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Getting Started in 2001 Rework
December 31, 1969 |Estimated reading time: 6 minutes
By Mark Pope
As space on circuit boards continues shrinking, 0201 component use is becoming standard. Production processes are adapting to 0201 capability. But can these miniature components be reworked? Successful rework can be achieved by using proper tools, slightly adapted procedures and applicable operator training.
Seemingly every element in electronics manufacturing is shrinking. From machine footprints to printed circuit boards (PCB) and components, innovations continue reducing the dimensions with which the industry has to work (Figure 1).
Figure 1. As industry standards and the available space between components continue shrinking in high-density applications, the use of 0201 components becomes necessary. These small components can reduce up to 50 percent of board space without changing fundamental designs.
Enter 0201s. Hardly visible to the naked eye, these components quickly are becoming standard in high-density packaging applications particularly in cellular phone manufacturing, where it is predicted that some 20 billion 0201s will be placed on the world's 1 billion phones by 2003. This figure is not surprising because 0201s can reduce the typical mobile phone board by more than 50 percent without changing the fundamental design itself.
Questions are being raised as to the adaptability of production processes to suit 0201s. Can placement systems handle these components with the necessary high degree of accuracy and speed? Can reflow processes solder 0201s without tombstoning? Can dispensing equipment meet the tighter criteria demanded by miniature components?
These questions can and will be resolved through a combination of better industry standards and evolving equipment. However, it remains that first pass yields for assemblies with 0201s likely will be lower than those using larger components.
The likelihood of lower yields ultimately leads to another question: can misplaced or faulty 0201s be reworked manually? Is the rework of these components physically possible and economically viable?
Reworking 0201s threatens to be both challenging and time-consuming. However, given the value of a populated PCB and the fact that the base unit cost per 0201 is meager, it makes good economic sense to repair an expensive board by replacing an inexpensive component, even when taking into consideration the cost of extended rework time.
It is possible to rework 0201s using the same well-understood, step-by-step process that already is in place. However, successful rework can be achieved only by using proper tools, slightly adapted procedures and applicable operator training. When done well, scrap can be reduced and faulty PCBs can be transformed into saleable product.
Vision System ChoicesA vision system is obligatory when reworking 0201s. The available choices consist of either magnifying glass-type systems or microscopes.
Adjusting a magnifying system to find the correct distance between the eye, lens and board to obtain clarity and appropriate magnification takes additional time; time that the operator could spend on the rework process. Arm-mounted lenses also are not an ideal choice because the mechanism itself will either get in the way or be knocked out of position by the operator.
Solid, stable and reliable, microscopes are a more suitable vision system. For most companies, a moderately priced unit is adequate, although companies conducting a high amount of 0201 rework are better served with a higher-priced, high-functionality microscope.
Desoldering and RemovalOnce the faulty component has been identified, the next step is to desolder and remove the component. Two methods are available: a hot air system or fine-point tweezers.
If choosing a hot air system, care must be taken to select one that generates low airflow. Usually 0201s are placed side by side on a PCB. Consequently, adjacent components will reflow partially, despite any effort the operator makes to control the hot air. With limited airflow systems, partially reflowed adjacents will not move out of position. Once the faulty 0201 has reflowed, it then is possible to remove the component with standard tweezers and not disturb neighboring devices. With high airflow systems, adjacent 0201s almost certainly will be moved, creating more work for the operator.
Using conduction tweezers, it is possible to grip each end of the 0201, reflow the solder at each end, and lift the component from the board. The ability to control thermal energy is critical in the selection of heated tweezers because excessive heat will damage the substrate and any adjacent 0201s.
In many cases, conduction tweezers are preferred to hot air systems because of the adhesive that often can be found on 0201s. In the initial assembly of double-sided PCBs, a small amount of glue is placed under the component to hold it in place as the board runs through the reflow process on the second pass. Heated tweezers provide the additional mechanical strength necessary to break the surface tension of the adhesive prior to component removal.
Cleaning and ReplacementThe next step is to clean the surface in preparation for reattachment. For some manufacturers, residual solder is used. For others, all solder is removed and then replaced. If employed correctly these methods are equally effective.
On a standard PCB with larger components, solder braid generally is used to clean the pads. With a PCB populated with 0201s, however, such a technique is not possible because of space constraints. A "mini hoof tip" should instead be employed to smooth the pad and "refinish" the solder. This style of tip works well to reflow the pads and eliminate rough surfaces, creating the perfect environment for secure 0201 reattachment.
To avoid reattachment problems, it is essential that the correct amount of solder be applied to each pad. Solder should be added to the hoof tip, not to the PCB. The pads then will use a capillary action to draw solder from the tip to the pad. Once completed, flux residue should be removed with a cleaner and the 0201 should be replaced using standard tweezers. The hot tweezers used to remove the component are not suitable for reattaching the 0201 because they can pull the component away from the board when they themselves are removed.
Both ends of the 0201 then are soldered, typically using a very fine solder tip (approximately 0.2 mm in diameter) to work within the tight spaces of a high-density PCB. Larger tweezers or tips are not recommended because they often obscure the operator's view of the component.
Final InspectionA two-dimensional microscope is essential for the final checking of the site following rework. For a more thorough inspection, a three-dimensional system is a cost-effective choice, allowing the operator an unimpeded view of the components and ensuring that all solder joints are solid without needing to move the PCB.
ConclusionAt first glance, reworking 0201s may seem an expensive option one that can take twice as long to complete (vs. "standard" components and assemblies) and requires an extremely high level of operator skill.
But the benefits far outweigh the expense. The cost of repairing vs. scrapping a PCB populated with 0201s makes high-quality rework essential. It also is likely that first pass yields will decline until process standards and equipment complement the demands of high-speed 0201 assembly, again driving the need for effective rework.
Companies are working to supply the tools needed to transition to 0201s, introducing smaller soldering iron tips, better balanced increasingly precise hot tweezers, and new systems such as hot air pencils with highly controlled airflow.
With such forethought coupled with process adjustments and real-world experience, 0201 rework will become common practice, and another demonstration of the industry's ability to adapt and evolve.
MARK POPE, technical manager, may be contacted at Metcal Inc., 1530 O'Brien Dr., Menlo Park, CA 94025; (800) 776-1778; Fax: (650) 325-2035; E-mail: mpope@metcal.com.