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Strategies to Minimize Rework Challenges
December 31, 1969 |Estimated reading time: 6 minutes
By Zulki Khan, Nexlogic Technologies
OEMs generally do not include a rework strategy as part of their product development plans. OEMs generally focus on getting products fully operational, functional, and into the market to maintain profitability and remain competitive. They generally are not thinking about whether or not components can be added, changed, or altered to enhance products. At times, the slightest oversight in an original design can have a significant effect on a subsequent reworked board. For example, engineers overlook an important connection during first-article design. At rework, the only cost-effective solution to correct the problem is to use a bulky 22 AWG jumper wire to connect those two points (Figure 1).
Figure 1. A jumper wire connects two points on a PCB after connection is overlooked during first-article design.
In part, a rework strategy targets minimizing the challenges a process poses, including differing thermal profiles; hybrid eutectic and lead-free boards; different surface finishes; advanced components like BGAs, chip-scale packages (CSPs), and QFNs; and removing fully functional components to remove a faulty one. A well-planned and executed strategy can more effectively deal with these and other rework challenges. A rework strategy can also play broader roles to save time and money.
OEM Benefits
If properly planned, an effective rework strategy can pay dividends. It can save OEMs several hours and hundreds of dollars on a prototype, and days and a few thousand dollars on production runs. It minimizes rework cycles to one, saving on additional and costly rework. It contributes to a product’s time-to-market and time-to-revenue, and reduces rework cycles to avoid PCB fatigue - leading to possible board and component damage. Lastly, it can improve product functionality, increase reliability, and provide greater life-cycle probability.
Figure 2. Component removed from a simple PCB with fewer components.
Rework should be considered an important aspect of design for assembly (DfA) to improve functionality and enhance products. Once a PCB is reworked, it should be functional and fully operational. Reducing the number of rework cycles should be the goal. Otherwise, subjecting a PCB to multiple rework cycles can fatigue it; tampering with the components can jeopardize their integrity. Performing a thorough debug process will subsequently contribute to reducing rework to a single cycle. This might involve a single page of rework instructions. Following these will end the rework process and yield a fully operational board.
Creating a Strategy
It’s a good idea to obtain OEM approval on a first article before proceeding with small production runs of 25-30 boards to reduce rework time and expenses. Issues that must be reworked can result from the OEM debug process at the first-article stage.
The rework strategy largely depends on the level of PCB complexity and the number of advanced packages populating the board. Some PCBs are relatively simple with a small number of components (Figure 2). A rework strategy can be straightforward; however, with trends toward high-pin-count, high-density packaging in surface mount assembly, a rework strategy often is more complicated and demands careful planning and execution. Regardless of its scope, a well-thought-out rework strategy begins with EMS providers and OEMs mapping out a first-article rework plan to include design for manufacturing (DfM) criteria. A process engineer defines all the steps that a rework project requires. Each step, related issues, and required assembly materials and processes are described in detail. Those details include such items as solder type, if the project calls for a no-clean or water-soluble solder, whether an active or passive flux is to be used, or if the PCB will be washed post-rework. Sometimes, a board cannot be washed due to it moisture-sensitive components. Therefore, those PCBs can be reworked with only no-clean solder to avoid washing and maintain component integrity.
Details like these are instrumental in a rework strategy. They comprise clear notes and instructions that rework personnel must follow precisely. Those notes should use simple language to avoid ambiguity and confusion; this makes it easy for the rework technician to follow step-by-step instructions. Whenever possible, figures and illustrations added to these notes make the rework process more efficient. Rework can focus solely on a component or group of components, on the PCB itself, or on both component and board levels. It involves the number and types of components, as well as other variables, such as whether or not rework components are thru-hole or surface mount, as each requires a different rework method. The rework process then splinters into whether or not it is targeted at removing and replacing one or more components. In some cases, rework may require removing thru-hole components on a PCB before accessing a surface mount device that must be removed and replaced (Figure 3). It also may involve cutting a trace, drilling a via to break a connection, breaking a connection between two or multiple points on a board, or adding a jumper wire to create a connection. Further, the type of component - thru-hole or SMT - calls for different tooling or types of tools.
Complex PCBs
When complex PCBs are populated with small components, such as BGAs, QFNs, and CSPs, a detailed rework strategy must specify the types of tooling needed for component removal and replacement. For example, a special nozzle is needed to remove BGAs (Figure 4). The rework process for components using lead-free solder is similar to that for tin/lead components, although removal and replacement tooling may be slightly different. The key steps in the remove-and-replace rework process include removing the defective component, preparing the site, solder paste/flux deposition, and component replacement. While those steps serve as the basic strategy structure, various tools and processes affect the specific rework project.
Figure 3. One or several thru-hole components must be removed with the aid of a solder pot.
Thermal profiling is a critical aspect of a BGA/CSP rework process, especially for high rework yields. Special thermal profiles must be defined to remove these components, especially if they are lead-free. Specialized BGA rework stations are used that feature bottom heating to bring the PCB temperature up to an appropriate level. At this temperature, lead-free BGA removal and replacement is performed without damaging the component or PCB. An excessively high thermal profile may cause SMT pad lifting prior to component removal. A high profile also may burn and damage the board and surrounding components mounted on the PCB. When a CSP is replaced, the proper thermal profile ensures that the component and board are not overheated, and all solder balls reflow correctly.
Conclusion
Subsequent board layout calls for implementing a rework strategy by maintaining a disciplined record of component sequencing and back-annotating to the original schematic. Carefully documenting board and component layout can ensure that rework is performed more efficiently, cost effectively, and accurately. Component sequencing reference designators at the PCB-layout level are not a normal practice or an industry requirement, but can ensure that rework technicians in the lab or the field locate faulty components on the PCB easily.
Figure 4. Special nozzle used for BGA removal.
Sequencing is a vital rework element that can save OEMs time and money. There may not be spares readily available at an RMA depot, and rework must be performed quickly to get the repaired PCB back to its system. In such cases, it is important to quickly locate rework areas, repair the PCB, and return it.
After layout, a PCB is sequenced and back-annotated to the original schematic to reflect changes made during the debug process. In such cases, the debugging engineer likely designed that specific board and; therefore, is familiar with its functionality and changed values. Back annotating PCB schematics represents the exact board, which is different from the original layout. During the debug-and-rework processes, for example, two components were added; traces were cut; and some components were connected at their top edges. A sequenced and back-annotated schematic lets rework technicians understand the current version of a given board and efficiently debug and test it. By not performing this sequencing, rework time and cost can double or triple. Inordinate rework costs can be incurred with complex SMT boards populated with more than 3,000 components. At that level, rework takes on a whole new meaning. Unless a well-defined strategy is implemented, rework can cause confusion, product delays, and high OEM costs.
Zulki Khan, president and founder, Nexlogic Technologies, Inc., may be contacted at (408) 436-8150; zk@nexlogic.com.