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Selecting a Rework System
December 31, 1969 |Estimated reading time: 10 minutes
When selecting a rework system, no issue is as important as thermal management. without it, components other than those that are the focus of the rework process can be adversely affected. This article gives guidelines on evaluating and choosing a rework system.
By Chris Underhill, Ph.D., Finetech USA
A reflow oven is simply a thermal management system (TMS), and a very sophisticated one at that. Much is also spoken about lead-free implementation in relation to reflow. The lead-free effect requires nothing more than to demand that the TMS improves its performance to track solder paste requirements. But this is not an easy task; ask any oven user or vendor. How does this relate to choosing a rework system? Rework equipment is supposed to be able to closely replicate the environment seen by the board, paste, and component on its initial pass through the oven, requiring the rework system to mimic an oven. This is a tough assignment for a bench-top unit, with a price tag a fraction the cost of an oven, which must be capable of handling the smallest passives to the largest BGAs on a wide range of boards.
Why then, when evaluating a rework system, would anyone concentrate on any issue other than thermal management? If a system cannot pass this initial “blink” test, the evaluator should move to equipment that can. Once a short list of vendors is completed, other secondarily important issues can be evaluated. But, if it’s not hot in the right places and cooler in the others, no number of additional features will help achieve the goal of reworking successfully to the required paste specifications.
Rework articles typically focus on applications that can be placed under the heading of “advanced rework,” or on specific system features, such as IR vs. hot gas; type of vision used; nozzle design - all of which form part of a rework solution. But how do you answer the question, “How do you select a rework system?” When beginning the selection process, consider these questions:
- Will this be your first rework tool?
- Is it for a single unit at a single location, multiple tools at one location, or various systems located in different sites throughout the world?
- Are all of these installations expected to run identical profiles and achieve equivalent results?
- Is it for general rework applications?
OEM requirements usually are different from those of contract manufacturers (CMs), the former faces known demands, while the latter has to deal with whatever comes through the door. Having worked with a particular tool vendor in the past means that they will be included on the short list, depending on past experience. But, other vendors should not be excluded based solely on this familiarity. Even though the learning curve may be higher for a new tool, long-term benefits may outweigh added training required. Other vendors can bring additional capabilities, some of which may be considered unimportant or not technically viable by an existing tool supplier.
The primary goal is to find a technically valid solution. Only then should other issues be considered. It is, after all, the technical viability that will make money. Secondarily important issues, such as application knowledge, training, support, and nozzle availability should not be taken lightly, and are crucial to the long-term success of the machine and a company’s business plan. But their true value will be felt only after having realized the primary technical goal.
Most selection processes use a matrix that is scored with a weighting system for issues deemed most important to the end user. No matter how this performance matrix is constructed, the outcome is predictable. The system and vendor that consistently score highest on issues that the customer weights the highest will win. That the decision is pre-ordained might seem trite. However, this outcome points out how important the weighting process is to the final result.
Unless due diligence is paid in evaluating the technical details that most affect your anticipated work profile, the choice will be subjective. Without proper research, the decision could be based on features that may or may not be crucial to your rework goals. For example, choosing a vendor just because you know how to get the best out of your present system dismisses any advantages of learning something new.
Is cost a limiting factor? A survey of available “non-hand-tool” rework systems generally segments the market into the “sub-$35K” and “$60K and up” groups. Exceptions to this exist due to the cost of additional modules; but overall, this differentiation holds true. Where should you look? If this is your first rework system, the demands may be basic - not too large a board, not too small a component, and not too fine pitch. For this type of application, a lower-cost unit could meet your needs. Why pay more if the requirements can’t be met? However, outside of these basic needs, best-of-breed principles should be followed, and time should be invested to execute a comprehensive market analysis.
Figure 1. Critical measurement locations.
It is a straightforward issue that comes down to temperature. Exceeding the melting point of the solder paste/sphere is good. In fact, it’s essential. But it must be localized to the component being removed or replaced. Heat is delivered using a combination of time, flow rate, and the system’s ability to localize the heat where it is needed. The total thermal environment seen by the interface comes at the component from both sides.
Look at any process profile, either from an oven or a rework system, and they look very similar at first glance. They rise at a rate around 3°C/sec., follow a flat soak zone, spend 30-90 sec. above liquidus, and ramp down to room temperature. This may lead you to believe that all rework systems are essentially the same, that any hot-gas system could achieve this general profile shape. It may achieve the general shape, but when multiple thermocouples are located in various locations on the board (Figure 1), and the temperatures are measured simultaneously, critical differences between systems will be evident.
