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Double-sided Lead-free Rework
December 31, 1969 |Estimated reading time: 5 minutes
By Paul Wood, OK International/Metcal
Reworking lead-free assemblies requires care to prevent higher temperatures from damaging components and solder joints around the site, including the underside of the board. There is no value in successfully repairing one area of the assembly if the PCB underside or other areas become degraded through excessive or uncontrolled heating.
Operators reworking lead-free assemblies are under pressure to maintain or increase productivity; process-control improvements must be achieved without reducing throughput. With hot-air rework systems ramp rate, needs to be higher to reach the higher lead-free reflow temperatures.
A number of changes have come about as a result of the transition from tin/lead (Sn/Pb) to lead-free soldering processes. Sn63Pb37 solder melts at 183°C and requires a peak reflow soldering temperature between 210° and 220°C for an acceptable solder joint to form. With widespread understanding of hand soldering processes, reworking of tin/lead assemblies has become relatively straightforward and often is taken for granted.
However, the same cannot be said of lead-free rework. The most popular lead-free SAC solder alloys have a melting point range of 217° to 221°C. This requires a peak reflow soldering temperature of 235°?250°C. These higher temperatures require better process control to prevent thermal damage to components from excessive heating. Therefore, the lead-free soldering process window will be smaller than for tin/lead solders. These and related issues involved in producing high-quality lead-free rework often are not fully understood. Overall, quality must be engineered into a lead-free rework process.
Collateral Damage
High soldering temperatures have the potential to damage nearby components, including those on the underside of the board. Where components are located beneath a large topside-mounted BGA – which has a high thermal capacity – the energy required to desolder or reflow the BGA may also cause the underside solder joints to partially or fully melt. Small passive surface-mount devices are particularly vulnerable, since their solder joints have a low thermal mass and can melt quickly, before the higher-mass topside component is reflowed. Devices with plastic bodies, such as connectors, can melt or become deformed by excessive heat. To establish a high-quality lead-free reflow process, measures must be in place to protect these joints and components, and prevent them from reaching excessive temperatures.
These sites must be inspected after rework activity is complete, to verify that they have not been damaged. It is surprising how often this important precaution is not taken. Standards such as IPC-A-610D and IPC J-STD-001D provide the inspection criteria for lead-free solder joints.
Reworking Array Packages
Convection-based hot-air array-package rework is the most effective method for reworking BGA devices, including desoldering and re-attaching the device. Ideally, the system should control time, temperature, and airflow parameters, so users can establish thermal profiles. This allows preheat, soak, ramp, reflow, and cooling stages of the reflow profile to be optimized for a given application.
The major challenge for an array-package rework system is managing delivery of heat to the desired rework site without either damaging the component being reworked or, as mentioned, causing other components to become detached or damaged. IPC recommends a 260°C maximum temperature for the lid and body of plastic components. However, opinion varies within the industry, with some component manufacturers recommending 250°C. In Japan, assemblers are suggesting a peak temperature of 245°C as a more appropriate maximum. Since these temperatures are close to the reflow temperature of SAC alloys, any rework system must manage peak temperatures closely.
Under these constraints, the distance between the nozzle with hot air output and the connections being reflowed becomes an important parameter. If the nozzle-to-PCB distance changes, the profile can be affected by 10°C or more. PCB warping during heating can, for example, affect the repeatability of the rework process. Undesirable when reworking a tin/lead assembly, this is unacceptable with lead-free, given its smaller process windows. Therefore, it is important that the nozzle-to-PCB distance be established when setting up a process. In higher-temperature lead-free processes, a board support is necessary to prevent PCB warping, maintaining a consistent nozzle-to-board distance. Boards are more prone to warping in a lead-free process, since the nozzle temperature will ramp more quickly and board temperatures reach points well above the glass transition temperature (Tg) of the PCB for longer periods. In the future, changing board material to a medium with a higher Tg will reduce the probability of PCB warping.
Equipment Requirements
When considering the heating strategy, the most important objective is to prevent overheating of the board and components. There is no value in successfully repairing one area of the assembly if the PCB underside or other areas become degraded through excessive or uncontrolled heating.
Lead-free double-sided assemblies require higher peak temperatures, up to 371°C, with increasing levels of temperature control. This applies to array-package rework stations, soldering stations, and soldering iron tips that may be used for touch-up on the board underside of the BGA being reworked. Soldering irons should not overshoot their temperature set point. Selection of consumables also must be adjusted.
Rework processes will benefit greatly with the application of under-board heating to prevent warping. Under-board and topside heating combine to raise PCB temperature as a whole to a point just below the solder reflow temperature. This allows the topside nozzle to complete heating with lower temperatures and, as a result, minimizes the thermal deltas and shock to the board.
Rework stations are upgraded for lead-free with dual-stage preheaters. These are capable of maintaining a higher temperature in the central region directly under the rework site, while keeping parts of the assembly outside this area at a lower temperature to minimize board warpage. Convection heaters also are becoming the preferred heating technology, since their faster response without thermal overshoot aids operators to achieve a short time-above-liquidus (TAL) and time-at-peak duration.
Component manufacturers are making tin/lead finished parts obsolete and replacing these with lead-free equivalents. Equipment used in programs where this transition is occurring may need to be upgraded. Military and aerospace equipment rework often encounters circumstances where parts only are available in lead-free finishes. This typically is supported for much longer periods than commercial products, thus it can be anticipated that lead-free finish issues will only worsen. Care is necessary in reworking mixed – tin/lead and lead-free – assemblies.
Conclusion
Lead-free rework requires changes to equipment and processes that mean more than just increasing the source temperature. Increased process temperatures can affect underside components and create unnecessary risks of defects. Guidelines for double-sided rework include using under-board support; establishing an optimum nozzle-to-PCB distance and using it consistently; inspecting underside solder joints in the vicinity of the reworked component; ensuring rework equipment supports higher temperatures and tighter process control when working with lead-free finishes; and fully training operators to understand the changes in material properties and process demands for lead-free rework.
Paul Wood, market development manager, APR/BGA products, OK International/Metcal, may be contacted at pwood@okinternational.com.