The current era has been witnessing a high speed of integration and the massive production and manufacturing of lead components to be miniaturized. When it comes to SMT (Surface Mount Technology) assembly, components have to go through a series of processing, including pre-processing, placement, soldering, test and packaging. During the whole process, the occurrence of ESD (Electrostatic Discharge) may call for damage to different extent, reducing the performance of SMT assembly. In the process of soldering, however, the humidity inside components may bring forward vapor and pressure, causing cracks inside components and they may be diffused. As a result, shorts may be caused finally, reducing the reliability of products. Therefore, the damage caused by ESD on soldering joints is so severe that sufficient attention has to be paid to that, which is what is talked about in this article.
Generation Source of ESD
Electrostatic electricity mainly derives from three sources: friction, induction and human body.
a. ESD by Friction
As a type of electric energy, electrostatic electricity usually stays at the surface of objects and its generation derives from the imbalance between positive and negative charges within a partial range. Generally speaking, electrostatic electricity is generated through the transfer of electrons and ions. When friction takes place between two insulating objects, the electric energy on the surface of one object will be transferred to the surface of the other object that then features negative charges. ESD by friction mostly is correlated with object surface cleanliness, size and the environment.
b. ESD by Induction
The objects that are capable of arousing induction are conductors and dielectric medium in the field of static electricity. When conductors are available around charged objects, polarization will be caused on conductors due to the effect of electrostatic field. As a result, inductive charges will occur with opposite polarizations and equivalent values, which will add the possibility of ESD.
c. ESD by Human Body
Human body is regarded as the main source of ESD during the whole process of SMT assembly. When humans move, the friction caused by shoes or clothes will lead to charged objects. Additionally, temperature, high-speed movement and fractures will also lead to ESD.
As a matter of fact, static electricity is often met in people’s daily life, but it features little damage. If, however, ICs and high polymer materials are frequently used, the damage of ESD will be improved. Therefore, it’s greatly significant to come up with ESD protection scheme in SMT assembly process.
Damage of ESD on Solder Joints
Soldering joints play such a key role in the whole system of electronic products that they carry out the responsibility of connection during circuit running. Currently, circuits become increasingly highly-densed with soldering joint count substantially increasing. If something is wrong with a single soldering joint, the whole circuit system will be failed. Soldering joint defects mainly come from the defects that take place in the process of SMT assembly process. Once the environment temperature fluctuates or the current of the whole circuit becomes unstable, components will become so hot that thermal fatigue will further decrease the performance of electronic products. Moreover, when the temperature around solder joints changes, the thermal stress will be generated inside solder joints.
Another defect of solder joints comes cavity. Cavity refers to some tiny air bubbles within soldering joints and it derives from compound expansion or the air residue in solder paste. Although visual inspection can be used to find out some exterior cavities of solder joints, the cavities made during the soldering process will become such a huge threat that it is difficult to expose them out. However, cavities are formed due to a series of natural processes, which means that they are unavoidable. On one hand, cavities can stop fractures from being formed or from changing the expansion routes of fractures. On the other hand, cavities will shrink the shelf life of end products.
Measures to Minimize the Negative Effect of Solder Joints
ESD protection tops the list of the problems that will possibly occur during SMT assembly process. Operators have to be equipped with sufficient knowledge concerning ESD protection, totally aware of the aim and specific measures of ESD protection. Besides, all the operators have to be trained to feature know-how and maintain high consciousness concerning ESD protection.
• ESD Protection Measure#1: Conductors and Insulators
If static electricity is generated on conductors, it should be leaked in time through a couple of channels such as some equipment and devices. The channel of electrostatic leakage can be the grounding with conductors so that electrostatic charges can be released from conductors. Anti-static wires can be built to contribute to ESD protection and they should be established independently to separate power grounding wires and anti-static grounding lines. Since charges are unable to move in insulators, the ESD protection measure for conductors doesn’t work on insulators. Thus, charge neutralization is usually applied for ESD protection of insulators. Ion fan is used to generate positive and negative ions so that electrostatic charges can be neutralized. Within the same environment, the lower environment temperature leads to improvement of humidity that also affects the conductance of conductors. When the electrical conductivity constantly rises and humidity also goes up, it’s better to protect against ESD within such environment. As a result, it’s an optimal solution to improve humidity and decrease temperature as ESD protection measure within a dangerous field.
• ESD Protection Measure#2: ESD Protection Device
To largely stop static electricity from being generated and to release the existing static electricity, ESD protection devices should feature sufficient conductivity and be capable of controlling the speed of ESD leakage to avoid static electricity from being leaked too much.
• ESD Protection Measure#3: Protective Circuit
It’s quite normal to add protective circuit to ESD protection measures. However, protective circuit is not accepted in miniaturized and high-density circuits.
As far as ESD protection measures are concerned, so many elements have to be taken into consideration when making a decision so that the optimal ESD protection measure can be applied.
P.S. Some Tips for Optimal Solder Joint Generation in SMT Assembly Process
As electronic products become constantly miniaturized and multi-functional, it becomes increasingly difficult to analyze the causes for defects during SMT assembly process. Solder joints issue has been a key issue in SMT assembly process so solder joints issues can be dealt with by appropriately setting reflow soldering temperature curve.
Cavity is capable of accelerating fracture’s expansion speed and the strength of solder joints will be reduced. The application of blind vias on PCB (Printed Circuit Board) will lead to cavities, which will be also caused by inappropriate technology application. Air is available in blind vias and its volume will become enlarged after being heated. Besides, some air is expelled when solder paste is being melted and unexpelled air will become cavity in solder joint.
In addition, the generation of cavity is directly related with soldering temperature. Preheating temperature should be prolonged to the range from 90 to 120 seconds so that the air and water contained in solder paste can be totally vaporized, which is capable of reducing the opportunity of cavity advent. Or, to reduce the surface tension of solder paste is capable of expel bubbles in high-temperature environment.