This is the general approach adopted by iNEMI in an initiative to evaluate and compare rework systems in their repeatability and ability to mimic a reflow oven. It uses a device* that has embedded thermocouples carefully located to measure temperatures at critical locations. What happens away from the component, in many cases, is as important as what is happening under the nozzle. Thermocouples are placed at the extremities of the board to ensure that the delta (∆) is within an acceptable range to avoid board warpage. They also should be placed on the top and bottom of the PCB. There is no point reworking the top surface, while at the same time, inadvertently disturbing components on the bottom. Therefore, the way to correctly evaluate individual system performance is to heat, probe, and measure. Adopting such a detailed evaluation philosophy is not a requirement in making an individual system selection, but it will take the subjectivity out of the equation, allowing data to be collected from identical locations for all tools. Two heat sources contribute to this energy density - top and bottom.
Bottom Heat
Under-board heating is “macro” heating for a rework system, and generally provides an elevated ambient board temperature (soak). It should be high enough that the more localized top heat does not need to be excessively intense, but not so high as to cause damage to sensitive top- and bottom-side components. The key issue is uniformity, most critically measured from edge-to-edge on the board, which is a good indicator of the heater’s ability to provide a uniform base temperature that will be supplemented by the more localized top heating. Thermocouples can be placed, profiles run, and measurements taken and compared. A quick survey of supplier specifications for bottom heaters may lead one to believe that the unit’s power specification vindicates the system’s ability to heat the board. This, however, is the wrong parameter on which to focus. It is the efficiency of the under-heater unit that should be judged; merely increasing power solves nothing. Diffusing the heat flux to ensure a timely, well-saturated board should be the focus.
Top Heat
Top heating is the more critical heating element, or the “micro” environment produced around the active component that ideally should be localized. It is not only necessary to deliver the heat flux where you want it and in a timely manner. But it is similarly important to preclude heat from where it is not needed, ensuring neighboring component-placement integrity. Therefore, IR top-heating systems are excluded from the following discussions due to their inability to adequately focus heat flux where it is needed without impinging on nearby components.
Figure 2. Top-heat options control.
Nozzles used in the rework process are steering devices for the gas-flow heat generated in the heater cartridge. All manufacturers claim to have unique nozzle designs; and most do an excellent job designing complex nozzles to direct the heat to where it is needed. However, the results can be only as good as the temperature uniformity and flow-rate homogeneity produced by the heating element and flow-control electronics. There are two competing hot-gas-producing philosophies (Figure 2) used to drive heating elements - both of which have the same goal of producing a flux of temperature-controlled heated gas (air or nitrogen), and steering it toward the device and the interface between solder spheres and the PCB. It also must be able to control the gas volume. Large components require a higher flux than small ones, which may get displaced if an excessive flow rate is used.
Figure 3. Advantage of a bypass.
Some form of PID electronics control is used in most systems. Effects of this can be seen in a system’s ability to conform to desired ramp rates and the degree to which overshoot and undershoot of set temperatures is experienced (Figure 3). To overcome this over-saturation effect, the gas-bypass solution enables short bursts of ambient gas to be routed into a mixing chamber prior to passing through the nozzle.The result of such a system is to minimize over/undershoot significantly - the system can track set temperatures directly. If the temperature of the gas in the nozzle cannot be controlled uniformly, then the component will experience non-homogeneous heating. As complexity increases, elements contributing to the heating cycle become crucial. Improvements to any of these elements will enhance a system’s overall performance.
Finally, is the rework process repeatable? Can operators achieve repeatable results independently? Do profiles that run multiple times on one system generate the same results? Do standard profiles that run on different machines achieve the same results? This can be an exhaustive analysis, but essential if the rework scope is more than one system running an occasional repair. It is a system’s thermal-management capability that dictates whether “System A” requires “Profile A,” and “System B” requires “Profile B” to achieve like results for the same board-and-component combination. The ability to comply with this speaks volumes about a vendor’s quality control, and the degree to which they can produce the same tool consistently.
Conclusion
Three issues must be satisfied as a pre-qualification process:
Board size - Can largest and smallest sizes required be accommodated? Accuracy - Does the system have the accuracy needed, better than 0.5 mil. for smallest-sized components? Resolution - Can the system resolve smallest (0201 or less) and largest(>50 mm) components with the same optical system?Assuming compliance with the above, attention should focus on each system’s thermal-management capability. Executing the follwing steps when making such an evaluation will ensure that the system meets your technical needs:
- Develop a matrix - weight-based on technical needs.
- Evaluate the complete marketplace.
- Have vendors bring equipment to your facility.
- Use your own test material.
- Ignore old biases, measure performance, show me.
- Don’t surprise vendors. Give them enough information to bring out their best, and keep an open mind.
* Rework Rider, ECD, Milwaukie, Ore.
Chris Underhill, Ph.D., general manager,Finetech, may be contacted at (480) 893-1630; e-mail: chris@finetechusa.com.