Why they occur, and what to do about them.

Soldering, the process of joining two or more metals through the application of heat, has been around for millennia, and is the primary means of making physical and electrical connections between the leads of electronic components and the metal pads on a printed circuit board. To make the connection, the solder must be molten so that it can wet the metal surfaces that need to be connected. When solder cools enough to solidify, it forms a joint, making the connection. Reaching a specific temperature (the precise temperature varies depending on the alloy used) is essential, however, because unless the solder melts to its liquidous form, it cannot wet to the mating surfaces. Solder that doesn’t melt, even if present, is referred to as “cold.” Even if the solder joint appears fine visually, it lacks the strength and integrity of a proper joint and could fail.

Here, we focus on understanding cold solder joints, what causes them, and design and manufacturing practices to prevent them.

Solder is simply an alloy, composed usually of tin with other metals, depending on the desired properties, melting point or other characteristics. It can contain copper, silver, lead, antimony, indium, bismuth or other metals. Regardless of the attachment process used – surface mount technology (SMT) or through-hole technology (THT) – soldering is integral to the assembly process.

PCB assemblers use various methods to apply solder to the board, including wave soldering, reflow soldering, selective soldering and hand soldering. Despite the utilized method, the formation of cold solder joints is a common issue and can significantly impact the performance and integrity of the PCB and increase assembly costs, in part through the need for rework.

A cold solder joint occurs when the solder fails to melt properly and bond with the components as intended. This can result in weak or unreliable connections that may break or cause malfunctions in the electronic device. Cold solder joints typically have a dull, grainy appearance instead of the shiny, smooth finish of a well-formed solder joint (Figure 1).

Figure 1. Cold solder joints tend to look dull and grainy instead of shiny and smooth.

Many perfectly good solder joints are not necessarily shiny, depending on the solder alloy used. With a cold solder joint, the solder does not completely melt, or it does not flow sufficiently to cover the component lead and PCB pad. The connection will not be as strong and conductive as necessary for reliable circuit operation.

Causes of cold solder joints. Cold solder joints form for a number of reasons during the soldering process:

• Insufficient heat. If the PCB is not preheated sufficiently, the components and PCB pads may be unable to reach an adequate temperature for soldering. This can hinder solder from flowing correctly, leading to cold solder joints. The temperature in the preheating zone must be precisely regulated.
• Inconsistent solder wave height. The flow and height of the solder wave in a wave soldering machine should be uniform for effective soldering. A low wave height can prevent solder from making good contact with all component leads, resulting in cold joints or insufficiencies or “starved” joints. A stable wave height ensures that all leads are equally coated with molten solder.
• Excessive soldering speed. If the PCB travels through the solder wave too rapidly, the solder will not have sufficient time to flow and adhere to the component leads. Excessive soldering speeds can cause insufficient solder joints.
• Oxidized or dirty pads/leads. Dust, oil or oxidation on the component leads or PCB pads can hinder solder from sticking or wetting. Cleaning the pads and leads thoroughly before soldering is necessary to form strong joints.
• PCB design error. Without anticipating the impact, PCB designers often use a direct connection when electrically connecting the PTH component holes with large copper shapes on inner layers. During wave soldering, molten solder rises in hole barrels, during which time the heat of the molten solder repeatedly dissipates into the inner layers. A direct connection (Figure 2) increases the rate of heat dissipation that limits the rise of molten solder paste in the PTH barrel and ultimately results in a cold and weak solder joint.

Figure 2. A direct connection from the PTH component holes on innerlayers increases the heat dissipation rate and causes cold joints.

Preventive Measures

Cold solder joint prevention during wave soldering involves paying close attention to detail throughout the process, from designing the PCB through post-soldering inspection. To mitigate these issues, several preventive measures can be implemented to ensure optimal soldering results:

• Optimize preheating. Ensure the preheating area is adjusted to the right temperature and there’s time to enable the PCB for heating to the ideal soldering temperature. This reduces thermal shock and enables the solder to flow with ease.
• Ensure consistent solder wave height. Check the solder’s wave height regularly and maintain it appropriately. The wave height should be calibrated so that component leads are fully submerged in the molten solder and do not result in splashing or bridging.
• Control soldering speed. Adjust the conveyor speed as the PCB travels through the solder wave to ensure components have sufficient time to develop good contact with the solder. Excessive speed may cause an inadequate solder connection, while a pace too slow can result in too much solder deposition and problems with component placement.
• Use high-quality solder and flux. Use the highest-quality solder with a suitable flux so that the solder melts evenly and forms reliable bonds. Poor-quality solder or dirty flux can prevent good soldering and result in poor connections.
• Best design practices. During PCB design, when connecting PTH component holes with copper shapes of GND or power layers, always use a thermal connection (Figure 3). A thermal connection reduces the rate of heat dissipation in plane layers during wave soldering, enabling the solder to remain molten for a longer time while soldering. This permits molten solder sufficient time to flow throughout the hole barrel, from the top of the hole to the bottom, creating a strong and solid solder joint.

Figure 3. A thermal connection reduces the heat dissipation rate in plane layers during wave soldering, permitting solder to flow completely through the barrel.

Akber Roy is chief executive of Rush PCB, a printed circuit design, fabrication and assembly company; roy@rushpcb.com / https://rushpcb.com