Seven Factors Determining Reliability of Lead-Free Soldered Interconnects

2018-12-25 16:12

 With more and more lead-free electronic products coming to the market, reliability issues have become the focus of many people's attention. Unlike other lead-free related issues (such as alloy selection, process window, etc.), we often hear widely divided opinions when it comes to reliability. In the beginning, we heard many "experts" say that lead-free is more reliable than tin-lead. Just when we believed it was true, another "expert" said that tin-lead is more reliable than lead-free. Which one should we believe? It depends on the specific situation.
  Lead-free solder interconnection reliability is a very complex issue, it depends on many factors, we briefly list the following seven factors:
  1) Depends on the weld alloy. For reflow soldering, the "mainstream" lead-free solder alloy is Sn-Ag-Cu (SAC), while for wave soldering it may be SAC or Sn-Cu. SAC alloys and Sn-Cu alloys have different reliability properties.
  2) Depends on process conditions. For large complex boards, the soldering temperature is usually 260(C, which may have a negative impact on the reliability of the PCB and components, but it has less impact on small boards, because the maximum reflow temperature may be lower
  3) Depends on PCB laminate material. Some PCBs (especially large and complex thick boards) may experience delamination, laminate cracking, Cu cracks, CAF (Conductive Anode Filament) failure due to higher lead-free soldering temperatures depending on the properties of the laminate material Wait for the failure rate to increase. It also depends on the PCB surface finish. For example, it has been observed that the joint between solder and Ni layer (from ENIG coating) is more prone to fracture than the joint between solder and Cu (such as OSP and immersion silver), especially under mechanical impact (such as in drop test ). Also, more PCB cracks occur with lead-free soldering in drop tests
  4) Depends on components. Certain components, such as plastic packaged components, electrolytic capacitors, etc., are more affected by increased soldering temperatures than others. Secondly, tin wire is another reliability issue that is more concerned about fine-pitch components in high-end products with long service life. In addition, the high modulus of SAC alloys can also put more stress on components, causing problems for low-k dielectric components, which are often more prone to failure
  5) Depending on mechanical load conditions. The high stress rate sensitivity of SAC alloys requires more attention to the reliability of the lead-free solder interface under mechanical impact (such as dropping, bending, etc.), at high stress rates, excessive stress can cause the soldered interconnect (and/or PCB) to break easily .
  6) Depending on thermomechanical loading conditions. Under thermal cycling conditions, creep/fatigue interaction can lead to solder joint failure (ie microstructure coarsening/weakening, crack initiation and growth) through damage accumulation effects, and the creep stress rate is an important factor. The creep stress rate varies with the magnitude of the thermomechanical load on the solder joint, so that SAC solder joints can withstand more thermal cycles than Sn-Pb solder joints under "relatively mild" conditions, but under "more severe" conditions Under the thermal cycle than Sn-Pb solder joints to withstand less. Thermomechanical loading depends on temperature range, component size, and CTE mismatch between component and substrate. For example, it has been reported that components with Cu leadframes experience a higher number of thermal cycles in SAC solder joints than Sn-Pb solder joints with 42 alloy leads on the same board that passed the thermal cycle test. Framed components (whose PCBs have a higher CTE mismatch) will fail earlier in SAC alloy solder joints than Sn-Pb solder joints. Also on the same board, the solder joints of the 0402 ceramic chip experienced more thermal cycles in the SAC than the Sn-Pb, while the opposite was true for the 2512 component. As another example, many reports have reported that solder joints for 1206 ceramic resistors on FR4 fail later in lead-free soldering than Sn-Pb when thermally cycled between 0°C and 100°C, while at temperature extremes At -40°C and 150°C, this trend is just the opposite.
  7) Depends on the "acceleration factor". This is also an interesting and very closely related factor, but it would complicate the whole discussion a lot since different alloys (like SAC vs. Sn-Pb) have different acceleration coefficients. Therefore, the reliability of lead-free solder interconnects depends on many factors. These factors are intricate and interact, and their detailed discussion can be found in the newly published book "Lead-Free Soldered Interconnect Reliability"

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