Welding quality reliability
The "light, thin, short, and small" of electronic products puts forward higher requirements for the miniaturization and assembly density of components. Under such requirements, how to guarantee the quality of solder joints is an important issue. As a direct result of welding, solder joint quality and reliability determine the quality of electronic products. That is, in the production process, the quality of the assembly is finally expressed.
At present, in the electronics industry, although great progress has been made in the research of lead-free solders, it has begun to be applied worldwide, and environmental issues have also attracted widespread attention. However, due to many reasons, Sn-Pb solder alloys have been used. Soldering technology is still the main connection technology for electronic circuits. This article will give a more comprehensive introduction to the solder joint quality and reliability of Sn-Pb solder alloys.
1 solder joint appearance evaluation
Good SMT solder joints should be soldered to the device for the life cycle of the device, with no mechanical or electrical failure. Its appearance is as follows:
(1) Good wetting
(2) The proper amount of solder and the solder completely covers the soldering area (or soldering end) of the pad and the lead, and the component height is moderate;
(3) A complete and smooth and shiny surface. In principle, these guidelines apply to all types of solder joints that are soldered by all soldering methods in SMT. In addition, the edge of the solder joint should be thin. If the soldering surface is large enough, the wetting angle between the solder and the surface of the solder pad is preferably 300 or less and the maximum is 600 or less.
2 Failure modes of solder joints during the life cycle
Taking into account the relationship between failure and time, failure forms are divided into three different periods, as shown in Figure 1.
(1) In the early stages of failure, failure of a large number of solder joints with poor quality occurs, and some solder joints are also damaged due to improper process operations and handling. Can be optimized through the process to reduce early failure rate.
(2) Stabilization failure rate phase, where most of the solder joints are of good quality and the failure rate (failure rate) is low and relatively stable.
(3) At the end of the life cycle, the failure is mainly caused by cumulative destructive factors, including chemical, metallurgical, and thermal-mechanical properties, such as the metallurgical reaction between the solder and the metal to be welded, or heat. Mechanical stress causes solder joint failure. The failure is mainly determined by the characteristics of the material, the specific structure of the solder joint and the load.
3 solder joint failure mechanism caused by welding process
Some unfavorable factors in the welding process and subsequent improper cleaning processes may lead to solder joint failure.
3.1 Thermal Stress and Thermal Shock
The rapid thermal change during wave soldering causes a temporary temperature difference to the component, which subjects the component to thermal-mechanical stress. When the temperature difference is too large, stress cracks occur in the ceramic and glass parts of the device. Stress cracking is an unfavorable factor affecting the long-term reliability of solder joints.
After the solder is cured, the PCB must also be lowered from 1800C to room temperature. Due to the difference in thermal expansion coefficient between the PCB and the component, sometimes the ceramic component may also be broken.
The glass transition temperature of PCB is generally between 1800C and room temperature (FR-4 is about 1250C). After welding, the welding surface is forced to cool so that both sides of the PCB will be at different temperatures at the same time. As a result, when the soldering surface reaches the glass transition temperature or below, the other side is still above the glass transition temperature, and thus the phenomenon of PCB warpage occurs. When the PCB is warped badly, the above components will be damaged.
3.2 Dissolution of metals
In the assembly of thick and thin film hybrid circuits (including chip capacitors), there is often a phenomenon of gold etching and silver etching. This is because the tin in the solder and the gold or silver in the gold-plated or silver-plated pin form compounds, which leads to a decrease in the reliability of solder joints.
In many cases, when the solder cools from the soldering temperature to the solid state temperature, dissolved metal precipitates and a brittle metal compound forms in the solder base. Copper forms needle-like Cu6Sn5, silver forms flat Ag3Sn, and gold forms AuSn4 cubes. One of the common characteristics of these compounds is that they are very brittle, have extremely low shear strength, and the components can easily fall off. If the content of gold and silver is small, the amount of the produced compound will not be much, and these compounds will not cause too much damage to the mechanical properties of the solder joint. However, when the content is high, the solder may become brittle. Take gold as an example. When the reaction time and temperature are sufficient, all the gold will react with tin. Therefore, the content of gold in solder joints should not exceed 3%-4%.
3.3 Overheating of Substrates and Components
Various materials, such as plastics, are generally unstable at the soldering temperature, and substrate peeling and discoloration often occur. Paper-based phenolic resin sheets often peel off, suitable for infrared reflow soldering, and FR-4 (epoxy glass substrates) often change color in infrared reflow soldering.
The word "popcorn" is specifically for large chip ICs. The IC plastic package is susceptible to moisture absorption. When the heating time is too long, the moisture will be released. During reflow soldering, moisture is vaporized and accumulated into a bubble at the weak interface of the package at the bottom of the chip, and the package is exposed to the pressure of the bubbles and cracking occurs. This phenomenon is related to the size of the chip, the thickness of the plastic under the chip, and the quality of the bonding between the plastic package and the chip, especially in relation to the amount of moisture. In wave soldering, bursting hardly occurs. The current solution is to dry the IC first, then seal it and keep it dry. Or pre-baking above 1000C for several hours before use.
3.4 Damage caused by ultrasonic cleaning
Ultrasonic cleaning is effective for removing residual flux on the PCB. The disadvantages are controlled by the size of the ultrasonic power. If the ultrasonic power is too low, it will not work, and if the ultrasonic power is too high, the PCB and the components above will be destroyed.
Ultrasonic cleaning may cause two kinds of damage consequences:
(1) The impact of small droplets on the surface is like sandblasting, similar to surface weathering.
(2) In the cleaning bath, the ceramic substrate is excited by the ultrasonic load to exhibit a harmonic state. On the substrate, the pins of the surface mount component are periodically applied at the common harmonic frequency, which eventually leads to fatigue fracture at the bend of the pin.