Analysis of Causes and Solutions for Solder Bridging Defects in Wave Soldering Process

Abstract

As the core soldering process for batch assembly of through-hole printed circuit boards, wave soldering is widely used in the electronics manufacturing production process due to its high production efficiency and suitability for mass manufacturing. Solder bonding (bridging) is a soldering defect that occurs frequently during wave soldering production. This defect occurs when two or more electrically isolated solder joints are connected by solder, directly causing electrical short-circuit faults on circuit boards and seriously affecting the yield and reliability of electronic products. This paper analyzes the mechanism of solder bonding defects from multiple dimensions including soldering temperature, preheating process, flux, equipment parameters, PCB design, component assembly, and solder quality, and proposes corresponding process optimization and improvement plans, providing reference ideas for solving the problem of poor solder soldering in the electronics manufacturing industry.

Keywords: wave soldering; soldering; bridging; printed circuit board; soldering defects; process optimization

1. Introduction

Modern electronic equipment is continuously developing towards miniaturization and integration, with increasing component layout density on printed circuit boards (PCBs), which places higher demands on the stability of wave soldering processes. Wave soldering relies on molten solder to form flowing peaks that contact the bottom solder joints of the PCB to complete the soldering operation. In mass continuous production mode, due to the coupling of multiple factors such as equipment status, process parameters, material characteristics, and PCB design, various soldering defects such as solder joints, cold solder joints, missed soldering, and solder balls are highly likely to occur. Among these, solder defects directly cause circuit short circuits. If they flow into downstream finished product processes, they not only cause product failure but also increase repair costs and restrict production efficiency. Therefore, systematically identifying the triggering factors for wave soldering defects and implementing targeted improvement measures is of practical engineering significance for enhancing PCB assembly soldering quality.

2. Analysis of the main causes of wave soldering defects

The essence of tin is the phenomenon where molten solder unexpectedly spreads and migrates between solder joints, forming conductive bridging after cooling. The defects are often not caused by a single factor but are the result of the combined effects of process, material, and structural design.

2.1 Improper control of solder melting temperature

Solder temperature is a key process parameter affecting the surface tension and flowability of solder. When the solder bath temperature is set too high, the surface tension of the molten solder decreases, significantly enhancing flowability. At the moment the wave peaks contact the PCB, low-tension liquid solder is more likely to spread between adjacent pads and component pins, preventing timely shrinkage separation before cooling, and after cooling and curing, solder bridging forms between solder joints. However, solder temperature cannot be simply lowered; too low temperatures can lead to cold solder joints and other soldering defects, and temperature range imbalance is a common adverse factor on site.

2.2 Insufficient PCB preheating process

The preheating process in wave soldering activates the flux and reduces the temperature difference under PCB heating. If the preheating temperature is too low or the preheating time is insufficient, the body temperature of the PCB substrate, pads, and component pins is much lower than the molten solder temperature. When the PCB contacts wave solder, the temperature difference between hot and cold is too large, causing abnormal fluidity changes after the solder contacts the low-temperature workpiece; At the same time, the flux cannot be fully activated, causing uncontrolled solder wetting, with liquid solder easily overflowing to surrounding areas, increasing the probability of solder bonding between adjacent pins and pads. Conversely, excessive preheating can also cause flux to evaporate and fail prematurely, also triggering poor soldering, so the preheating window must be precisely controlled.

2.3 Flux quality and spraying amount abnormalities

The main function of flux is to remove the oxide layer on the surfaces of pads and pins, improving solder wetting performance. On one hand, excessive flux spraying causes excess liquid flux to accumulate in the gaps between pads at the bottom of the PCB, causing molten solder to diffuse along the flux spread path to adjacent solder joints; On the other hand, if the flux itself does not meet physical and chemical quality standards and the active component ratio is unbalanced, it excessively increases solder wetting fluidity and promotes solder spread across solder joints. Additionally, uneven flux spraying and localized accumulation can also cause concentrated solder interconnection issues in certain areas.

2.4 Unreasonable process parameters for peak height

Wave height determines the contact depth and contact area between the PCB and molten solder. If the peak height is set too high, the PCB immersed too deeply in the solder peaks increases the contact range between the solder and the PCB bottom solder joints and pins, making it easier for liquid solder to fill the gaps between the pins; At the moment the PCB peels off the wave, the solder cannot rely on its own surface tension to complete separation and breaking, leaving residual solder to connect adjacent solder joints, resulting in batch solder connection defects. Too low a wave height can cause missed solder joints and poor pad wetting, requiring parameter tuning to match the board thickness and component type.

2.5 PCB pad structural design defects

PCB hardware design is the fundamental influencing factor. When the spacing between pin pads is too small and the physical gap between solder joints is insufficient, leaving limited space for solder retraction and separation, bridging is very likely to occur under conditions with high solder flow; Unreasonable pad size design, such as pads being too large, can also expand the solder spread range. If high-density through-hole boards are not adequately designed during the initial design phase and do not fully consider wave soldering process compatibility, it is difficult to completely eliminate the risk of solder bonding by simply adjusting equipment parameters later.

