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Step 10: Rework and Repair
December 31, 1969 |Estimated reading time: 7 minutes
Given the higher temperature requirements for lead-free soldering and the more delicate nature of array package components, rework stations must now feature excellent profiling and tolerance, as well as offer easy calibration if optimum results are to be achieved and thermal damage avoided.
By Paul Wood
The main concerns surrounding rework and repair are time, cost, quality and repeatability. Too much time means excess cost, while poor quality has never been acceptable and repeatability requires good process control. But the ability to duplicate precise heating profiles across operators, facilities and countries is the foundation for the entire rework and repair process, and feeds into the time, cost and quality equation.
Today's manufacturers have to deal with small, sensitive array packages with complex profiles and hundreds of connections that can only be seen with sophisticated vision systems. Operator turnover also is often high; conversely, experience with array packages is essential if time and quality goals are to be achieved. With lead-free solder as an added variable, reflow temperatures are higher, time above the higher reflow temperatures and joint appearance both are different, and the need for process control is even greater than for eutectic solders. However, with the right thermal profiles, equipment and knowledge, reaching time, cost and quality goals with array packages and lead-free solder is possible.
Figure 1. Convection heating is essential for effective lead-free rework.
Reworking array package components using lead-free solder is similar to the process using leaded components with eutectic solder: establish the thermal profile, remove the failed component, clean and prepare the site, place a new component with flux or solder paste, reflow, and inspect. However, convection, not radiation, is the heating method of choice, as in the assembly process, as it allows greater process control, without which repeatability is impossible. (Figure 1)
Different lead-free compositions exist and will be fine-tuned as time and processes mature. The most common are based on tin alloyed with small amounts of silver, copper or bismuth, with melting points ranging from 206° to 221°C. Solder peak temperatures are higher at 217° to 235°C.
The operating window for lead-free is being squeezed by component suppliers and solder manufacturers. The maximum solder temperature peaks around 235°C, but the component suppliers' maximum temperature is 265°C, with most ranging from 240° to 250°C — and these are close to the 225° to 233°C soldering temperature. Time above reflow also is reduced from 60 to 90 sec for eutectic tin-lead solder down to 15 to 30 sec for lead-free. So rework systems must be capable of ramping up and down quickly to achieve this small temperature peak.
Using convection makes it easier to establish a repeatable thermal profile that will not overheat the package or hold it above reflow for too long. Establishing the ideal profile takes experience, patience and knowledge of lead-free. In addition to the standard pre-heat, soak and reflow (plus cool down) zones, lead-free demands an extra ramp zone and more precise heating control.
Figure 2. Using five thermocouples to quickly establish a lead-free profile.
The higher temperatures needed for lead-free, coupled with the thermal sensitivity of BGAs and CSPs, demands precise temperature and addition of a ramp stage where temperatures rise at a rate that will not harm packages. That is why today's more sophisticated rework systems employ four heating zones and one cooling zone. The higher temperature requirements and thermal sensitivity of array packages can be problematic without the ability to ramp temperatures at a rate that will not harm components. Having a controllable pre-heater allows for efficient pre-heating that avoids the thermal damage that is risked when working with expensive but sensitive packages unsuitable for heating above 235°C with quick reflow times.
Thermal profiles for lead-free are different from those of eutectic solder, as tolerances are tighter and some type of repeatability and process control is required in the rework station. A typical lead-free profile would be to pre-heat to 140°C in the 100s, followed by a soak zone below 170°C for the 90s, then a ramp-up to 225°C in the 100s, reflow up to 235°C for the 20s and then cool down for 60 sec. The differences between this and a tin-lead profile are substantial, and the key is system control, with the ability to ramp up faster and cool down quicker. (Figure 2)
Another factor to consider with lead-free is the temperature difference (ΔT) across the soldering area. A ΔT of 10°C is considered acceptable to produce a good tin-lead joint, but is halved to 5°C for lead-free, which is difficult to achieve in practice. The second delta is usually 10°C from lid to solder ball, and also the underside of the BGA, which is the bottom surface of the printed circuit board (PCB) under the component on the opposite surface of the board. (Figure 3)
Figure 3. The new 5°C delta for lead-free is critical for thermal strength.
