In copper joint soldering, poor contact caused by cold solder joints directly affects the reliability and safety of the circuit. The root causes are often related to surface preparation, temperature control, solder selection, and operating techniques. Solving this problem requires collaborative optimization across multiple stages to build a complete soldering quality control system.
Clean soldering surfaces are fundamental to avoiding cold solder joints. Copper joint surfaces are prone to oxide layers, oil, or impurities. These substances hinder solder wetting of the substrate, resulting in physical adhesion rather than metallic bonding at the solder joint. Before soldering, surface contaminants must be thoroughly removed with alcohol, acetone, or a specialized cleaning agent. For stubborn oxide layers, mechanical polishing or chemical etching methods can be used. For example, use fine sandpaper to polish the copper joint surface in one direction until a metallic luster is revealed, while avoiding over-polishing that would exceed surface roughness limits. Furthermore, maintaining a clean environment during soldering is crucial to prevent secondary contamination from dust, fingerprints, etc.
Precise control of soldering temperature and time is key to avoiding cold solder joints. When the temperature is too low, the solder cannot melt sufficiently, making it difficult to form a reliable intermetallic compound (IMC); when the temperature is too high, it can lead to copper joint oxidation, solder volatilization, or component damage. Typically, the soldering temperature for copper joints needs to be adjusted according to the type of solder (e.g., tin-lead solder has a melting point of approximately 183°C) to ensure that the solder spreads evenly after melting. Soldering time is equally important; too short a time will result in incomplete IMC layer formation, while too long a time may cause thermal stress or component damage. In practice, the suitability of temperature and time can be judged by observing the solder flow pattern; for example, the solder should present a smooth arc rather than a granular shape.
The compatibility between solder and flux directly affects the soldering quality. High-quality solder with adequate tin content and low impurity content should be selected, such as tin-lead alloy solder with a tin content of over 60%, which has better fluidity and wettability. The role of flux is to remove oxide films and enhance wettability; acidic or neutral flux should be selected according to the material of the copper joint. Acidic fluxes are more active but may leave corrosive residues; neutral fluxes are less active but leave less residue, making them more suitable for applications with high reliability requirements. Before soldering, check the flux's expiration date and active ingredient content to ensure it effectively cleans and fluxes.
Proper soldering technique is crucial to avoiding cold solder joints. Keep the soldering iron tip clean; before starting work each day, use a damp sponge to remove oxide layers and residual solder slag to prevent heat transfer obstruction. During soldering, the soldering iron tip should simultaneously contact both the copper joint and the leads to create a heat conduction path, avoiding uneven heating on one side. Add solder from the opposite side of the soldering iron tip, using surface tension to evenly cover the pads. Furthermore, after soldering, keep the copper joint stationary until the solder has completely solidified before moving it to prevent vibration from causing cracks or cold solder joints.
Post-soldering quality inspection is the last line of defense for ensuring reliability. Visual inspection reveals the solder joint's gloss, shape, and wetting angle. A normal solder joint should have a smooth, curved shape, free from cracks, holes, or buildup. Electrical performance testing involves measuring the resistance of the circuit with a multimeter; the resistance of a cold solder joint is typically 10-100 times higher than that of a normal solder joint. For hidden solder joints, X-ray inspection can be used for layer-by-layer testing to ensure there are no internal pores or incomplete fusion defects. Furthermore, environmental testing (such as temperature cycling and vibration testing) verifies the reliability of the solder joint under actual operating conditions.
Avoiding cold solder joints in copper joint welding requires coordinated optimization across multiple aspects, including surface cleaning, temperature control, solder matching, operational procedures, and quality inspection. Strict process control and quality verification can significantly reduce the risk of cold solder joints and improve the reliability and stability of copper joint welding.
