For any successful printed circuit board (PCB) assembly, it is imperative to have quality solder joints. PCB Trace Technologies Inc recommends understanding the factors that affect solder joint quality, as this allows proper troubleshooting of problems and producing reliable electronics. The proper wetting and spreading of molten solder on copper pads and component leads determines the creation of solder joints.
The process of wetting forms a metallurgical link between the solder and the copper pad. Molten solder spreads by a process of surface tension over the surface of copper, and the presence of flux helps this action. The process of wetting and spreading together define the solder-copper-flux combination system.
In any electronic system, the reliability of solder joints often forms a source of contention. This is due to a number of variables affecting the stability of solder joints, any one of them capable of significantly lowering the lifespan of the joint. Therefore, identifying and minimizing probable origins of solder joint failure all through the assembly process could help avoiding the expense involved within the lifespan of the product.
Factors Affecting Quality of Solder Joints
Temperature
Solder is an alloy of many metals. When heated, the constituent metals combine and generate a hot eutectic mixture that wets the surface of the copper pads for SMT components. As with any liquid, the surface of molten solder is under surface tension, causing it to form a convex fillet as it wets the copper pad. On encountering the edge of the component, surface tension draws the solder upwards, changing the shape of the fillet to a concave shape. On withdrawal of the heat source, the solder cools. However, if the temperature of the heating source is too low, the solder will not melt properly. This will not allow the eutectic mixture to blend properly and prevent the solder from effectively wetting and forming a concave fillet. This will result in the solder hardening as a cold junction, and the joint will not have any strength.
It is possible to identify a cold joint from an inadequate eutectic mixing. Usually, the surface of the solder is dull, with pockmarked appearance, and poor mechanical strength. Two factors can lead to inadequate heat on the junction during soldering:
- Inadequate source temperature setting for the type of solder in the machine.
- Excess heat evacuated from the solder joint, leading to a lowering of the eutectic temperature while soldering.
The second factor could be the result of a large ground plane linking the copper pads on the PCB, leading to a cold joint. If the designer has not provided a thermal relief, the via attaching the pad to the plane will conduct heat away from the pad to the plane. Unless the design process has provided an effective thermal relief, the joint may develop fractures later, resulting in failure.
Solder Type
Solder alloys come in various combinations. Earlier, tin-lead solder combinations were popular, and their melting point was 180 – 190 °C, close to the glass transition temperature of many PCB laminates. However, because of the negative effects of lead on health, the industry has transitioned towards lead-free solder to comply with RoHS directives. Two of the most common lead-free solders melt at higher temperatures, between 220 and 450 °C, necessitating a shift towards PCB substrates with higher glass transition temperatures.
Irrespective of the solder type, the temperature of the solder and its wetting ability will affect the strength of the joint. Moreover, depending on the solder type, an improper flux type may cause the generation of oxides, resulting in a reduction of the strength of the solder joint.
Stress-Related Events
The solder joint may undergo an excessing loading from a mechanical event like a drop or shock, such as that during board depanelization, in-circuit testing, connector insertion, or PCBA insertion. Such events could lead to failure of the joint.
It is difficult to avoid mechanical overstress as these events are mostly unpredictable. Most shock testing studies have demonstrated that such failures exhibit a random distribution.
Failure due to overstress manifests itself often as a crater in the pad, or as a joint fracture in the intermetallic connection. A crater in the pad forms as a result of a breakage in the laminate just below the copper pad forming the joint. A fracture in the intermetallic connection is where the copper pad and solder meet. The intermetallic connection being the most fragile section of the joint, is particularly prone to overstress.
Finer pitch components exhibit more of this sort of overstress failures, with BGAs being the most notorious. Pad cratering is often involved with trace fracture, causing a major problem. Fatigue cracks may also cause breaks in solder joints, but these are often along the length of the joint. On the other hand, overstress failures are mostly along the intermetallic connection.
Reflow Profile
Setting a proper reflow profile for a particular PCBA is of paramount importance for obtaining quality solder joints. This is especially true for the preheat zones, leading to the actual soldering temperature. A gradual increase in the preheating temperature settings is important, as it prevents a thermal shock due to a sudden temperature jump. The length of time the board assembly remains in the soldering zone is also important, as this determines the soldering temperature that actually melts the solder.
Oxidation and Flux
The entire process of soldering has a basis in chemistry, and grasping some fundamentals helps in understanding the role of flux in the process of soldering. During the soldering process, the hot eutectic mixture has a tendency to react with oxygen to form oxides. Most metal oxides exhibit a lower mechanical strength compared to that of pure metals.
The presence of flux prevents oxidation and attracts the molten solder to the target location. Flux also helps to remove the presence of grease and oils from the region. Most solder manufacturers specify the type of flux that matches the different types of solder they offer.
Occasionally, the joint site may have contaminants like dirt, grease, or oxides. Flux helps to clean the metal surfaces chemically, while also preventing oxidation. Rosin flux is commonly in use, as it has the properties to improve the mechanical strength and the electrical conductivity of the solder joint. Often, assemblers also use a wetting agent to lower the surface tension and improve the wetting ability.
Conclusion
PCB Trace Technologies Inc has an expert team of technicians and equipment operators to achieve a high quality of solder joints. We guarantee all our products to international IPC-610 standards. If you have an important upcoming project requiring PCB assembly, please reach out to us. We take great pride in our work.
