Solder bridging is one of the most common defects in wave soldering, it refers to a short circuit formed between adjacent pads due to the improper connection of molten solder. This defect can lead to electrical misfunctional, failure and even safety hazards. Furthermore, the subsequent repair operation may cause new issues, such as pad lifting or via barrel cracking. Bridging is the result of the combined interaction of multiple factors, including pad spacing, the condition of the pad’s surface, solder purity, flux activity, PCB thermal distribution, solder immersion depth, and the height of component leads.
No. 1 – Spacing Between Adjacent Pads:
Radius of Curvature and Internal Pressure Effect: The spacing between adjacent pads directly influences the radius of curvature. The radius of curvature is a function of the spacing: as spacing increases, the radius of curvature increases and the additional internal pressure decreases, making the possibility of “bridging” less likely. Conversely, when the spacing between adjacent conductors narrows, the radius of curvature decreases. It increases the internal pressure and load of the molten solder to spanning the adjacent pads, thereby increasing the internal pressure. Consequently, the molten solder flows across the solder mask dam, accumulates in the bridging zone, and forms a “bridge.”
Engineering Key Points:
In fine-pitch designs (such as QFP or BGA peripheral leads), pad spacing is the primary concern for preventing bridging. IPC standards specify the requirements of pad spacing for various package types. These should be strictly followed during design, to avoid the risk of short circuits that may caused by excessively tight spacing.
Core Mechanism: Spacing → Radius of Curvature → Additional Internal Pressure → Solder Flow Direction → Bridging/No Bridging
No. 2 – Cleanliness of Pad Surface: The Interplay Between Wetting Force and Bridging
The cleanliness of the pad’s surface reflects the ability of molten solder and wetting the solder paste. This is typically quantified in terms of wetting force (adhesion force). A field generated by atomic attraction which exists at the surface of solder pad. Only when the pad surface is sufficiently clean, so the atoms of the solder and on the pad surface, could approach each other closely enough and achieve molecular bonding under the high-temperature conditions during reflow soldering.
If oxidization or contaminants adhered on the pad surface, that will impede the molten solder atoms from approaching the pad surface, thereby preventing adequate wetting.
The mechanism of wetting force that influences bridging: the wetting force exerted by molten solder — located between adjacent pads — on the pad surface directed towards the pads. The force tends to draw the intermediate solder toward the pads, thereby prevent bridging.
Conversely, the negative pressure generated within the liquid solder, it trends to draw the solder from the pads toward the bridging zone — an effect that directly opposes the wetting force.
If the pad surface is clean, the wetting force is strong and it’s counteracting effect is significant, that will make “bridging” unlikely. If the surface is contaminated, the wetting force is minimal or non-existent, consequently, the counteracting effect is weak or lost entirely, that will lead to a high probability of bridging.
No. 3 – Solder Purity: Impaired Fluidity Due to Increased Viscosity
The surface tension of the liquid solder is related to it’s purity. When impurities are introduced into a pure liquid solder, the viscosity of the molten solder increases significantly, even though there may be a tendency to decrease the surface tension sometimes. The increased viscosity leads to a remarkable deterioration in the fluidity of the molten solder, producing an effect that has a similar impact of increasing surface tension.
During wave soldering process, the solder in the pot is continuously contaminated by impurity metals (such as copper). Once these impurities accumulate to a certain concentration, they alter the physicochemical properties of the alloy and cause a significant increase in solder viscosity, that will cause more bridging.
Recommendations for Process Control:
① Regularly monitor impurity situation of the solder pot, particularly copper content (resulting from the dissolution of copper from PCB pads).
② Consider replacing the solder partially or performing purification, when copper content exceeds 0.3 wt%.
③ Establish a solder purity monitoring log, to record the correlation between test results and bridging defect rates.
④ For wave soldering equipment, it is recommended to conduct regular checks for impurity concentrations every week.
No. 4 – Flux Activity and Preheating Temperature: Oxide Layer Removal and the Activity Window
Pad surfaces are typically covered by a thin oxide layer, therefore, before transport to wave soldering, the active agents in the flux must melt down this layer and expose the clean surface. If the flux activity fails to achieve this goal, it may cause insufficient wetting and leading to bridging.
The effectiveness of flux activity is closely linked to the preheating temperature. If the preheating temperature is too low, the flux’s chemical activity is not fully realized, and the oxide layer on the pad surface hinders solder bonding, increase the possibility of bridging. Conversely, excessively high preheating temperatures also are detrimental to the prevention of bridging.
Failure mechanisms that associate with excessively high preheating temperatures:
① Solvents in the flux evaporate or decompose rapidly, and pad surfaces — having already been activated and cleared oxides — lose their protective coating and going through re-oxidation.
② Due to flux evaporation, by the time the PCB enters the solder wave oven, the flux has lost it’s ability to reduce the surface tension of the liquid solder.
