With the increasing prevalence of lead-free PCB electronic products in the market, reliability issues have captured the attention of many. Unlike other lead-free-related concerns (such as alloy selection and process window), opinions on reliability vary significantly. Initially, numerous “experts” claimed that lead-free options were more reliable than tin-lead. Just when we began to accept this, another “expert” asserted that tin-lead is actually more reliable than lead-free. Which perspective should we trust? It ultimately depends on the specific circumstances.
The reliability of PCB lead-free soldering interconnections is a complex matter influenced by various factors. We outline the following seven considerations:
1) It depends on the solder alloy. In reflow soldering, the predominant lead-free PCB solder alloy is Sn-Ag-Cu (SAC), while wave soldering may utilize either SAC or Sn-Cu. These alloys exhibit different reliability characteristics.
2) It depends on the process conditions. For large and complex circuit boards, the soldering temperature typically reaches 260°C, which can negatively affect the reliability of PCB components. However, smaller circuit boards are less impacted, as their maximum reflow soldering temperature may be relatively lower.
3) The performance of a PCB is influenced by its laminate material. Certain PCBs, particularly large and complex thick circuit boards, may experience issues like delamination, laminate cracks, copper cracks, and CAF (conductive anodic filament) due to the elevated lead-free soldering temperatures associated with the laminate’s properties. Consequently, the failure rate has increased. Additionally, the PCB’s surface coating plays a significant role; for instance, observations indicate that the interface between the solder and the nickel layer (from ENIG coating) is more prone to fractures compared to the solder and copper connections (like OSP and immersion silver), especially under mechanical stress such as drop tests. Furthermore, lead-free soldering can lead to increased cracking in PCBs during these drop tests.
4) The impact on reliability also varies by component type. Certain components, including plastic-packaged devices and electrolytic capacitors, are more sensitive to increased soldering temperatures than others. Additionally, the reliability of fine-pitch components in high-end products with long lifespans is particularly affected by tin wire. The high modulus of SAC alloys can exert greater pressure on components with low dielectric constants, making them more susceptible to failure.
5) Mechanical load conditions are also a determining factor. The sensitivity of SAC alloys to high stress rates necessitates careful consideration of the reliability of the PCB’s lead-free soldering interface under mechanical impacts, such as drops or bending. Excessive stress under high strain rates can lead to solder interconnection or PCB fractures.
6) Thermomechanical load conditions are crucial as well. Thermal cycling can induce creep and fatigue interactions that cause solder joint failures due to damage accumulation, including structural coarsening and crack propagation. The creep stress rate, which varies with the thermo-mechanical load on solder joints, affects the performance of SAC versus Sn-Pb solder joints. Under “relatively mild” conditions, SAC joints can endure more thermal cycles than Sn-Pb, but under “more severe” conditions, they may perform worse. For example, reports indicate that on the same circuit board passing thermal cycle tests, components with copper lead frames in SAC solder joints experience more cycles than those with Sn-Pb solder joints, while components using 42 alloy leads tend to fail sooner in SAC joints due to higher CTE mismatches. Interestingly, 0402 ceramic chip devices show a greater thermal cycle endurance in SAC than Sn-Pb, whereas 2512 components exhibit the opposite trend. Additionally, numerous reports suggest that 1206 ceramic resistors on FR4 demonstrate better thermal cycling performance in lead-free soldering compared to Sn-Pb between 0°C and 100°C, with an opposite trend at extremes of -40°C and 150°C.
7) The “acceleration factor” is another intriguing and closely related aspect that complicates the discussion, as different alloys, such as SAC and Sn-Pb, possess distinct acceleration coefficients. Thus, the reliability of PCB lead-free solder interconnections is influenced by a multitude of factors.
The reliability of PCB lead-free soldering interconnections is a complex matter influenced by various factors. We outline the following seven considerations:
1) It depends on the solder alloy. In reflow soldering, the predominant lead-free PCB solder alloy is Sn-Ag-Cu (SAC), while wave soldering may utilize either SAC or Sn-Cu. These alloys exhibit different reliability characteristics.
2) It depends on the process conditions. For large and complex circuit boards, the soldering temperature typically reaches 260°C, which can negatively affect the reliability of PCB components. However, smaller circuit boards are less impacted, as their maximum reflow soldering temperature may be relatively lower.
3) The performance of a PCB is influenced by its laminate material. Certain PCBs, particularly large and complex thick circuit boards, may experience issues like delamination, laminate cracks, copper cracks, and CAF (conductive anodic filament) due to the elevated lead-free soldering temperatures associated with the laminate’s properties. Consequently, the failure rate has increased. Additionally, the PCB’s surface coating plays a significant role; for instance, observations indicate that the interface between the solder and the nickel layer (from ENIG coating) is more prone to fractures compared to the solder and copper connections (like OSP and immersion silver), especially under mechanical stress such as drop tests. Furthermore, lead-free soldering can lead to increased cracking in PCBs during these drop tests.
4) The impact on reliability also varies by component type. Certain components, including plastic-packaged devices and electrolytic capacitors, are more sensitive to increased soldering temperatures than others. Additionally, the reliability of fine-pitch components in high-end products with long lifespans is particularly affected by tin wire. The high modulus of SAC alloys can exert greater pressure on components with low dielectric constants, making them more susceptible to failure.
5) Mechanical load conditions are also a determining factor. The sensitivity of SAC alloys to high stress rates necessitates careful consideration of the reliability of the PCB’s lead-free soldering interface under mechanical impacts, such as drops or bending. Excessive stress under high strain rates can lead to solder interconnection or PCB fractures.
6) Thermomechanical load conditions are crucial as well. Thermal cycling can induce creep and fatigue interactions that cause solder joint failures due to damage accumulation, including structural coarsening and crack propagation. The creep stress rate, which varies with the thermo-mechanical load on solder joints, affects the performance of SAC versus Sn-Pb solder joints. Under “relatively mild” conditions, SAC joints can endure more thermal cycles than Sn-Pb, but under “more severe” conditions, they may perform worse. For example, reports indicate that on the same circuit board passing thermal cycle tests, components with copper lead frames in SAC solder joints experience more cycles than those with Sn-Pb solder joints, while components using 42 alloy leads tend to fail sooner in SAC joints due to higher CTE mismatches. Interestingly, 0402 ceramic chip devices show a greater thermal cycle endurance in SAC than Sn-Pb, whereas 2512 components exhibit the opposite trend. Additionally, numerous reports suggest that 1206 ceramic resistors on FR4 demonstrate better thermal cycling performance in lead-free soldering compared to Sn-Pb between 0°C and 100°C, with an opposite trend at extremes of -40°C and 150°C.
7) The “acceleration factor” is another intriguing and closely related aspect that complicates the discussion, as different alloys, such as SAC and Sn-Pb, possess distinct acceleration coefficients. Thus, the reliability of PCB lead-free solder interconnections is influenced by a multitude of factors.
