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Decided to lead-free welding interconnection reliability of the seven factors

Decided to lead-free welding interconnection reliability of the seven factors

2018/12/25 16:13
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With more and more lead-free electronic products listed, reliability problems become the focus of many problems. And other lead-free related issues (such as alloy selection, process window, etc.) are different, in reliability, we often hear differences point of view. In the beginning, we heard many "experts" say lead-free than tin lead more reliable. We believe in, and "expert" said tin lead to more reliable than lead. What are we should believe in which one? According to the concrete circumstances .
Lead-free welding interconnection reliability is a very complicated problem, it depends on many factors, we simply list the following seven factors:
1) depends on the welding alloy. For reflow soldering, "mainstream" lead-free welding alloy is Sn - Ag - Cu (SAC), and wave soldering is may be SAC or Sn - Cu. SAC alloy and Sn - Cu alloy have different reliability performance .
2) depends on the process conditions. For large complicated circuit board, the welding temperature is usually 260 (C, it might give the reliability of PCB and components brings negative effect, but it with a small circuit board less affected because the biggest reflow temperature may be lower.
3) depends on the PCB laminate. Some PCB (especially large complex thick circuit board) according to the laminated material properties, may be due to lead-free soldering temperature is higher, and lead to delamination, laminated fracture, crack, Cu CAF (conduction anode wire must) failure and failure rate rise. It also depends on the PCB surface coating. For example, after observation found that welding and Ni layer (from ENIG coating) between joint than welding and Cu (such as OSP and immersion silver) between joint are more likely to fracture, especially in mechanical impact (such as drop test). In addition, in the drop test, lead-free welding will occur more PCB rupture
4) depends on the components. Some components, such as plastic packaging components, electrolytic capacitor, etc., by improving the influence degree of the welding temperature more than other factors. Secondly, tin wire is long service life in the high-end product fine spacing to pay more attention to components of another kind of reliability issue. In addition, SAC alloy high modulus will bring greater pressure components, low k dielectric coefficient of components to bring problems, these components are usually more easy to failure
5) depends on the mechanical load conditions. SAC alloy high stress rate sensitivity requirement pay more attention to lead-free welding interface in the mechanical impact reliability (such as drop, bending, etc.), and the high rate of stress, the stress will lead welding interconnection (and/or PCB) is easy to break.
6) depends on the thermal mechanical loading conditions. Under thermal cycling condition, creep/fatigue interaction will be through the damage accumulation effect and lead to solder joint failure (i.e. organization coarsening/weakening, crack appeared and expand), creep stress rate is an important factor. Creep stress rate with the solder joint thermal mechanical load amplitude change, thus SAC solder joints in "relatively mild" conditions can than Sn - Pb solder joints under more thermal cycle, but in "serious" conditions than Sn, Pb solder joints under less thermal cycle. Thermal mechanical load depends on the temperature range, components size and components and the CTE mismatch between basement level. For example, the report shows, in through the thermal cycle test the same piece of circuit boards, with Cu lead frame components in the SAC solder joints of the thermal cycle number than Sn, Pb solder joints, and use and alloy lead frame components (the PCB of CTE mismatch higher degree in SAC alloy solder joints than Sn, Pb solder will advance malfunction. Is in the same piece of circuit board, 0402 ceramic chip device of solder joint in the SAC by thermal cycle number more than Sn - Pb, and 2512 components the opposite. As another example, many report, at 0 ℃ and 100 ℃ between thermal cycle, FR4 1206 ceramic resistor of lead-free solder joints in the failure of the time to later than Sn - Pb, but in the temperature limit is - ℃ and 150 ℃, the trend was just the opposite.
7) depends on the "acceleration factor". This is an interesting and very close relationship between factors, but it makes the whole discussion becomes much more complicated, because of different alloy (such as the SAC and Sn, Pb) have different acceleration coefficient. Therefore, the reliability of lead-free welding interconnection depends on many factors. These factors are complicated, mutual influence, and the detailed discussion can see the latest edition of the book "the lead-free welding interconnection reliability".
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