No. 5 – Variations of Thermal Distribution on PCB Surface : Poor Wetting on High Thermal Areas
In actual production process, the following scenario often arises: despite optimal wave soldering parameters, good pad solderability, and appropriate solder/flux composition and properties, “bridging” still occurs in areas which populate high-thermal-capacity solder joints densely. Analysis indicates that this phenomenon is primarily caused by poor assembly design: clustering components with high thermal capacity, in a specific area that creates a zone of high thermal capacity. During wave soldering, under some specific conditions, certain areas may fail to absorb sufficient heat from the wave, then fail to reach the required wetting temperature. It results in lower temperatures on certain areas, where increase solder surface tension and viscosity of impair wetting, leading to “bridging.” For examples, the PCB which assembled with large transformers, IGBT, super capacitors and large dimension of GND copper, while they are designed for heat dissipation, but they also transfer and disipate the heat away quickly, from wave soldering, thereby affect the soldering process negatively.
– Engineering Solutions:
Increasing the wave soldering temperature or reducing the conveyor speed, then it could mitigate “bridging” in these areas. However, components with low thermal mass — such as plastic connectors — may lead to physical damage dure to excessive heat. Theoretically, a variable-speed automatic transporting system with low inertia could resolve this issue, but currently, widely applied wave soldering equipments are lack of such functionality.
– Practical Measures:
1) Optimize PCB layout by deploy components and avoid clustering.
2) Add thermal compensation pads or thermal vias in areas with high thermal mass.
3) Localized preheating: Extend the preheating time for these specific areas prior to wave soldering.
No. 6 – PCB Solder Immersion Depth: Variations in the Solder Wave Profile and Separation Zone
The PCB’s immersion depth into the solder wave, that reflects the magnitude of the fluid pressure and might exert solder on the board. While the upward pressure generated by immersion, as a lifting force during the formation of through-hole solder joints, it’s primary function is to ensure a steady supply of solder and effective heat transfer, rather than driving solder upward to achieve hole fill (solder penetration). The immersion depth must be matched to the PCB’s thickness and type. Excessive depth will increase risk to overflow molten solder onto the top of the PCB, but also lead to the occurrence of “bridging.”
No. 7 – The Protrusion Height of Component Lead: Shadow Effect and Bridging Threshold
The height of component leads which protrude beyond the solder pads, are critical factor that contributing to bridging between adjacent solder joints. This is particularly evident in dense arrays of solder joints (such as multi-pin connectors).
When leads protrude excessively from the board surface, the “shadow effect” caused by the leads elongates the solder detachment. It draws the trailing solder joints and leads into the detachment zone, thereby creating conditions to bridging. Conversely, when leads protrude at the standard height, the shadow effect from the leading leads is minimal, resulting in a narrow detachment zone that cannot span across two pads, thus make bridging less unlikely.
Safe Height of Pin Protrusion
Based on practical experience, the critical threshold for the safe height of multi-pin sockets (the height at which bridging does not occur) is approximately 3mm.
1) Formating process of the “shadow effect”:
Excessive pin protrusion → Leading pins obstruct the solder wave → The solder detachment zone (peel-off zone) elongates as trailing pins exit → Multiple solder joints are captured within the same detachment zone → Solder connects across the zone → Bridging occurs.
2) Core Points of Quality Control :
During component insertion, control the pin protrusion height within the range of 2.8-3.2mm.
② For dense solder joint arrays (e.g., multi-pin sockets, connectors), it is recommended to keep the height below 2.8mm.
③ Establish a sampling mechanism for pin height and include it in the First Article Inspection (FAI) process.
Extended Knowledge: Systematic Strategies to prevent Bridging Defects
Preventing bridging defects requires a coordinated effort across three dimensions: design, materials, and process.
Design Aspect:
① Ensure pad spacing meets IPC standards, prioritize components with fine-pitch leads.
② During PCB layout, distribute high thermal mass components around, to avoid clustering and heat concentration.
③ Design through-hole pads with appropriate ratio of hole-diameter-to-annular-ring, to control solder penetration.
Materials Aspect:
① Applying high-purity solder, monitor impurity levels regularly and control.
② Applying flux with moderate activity that matches the preheating temperature window.
③ Ensure the cleanliness of PCB pad surfaces and appropriate storage conditions.
Process Aspect:
① Set the preheating temperature within the flux’s active range (typically 90–130°C).
② Control pin protrusion height to ≤3.2mm (≤2.8mm for dense joint arrays).
③ Adjust solder penetration depth to 1/2–2/3 of the PCB thickness.
④ Optimize conveyor speed and wave height, to minimize the width of the solder detachment zone.
Conclusion:
A mindset of systematical engineering, the key approaches to preventing and resolving bridging: The root cause of bridging defects is the uncontrolled flow of liquid solder, when it detaches from the wave. A complex interplay of seven factors that combined together to determine will bridging occur or not: geometric constraints of pad spacing, the competition between wetting forces and PCB pad surface cleanliness, the viscosity links to solder purity, the chemical activity window of the flux, temperature gradients resulting from heat capacity distribution, fluid dynamic variations in solder wicking depth, and the shadow effect of lead height.
In practical engineering, resolving bridging issues that typically requires the simultaneous optimize the multiple factors and carefully balancing them, not only focuses on and adjust any solo parameter. Suggest the process engineers to establish a Design of Experiments (DOE) matrix for bridging defects, to evaluate factor interactions and identify the optimal process window systematically. When bridging issues happen, using professional analysis techniques to identify and pinpoint the root cause, then resolve the issues and prevent in future.
