleaned, decontaminated, and dried before use.

3) PCB deformation caused by pseudo soldering and prevention:

After the PCB is deformed, the coplanarity of component placement becomes poor, and some component pins and pads are suspended, resulting in pseudo soldering. BGA and QFP package components are more likely to form pseudo soldering.

There are generally two cases of PCB deformation: First, the incoming material is deformed, the incoming material is closed, and the PCB is accepted according to the standard.

4) The solderable layer on the PCB surface has different processing methods and metal materials, and its solderability indicators are also different. Therefore, if the solderability indicators are not qualified, pseudo soldering is also a major cause.

5) Some PCBs are warped and deformed during the high temperature period of reflow soldering. They will be flattened after cooling down, resulting in pseudo soldering and significant stress, and the possibility of later failure of the solder joints is very high.

3. Pseudo soldering caused by flux and solder factors and its prevention

1) Flux causes pseudo soldering and prevention

Poor flux performance will be unable to effectively remove oxides on component solder surfaces, PCB insertion holes, and pads, resulting in pseudo soldering of solder joints. This should pay particular attention to when changing the flux manufacturer or model. Especially when the new type of flux is used, a soldering test should be done.

The flux concentration should be checked frequently and updated according to the process specification.

2) Pseudo soldering caused by solder factors and its prevention

In the wave soldering process, the tin-lead solder is continuously oxidized at a high temperature of 250 °C so that the tin content of the solder continues to decrease, which deviates from the eutectic point resulting in poor solder fluidity, pseudo soldering, and insufficient solder joint strength.

4. Other factors cause pseudo soldering and its prevention

1) In the process of wave soldering and reflow solder cooling and solidification, the solder joints disturbed by PCBA jitter have low strength. The solder joints are very easy to open and fail in the use of customers. In electronic assembly, this situation is often attributed to In the field of pseudo soldering.

2) When the PCBA has a significant bending, during the product assembly, it is fixed on the base of the chassis, the PCBA is forced to be flat, resulting in stress, and the solder joints will crack over time, resulting in an open circuit. The method of prevention is to use a PCB with qualified flatness. If the hot plate is not laid flat when the wave soldering or reflow soldering is released from the furnace, it can be restored to flat by wave soldering or reflow soldering again; remember to correct it by artificial bending.

3) Another situation is that the PCBA is flat, but its fixed base is not flat, and the installation effect is the same as the result of the later generation of the above 2).

4) In the circuit board parts that are often subjected to external forces during use, such as the solder joints near the key switch on the PCBA, it is very easy to bend slightly during use, resulting in metal fatigue and failure of the solder joints.

5) The solder joints near the heating element on the PCBA are relatively easy to fail and open, and the effect is the same as the pseudo soldering.

6) Remember to prevent the PCB from bending during the production process; otherwise, there will be cracks in the metalized wall of the via hole, resulting in the phenomenon that the time is not connected, and it is easy to be misjudged with false welding.

7) In electronic assembly, reasonable soldering temperature and time are the fundamental guarantees for reliable soldering and the formation of good solder joints.