2.6 Component Assembly Positioning Deviation

The assembly accuracy of component inserts directly affects the position of the pins relative to the pads. During insertion operations, if the component pins shift toward the center of the pad, the pins are skewed or offset on one side, and the pins encroach on the gap space between adjacent pads. During soldering, molten solder spreads outward along the offset pins, crossing to adjacent solder joints, and after cooling and curing, forms a continuous solder connection. This issue occurs frequently in production scenarios where the precision of the insertion equipment is insufficient and manual insertion operations are used.

2.7 Accumulation of impurities inside the solder tank

After long-term continuous production and use, various impurities such as copper and metal oxides may mix into the solder tank. These impurities alter the physical properties of the molten solder, destroying its original surface tension and wettability, and causing deterioration of solder fluidity and cohesion. At this time, the fracture effect of solder decouples from the waves, making it easy to cause wire drawing and solder drag, which can lead to solder bonding between adjacent solder joints—a hidden factor that is often overlooked in long-term production.

3. Countermeasures for Improving Wave Soldering Defects

For the above various triggering factors, the approach of "prioritizing source design, assisted by process parameter optimization, and ensuring regular equipment maintenance" should be followed, and improvement plans should be implemented in combination with actual on-site production conditions.

3.1 Reasonably control the working temperature range of the solder

Strictly set the solder bath temperature according to solder alloy specifications, combined with PCB thickness and component heat resistance ratings, to keep the solder temperature within the specified process range and avoid ultra-high temperature operation. During production, regularly monitor the actual temperature of the solder bath to avoid deviations between the displayed temperature and the actual molten solder temperature; Temperature adjustment uses a small, stepwise adjustment method, synchronizing soldering status to ensure wetting effects and prevent secondary defects such as cold solder joints and cold solder joints introduced to solve solder bonding.

3.2 Optimize the process conditions of the preheating system

Adjust the temperature and conveying speed of the wave soldering preheating section to ensure that before the PCB enters the solder peak, the board surface, pads, and pins can be evenly heated, ensuring flux is fully activated and minimizing the temperature difference between the workpiece and molten solder. For circuit boards with large heat retention such as thick and multilayer boards, appropriately match preheating power to reduce temperature differences across different areas of the board; At the same time, avoid excessive preheating to prevent flux from evaporating prematurely and losing soldering function. A temperature curve tester can be used to collect the actual PCB heating curve and quantify and control the preheating process.

3.3 Control flux quality and spray amount

Select qualified flux products that meet process standards and have stable performance, and prioritize matching models specifically for wave soldering processes. Conduct regular inspections of the flux spraying system, adjust spraying pressure and nozzle position to ensure uniform flux spraying, strictly control spray thickness, and eliminate local flux buildup; Clean the printheads regularly to prevent blockages and uneven spraying. If solder is concentrated and outbreaks, try slightly reducing flux supply and compare and observe changes in soldering quality.

3.4 Scientifically adjust the peak working height

Adjust the wave peak height according to PCB thickness and component lead length, controlling the reasonable depth of PCB immersion in the wave peak, and reducing unnecessary solder contact area. At the same time, pay attention to the operating status of the wave motor to ensure the wave liquid level is stable and free from severe turbulence; excessive turbulence can also worsen solder spread. Considering soldering penetration, while ensuring the solder joints are fully wet, try to minimize the risk of solder connection caused by excessive wave peaks.

3.5 Optimize PCB pad process design

Conduct Manufacturability Analysis (DFM) during the PCB design phase to fully adapt to wave soldering requirements. Set reasonable safety spacing between through-hole pads to avoid spacing too small; Optimize pad hole diameter and dimensions, and, if necessary, use pad openings designed to prevent solder connection to reduce the probability of bridge defects at the hardware source. For high-density circuit boards, if conditions permit, local use of masking tooling and other auxiliary process methods can also be considered.

3.6 Improving Component Insert Positioning Accuracy

Automated insertion equipment is regularly calibrated to maintain the accuracy of movement mechanisms, reducing issues of component pin misalignment and skewing; Manual insertion stations should improve operational standards, strengthen employee training, and ensure component pins are centered and placed in the pad holes. During production, inspection frequency has been increased to promptly intercept semi-finished products with severe pin misalignment from entering the soldering process, reducing poor solder bonding caused by assembly issues.

3.7 Implement daily maintenance of solder tanks and solder quality control

Establish a solder tank maintenance system, regularly remove oxidized slag from the surface of the solder tank, and conduct sampling and testing of the solder composition according to cycles. When impurity content inside the solder exceeds the standard, promptly replace the solder or perform impurity removal to ensure stable melt soldering performance. This reduces damage to the surface tension of the solder by impurities, improves the fracture characteristics of the solder when it leaves the peak, and decreases solder defects caused by solder drag and wire drawing.

4. Conclusion

Wave soldering defects are process issues caused by multi-factor coupling, and defects in on-site production are rarely caused by a single cause. When batch solder defects occur, it is not enough to adjust just one equipment parameter; temperature curve records, PCB design documents, material inspection results, and equipment operation records should be comprehensively investigated. Priority should be given to easily adjustable items such as equipment process parameters and material quality. For repeatedly unresolved defects, trace back PCB design and upstream insertion processes. Through multidimensional coordinated control of design, process, and equipment maintenance, the occurrence of tinning defects can be effectively suppressed, improving wave soldering yield, reducing the cost of electronic product repairs, and ensuring the quality of circuit board assembly.