Easily implement lead-free solder rework in PCB assembly-EEWORLD

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Due to the lack of wettability and wicking, it is difficult to achieve rework of lead-free soldering. Is it impossible to achieve lead-free soldering of various components? This article will introduce a method that can easily achieve lead-free solder rework.
　　Rework is an important part of the mass production process of lead-free PCB assembly, especially in the initial transition period when each link in the supply chain forms an information curve. However, due to the need for warranty, rework still exists throughout the life cycle of the product.
　　It is found that lead-free solder alloys are generally not as easy to wet and wick as Sn/Pb solders, so rework of lead-free solder is more difficult, and its application in QFP is an obvious example. Despite these differences, successful rework methods (i.e. manual and semi-automatic) have been developed for lead-free solders (Sn/Ag/Cu or Sn/Ag) used for different components such as discrete components, area array packages, etc. by using flux gel, pen flux and wicking solder. Most rework equipment used for Sn/Pb can still be used for lead-free solder. Soldering parameters must be adjusted to accommodate the higher melting temperature and lower wettability of lead-free solders. Other preventative measures used in Sn/Pb rework, such as board drying when necessary, still apply to lead-free solder rework. Studies have shown that reliable lead-free solder joints with proper particle structure and intermetallic formation can be produced using appropriate rework processes.
　　Special attention should be paid to reducing the possibility of negative effects of the rework process on the solder joint, component and PCB. Due to the increase in soldering temperature, the Z-axis coefficient of thermal expansion (CTE) mismatch between the laminate, glass fiber and Cu places greater stress on the Cu layer, which may cause cracking of the Cu in the plated through hole, resulting in failure. This is a rather complex issue because it is determined by many variables, such as the number and thickness of PCB layers, laminate materials, rework temperature profile, Cu layout and via geometry (such as aperture ratio). There is still much work to be done to determine under what conditions laminate materials (e.g., high Tg, low CTE) can replace traditional FR4 and meet the requirements of lead-free soldering. This is not to say that lower cost materials (e.g., CEM, FR2, etc.) cannot be used in lead-free soldering. In fact, these methods are applied in mass production and examined on a case-by-case basis. The effect of rework on pad and mask adhesion must be carefully evaluated.
　　Similarly, the effect of rework on component reliability should also be carefully studied. Warpage and delamination are some of the possible problems. The recently released IPC-020B standard indicates that rework should be considered as an issue for higher temperature component ratings in lead-free soldering.
　　Electrochemical reliability is another important issue to consider. When flux residues dissolve in moisture condensation on the board, electrochemical reactions occur between conductor traces under electrical bias, resulting in a decrease in surface insulation resistance (SIR). If electromigration and dendrite growth occur, more severe failures will occur due to "shorting" between conductor traces. Electrochemical reliability is determined by the resistance of the flux residues to electromigration and dendrite growth in no-clean applications. Therefore, SIR testing based on IPC (per TM-650 2.6.3.3) or Telecordia testing must be performed to ensure that any reaction between the rework flux and the reflow/wave soldering flux and the rework flux does not introduce electromigration and dendrite growth risks in no-clean applications.
　　The issue of "component mixing" is a special problem involved in ensuring quality, especially during the transition period of this technology. Preliminary studies on the effect of lead on long-term reliability of lead-free solders indicate that the effect varies with the lead content in the solder joint, with the greatest impact at the intermediate lead content due to the formation of segregated phases (e.g., rough lead particles) in the tin grain boundaries between the continuously solidifying dendrites, where cracks initiate and propagate under cyclic loading. For example, it is shown that 2-5% lead content will have an adverse effect on the fatigue life of lead-free solder, but it may not be worse than the effect of Sn/Pb solder. Therefore, if lead-free solder is used to rework lead-containing boards, from the perspective of solder, the reliability of lead-free solder and Sn/Pb solder mixed together may not affect Sn/Pb solder. However, the effect of temperature on components (especially plastic-encapsulated components) is a problem worthy of attention. On the other hand, the reliability of solder joints formed by reworking lead-free solder boards with Sn/Pb solder may not be as good as the reliability of lead-free solder joints on other boards. As far
　　as the supply system is concerned, it is critical to clearly mark the soldering irons and soldering materials (core solder, flux gel, etc.) used for lead-free soldering during the transition period.
　　Rework will inevitably increase time and economic costs. Rework is only a measure to make up for the loss. The most important thing is to strengthen operator process training and try to minimize rework.

Reference address：Easily implement lead-free solder rework in PCB assembly

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