The wetting process and temperature profiles must be controlled to ensure the resulting joints are strong. Improved heating regulation and faster ramp-up are needed with lead-free — particularly in the under-board heater, which means that hot plates should not be used. Temperatures must be high enough to melt and form intermetallics, activate flux and optimize wetting, yet low enough to avoid damaging the PCB and component.
Inspection
Lead-free solder joints look grainy under inspection when compared to traditional soldering, and inexperienced operators often reject them for quality reasons. When lead-free is implemented, companies must set a new standard and train operators in proper inspection criteria. X-ray inspection works well because joint appearance is not then an issue, but visual systems are becoming more popular due to the expense of X-ray.
Conclusion
Rework is not going to disappear, as array packages and lead-free processes will continue to require post-production processes. While the basic rework steps remain the same, substantial temperature differences between eutectic and lead-free solders mean tighter processes, better temperature profiles and the use of precise rework systems with closed-loop process control are required if high-quality, low-cost rework is to be achieved.
Paul Wood, market development manager for Metcal, may be contacted at OK International, 1530 O'Brien Drive, Menlo Park, CA 94025, (650) 325-3291; Fax (650) 325-5932; E-mail: pwood@okinternational.com.
High-value Rework
By Chris UnderhillUsing automation, high-volume rework can be transformed into high-value rework where typical "expense" operation becomes an "income" business segment.
A rework process usually undoes some type of error — either human or machine-generated. While the operation may recover costs, it adds to product expense and directly or indirectly reduces margin. Such rework operations are traditionally manual and sometimes are done without the aid of even a manual rework tool.
The notion of "automated" rework begs the question, "Why not just fix the original problem in the first place?" This is not always possible.
Figure. Automated high-volume rework.
Under what circumstances does automation play a role in rework? The answer may simply be when hundreds or thousands of the same part need to be repaired. Such errors, if due to assembly, quite often are caught during the process. (Figure) But a whole new level of rework arises when:
- Component failure is discovered too late
- High-value products require revisions
- Design defects go unnoticed until the end user experiences them
- Re-engineering change orders can revive a once-obsolete product
- Firmware updates require the change of only a single die to reuse a product.
These tasks require a rework operation specifically designed to repair/replace components in volume, where yield becomes a function of the rework process, not just the steps that precede it. The answer is an automated rework process: a programmed sequence of repair steps including defective component de-solder and discard, residual solder removal, component placement, and re-solder. Since all steps are performed without operator intervention, the process is precise and the quality consistent. Vision alignment is critical, as is the ability to take in data maps to identify package(s) to be reworked.
In addition to the above, certain industries may require extra rework steps. Handheld electronic devices may involve shield-can removal/replacement operations. Passive devices (0201 and smaller) must be accompanied by the ability to dispense solder paste in small quantities. With elevated temperatures needed for lead-free work, the rework equipment must be capable of working for extended periods of time at temperatures typically 30° to 50°C above SnPb eutectic reflow temperatures.
Today's 0201 components often are located in a densely populated area of a PCB or module. The rework process must be accomplished not only in a precise manner for placement, but also in an exacting thermal manner to reflow solder for component removal and replacement. Surrounding components must not be disturbed, or additional defects will be created. Since more advanced technology is necessary to rework passive devices, equipment with high placement precision (in the order of 5 to 10 µm) is emerging, are automated systems.
Until recently, rework has been a singular problem — fix the board and move on. But with the advent of high-value boards and products, a new rework approach is mandated. Equipment suppliers recognize these challenges, engineering solutions and developing tools that address high-volume rework requirements, thus allowing manufacturers an opportunity to create a new value and income market segment.
Chris Underhill, general manager, may be contacted at Finetech Inc., 10201 S. 51st St. #127, Phoenix, AZ 85044, (480) 893-1630; E-mail: chris@finetechusa.com.