PCB Tech - Factors that determine PCB lead-free solder interconnection

With more and more lead-free PCB electronic products on the market, reliability issues have become the focus of attention of many people. Unlike other lead-free related issues (such as alloy selection, process window, etc.), we often hear very different views on 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.

The reliability of PCB lead-free soldering interconnection is a very complex issue, which depends on many factors. We simply enumerate the following seven factors:

1) Depends on the welding alloy. For reflow soldering, the "mainstream" PCB lead-free solder alloy is Sn-Ag-Cu (SAC), while wave soldering may be SAC or Sn-Cu. SAC alloy and Sn-Cu alloy have different reliability properties.

2) Depends on process conditions. For large and complex circuit boards, the soldering temperature is usually 260°C, which may have a negative impact on the reliability of PCB components, but it has less impact on small circuit boards because the maximum reflow soldering temperature may be relatively low.

3) Depends on the PCB laminate material. Certain PCBs (especially large and complex thick circuit boards) may cause delamination, laminate cracks, Cu cracks, CAF (conducting anode wire whiskers) due to the higher lead-free soldering temperature of the PCB due to the properties of the laminate material. The rate of failures such as failures has risen. It also depends on the PCB surface coating. For example, after observation, it was found that the joint between welding and Ni layer (from ENIG coating) is more susceptible to fracture than the joint between welding and Cu (such as OSP and immersion silver), especially under mechanical impact (such as in a drop test). ). In addition, in the drop test, PCB lead-free soldering will cause more PCB cracks.

4) It depends on the components. Certain components, such as plastic packaged components, electrolytic capacitors, etc., are more affected by the increased soldering temperature than other factors. Secondly, tin wire is another reliability issue that pays more attention to fine-pitch components in high-end products with long service life. In addition, the high modulus of SAC alloys will also put greater pressure on components and cause problems for components with low-k dielectric coefficients, which are usually more prone to failure.

5) Depends on the mechanical load conditions. The high stress rate sensitivity of SAC alloy requires more attention to the reliability of PCB lead-free soldering interface under mechanical impact (such as drop, bending, etc.). Under high stress rate, excessive stress will lead to easy soldering interconnection (and/or PCB). fracture.

6) Depends on thermomechanical load conditions. Under thermal cycling conditions, the creep/fatigue interaction can lead to solder joint failure through damage accumulation effects (ie, structure coarsening/weakening, crack appearance and expansion), and the creep stress rate is an important factor. The creep stress rate varies with the magnitude of the thermo-mechanical load on the solder joints, so that SAC solder joints can withstand more thermal cycles than Sn-Pb solder joints under "relatively mild" conditions, but under "more severe" conditions Lower than Sn-Pb solder joints withstand fewer thermal cycles. The thermo-mechanical load depends on the temperature range, the size of the component and the degree of CTE mismatch between the component and the substrate.

For example, there is a report showing that on the same circuit board that has passed the thermal cycle test, the components with Cu lead frames in the SAC solder joints undergo more thermal cycles than the Sn-Pb solder joints, and 42 alloy leads are used. The components of the frame (the PCB's CTE mismatch is higher) will fail earlier in the SAC alloy solder joints than the Sn-Pb solder joints. Also on the same circuit board, the number of thermal cycles that the solder joints of 0402 ceramic chip devices pass through in SAC exceeds that of Sn-Pb, while the number of thermal cycles for 2512 components is the opposite. To give another example, many reports claim that the solder joints of 1206 ceramic resistors on FR4 fail in lead-free PCB soldering later than Sn-Pb during thermal cycling between 0°C and 100°C. When the limits are -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 will make the whole discussion much more complicated, because different alloys (such as SAC and Sn-Pb) have different acceleration coefficients. Therefore, the reliability of PCB lead-free solder interconnection depends on many factors.