Introduction
The wave soldering machine process is the primary process responsible for defects in PCBA components, accounting for up to 50% of defects throughout the entire PCB assembly process. Defects that occur during wave soldering are a concentrated manifestation of issues present in previous manufacturing processes. This article will focus on defect phenomena common to both leaded and lead-free wave soldering.
I. Cold Solder Joints
1. Definition
A rough, granular surface, poor luster, and poor flow are the visual characteristics of cold solder joints. Essentially, any solder joint where an appropriate thickness of intermetallic compound (IMC) layer has not formed at the interface of the connection can be classified as a cold solder joint. When a normal solder joint is torn apart, the solder and base metal exhibit interlocking cracks, meaning that solder residues remain on the base metal and traces of the base metal are present on the solder.
2. Phenomenon
The interface of a cold solder joint neither undergoes wetting nor diffusion, resembling something glued with paste. The solder joint surface appears rough, with poor luster and a contact angle θ > 90°. At this point, an impermeable film obstructs the interface between the solder and the base metal, preventing the desired metallurgical reaction from occurring at the interface layer. This constitutes a visible cold solder joint phenomenon.
3. Formation Principle
3.1 Physical Phenomena Occurring During Wave Soldering Bonding Process
Taking the commonly used Sn-37Pb alloy solder as an example, under the influence of soldering temperature, it achieves bonding by first wetting the metal surface with molten solder, then utilizing diffusion to form an intermetallic compound (IMC) layer at the joint interface, thereby forming a unified structure.
During the soldering process, the wetting and cohesive force of the molten solder are related to the adhesion of the base metal. The weaker the cohesive force-i.e., when the adhesion between solid-phase atoms and liquid-phase atoms is greater than the cohesive force of the liquid-phase atoms-the more likely capillary phenomena will occur.
Therefore, to achieve the welding bonding process, wetting must first occur. Due to wetting, when the distance between the atoms of the liquid filler metal and the base metal is very close, the cohesive force of the atoms comes into play, causing the liquid filler metal and the base metal to merge into a single entity, thereby completing the bonding.
3.2 Intermetallic Compounds
Welding relies on the formation of an alloy layer at the joint interface to achieve bonding strength. This alloy layer is typically an intermetallic compound. Such compounds, where the metallic components of the alloy are combined in proportions based on atomic weight, are referred to as intermetallic compounds.
3.3 Influencing Factors
- Impurities on the surface of the base metal, such as oxidation or contamination by dirt, grease, or sweat stains, can result in poor weldability or even render the surface unweldable.
- Purchased PCBs, components, etc., with inadequate weldability that have not undergone strict incoming inspection tests before entering the user's warehouse.
- Poor storage environment and excessive storage duration.
- Excessively high solder pot temperature accelerates oxidation of the solder and substrate surfaces, reducing surface adhesion to liquid solder. Additionally, high temperatures erode the rough substrate surface, decreasing capillary action and impairing flowability.
4. Prevention of Cold Soldering
- Strictly control incoming inspection of outsourced and purchased components
- Optimize inventory period management
- Strengthen hygiene management during process handover
Staff should wear anti-static clothing, shoes, and gloves, and keep them clean at all times.
Most flux agents can only remove rust and oxide films, but cannot remove organic films such as grease. If components and PCBs become contaminated with grease or other pollutants during storage or production handover, it can cause tin and lead segregation and pinholes, reducing weld strength. Cracks may also form at lead segregation sites and solder joint interfaces, appearing normal externally but harboring factors that compromise reliability.
- Selecting the correct process specifications
- Clean all surfaces to be soldered prior to soldering to ensure solderability.
- Under considerations of both solder joint safety and reliability, stronger-active flux may be selected as appropriate.
II. Cold Welding
1. Definition
In wave soldering machine, if wetting occurs at the joint interface but the required diffusion process does not take place, and the intermetallic compound (IMC) layer at the joint interface is not clearly formed, it is defined as cold welding.
2. Phenomenon
The surface of a cold welded joint appears wetted, but no metallurgical reaction occurs at the interface between the solder and the base metal, and an appropriate thickness of the intermetallic compound (IMC) layer is not formed. This indicates that there are no issues with the solderability of the PCB and components. The fundamental cause of this phenomenon is the selection of inappropriate welding process conditions. It is an invisible defect phenomenon that is difficult to judge by appearance, thus posing significant risks.
3. Formation Principle
Cold welding primarily occurs when there is no significant atomic diffusion process at the interface between the liquid solder and the base metal. The fundamental cause of this phenomenon is insufficient heat supply during the welding process or insufficient contact time with the wave.
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