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New Challenges in the Rework of 0201 Passives
Neil O’Brien
Finetech
Phoenix, AZ
neil@finetechusa.com
Abstract
The continued drive toward miniaturization has seen component sizes decrease while board densities have increased. The world’s most advanced semiconductor packages are facing new challenges with the adoption of 0201 passive components. These challenging components will push high-speed assembly equipment to even more exacting parameters. However, failures will still occur during manufacturing, and rework and repair will be needed to salvage these dense and often expensive assemblies.
Often times, the 0201 component will be located in a densely populated area of a printed circuit board (PCB) or module. The rework process must be accomplished not only in a very precise manner for placement, but also in an exacting thermal manner to reflow the solder for component removal and replacement. The surrounding components must not be disturbed in this process, or additional defects will be created. When the surrounding components are also 0201’s, tight spacing between components and small mass leave little margin for process variables.
As more advanced technology is needed to rework these passive devices, equipment with high placement precision (in the order of 5 to 10 microns) is emerging. These systems have high magnification with good optics and lighting. In addition, specialized heating methods are employed. Many of these important equipment features and capabilities are also utilized in the high-accuracy world of micro-assembly, flip chip, MEMS and opto-electronics. This paper will look at the major considerations in successful 0201 component rework.
Table of Contents:
- 0201 Basics
- High Magnification, Precise Placement Accuracy and High-Quality Resolution
- Specialized Tooling
- Residual Solder Removal
- Solder Paste Dispensing
- Precise Thermal Management and Inert Atmosphere
- Lead-Free Considerations
- Conclusions
- Acknowledgments, References
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| Before discussing the 0201 rework process in detail, it is important to review the physical characteristics of this component type and its usage in today’s electronic assemblies. Figure 1 shows the nominal dimensions of the 0201. Figure 2 shows the 0201 comparatively in a photograph. |
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When viewing the size of an 0201 device against a soldering needle tip, it is apparent that the simplest form of manual rework (without machine assist) is not realistic.
It is easier to weigh this component by a quantity of one thousand, which results in a single 0201 weighing only 0.200mg. Essentially, the operator is working with a grain of sand! Furthermore, the failures that can occur in automated 0201 assembly and reflow are defined below. |
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1. Inaccurate positioning can result in the component being rotated or tombstoned (a device that has only one of its metallized terminations soldered to a board and the other is elevated above).
2. Inaccurate positioning can place the component on only one pad, or placement can crack the component.
3. Faulty solder paste printing can result in a variety of defects. Variations in paste volume and location can create unbalanced wetting forces during reflow. This contributes to defects such as rotated components, tombstoned components, bridges, and insufficient paste. (Rectangular pads are shown in the above illustration. Other pad shapes, such as triangular, have been studied to help reduce defects during reflow.)
4. Vibration and shock influence, in combination with wetting forces, can cause rotated or missing components.
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This paper will not go into the details of how these defects occur, but rather how to successfully repair the situation when it occurs. Two distinct rework methods are possible, depending on the failure:
Method One:
Remove the component and residual solder. If necessary, apply new solder paste to the pads. Finally, place and reflow a new component.
Method Two:
Remove the component. Place and solder the old component in the residual solder.
In order to achieve these methods in a precise and repeatable manner, an integrated machine-based solution will be examined. This solution should satisfy the following main criteria:
- high magnification with high-quality resolution
- high placement accuracy (10 micron or better)
- specialized tooling to handle small components in a
soldering environment
- the ability to remove residual solder when necessary
- the ability to precisely deposit new solder paste on pads when needed
- precise thermal management
- the ability to inert the atmosphere when needed
- (Pb) Lead free considerations
Today, the vast majority of component rework is done with a manual machine. Fully automated rework exists, but is typically used in a high-volume, dedicated rework process. In this situation, the failures are numerous and consistent. Often, lower cost and greater flexibility dictate a manual rework. This paper will focus on critical aspects of an integrated, repeatable solution that uses a manual alignment system, process observation, and a user-defined software profile interface. |
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| 2. High Magnification, Precise Placement Accuracy and High-Quality Resolution | back to top |
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| With pad sizes in the range of 0.3 x 0.3mm (300 micron square), component alignment becomes very important. When manually aligning a component, the optical specification is important in order to provide a large and clear image within the microscope view. Alignment accuracy and magnification factors normally associated with micro-assembly or flip chip die bonding have also proven useful for small passive rework. The 0201 size is comparable to components used in optoelectronics. The system used for this 0201 testing was specified at 5 micron placement accuracy. At maximum magnification, the component could be easily viewed within a field diameter of approximately 3.0mm.
Process observation is very beneficial to the rework operator. In the case of 0201 rework, process observation requires camera and magnification assistance. Figure 4 shows a process observation camera image. Without this assistance, the operator is unable to see what is happening during the removal or soldering process. A clear and magnified view of the component site allows the operator to make decisions about the most effective method of rework to be employed. |
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The extremely small 0201 passive cannot be handled like a typical surface mount technology (SMT) component. Normally, the SMT component is placed into solder paste or flux, and is then released from the tooling vacuum hold. Hot gas is delivered to the component area to reflow. The same nozzle can be used to both remove a component and replace it on the substrate. Reworking the 0201 with standard convection hot gas nozzles is not recommended. The airflow, even if adjustable, will disturb the component alignment and any neighboring 0201 components.
Our testing has proven that maintaining contact with the device and heating via conduction is a fast and repeatable method for reflowing small passives. The constant contact also eliminates tombstoning from the replacement process. Figure 3 shows the nozzle working principle. It is a variation of a thermode style of heating2. Instead of electrical pulses heating a thermode, the hot gas of the rework system is delivered behind the nozzle thermode surface.
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A software controlled heating profile conducts the energy to the component. The tooling is also designed to bring a small residual amount of gas flow out the nozzle perimeter to inert the soldering area. The soldering head has the ability to place or remove a component, as seen in Figure 4. |
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| As described earlier, rework Method One requires the removal of residual solder on the substrate and various approaches can be applied. Once again, due to the small size and densities involved, a machine-assisted approach will be taken. A specialized removal nozzle was employed to combine both component removal and residual solder removal. Re-use of the misplaced component in many cases is not practical. |
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| The bottom side of the tool is shown in Figure 5. Here hot gas is delivered in a low flow to liquefy the solder. A software profile is used specifically for this removal process. Once liquefied, vacuum is pulled through the center nozzle opening. The component and residual solder are pulled into a collection container for disposal. The operator adjusts the positioning table to bring the second pad under the vacuum nozzle for residual solder removal. |
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| Figure 6 shows the overlay microscope image of a tombstoned component and the center nozzle opening on the removal tool. Once aligned, the operator lowers the placement arm and begins the heating profile for component removal. |
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| With larger surface mount components, new solder paste can be applied via a stenciling process. Often a mini-stencil is used on the rework system itself. The 0201 component size and higher density substrate area has driven the need for an integrated, paste dispense capability. With typical pad sizes of 0.3mm x 0.3mm (0.012x 0.012”), a very precise volume of solder paste needs to be applied. In 0402 rework, a Type V solder paste is typically used for dispensing. In 0201 rework, a Type VI paste is recommended. In our testing, an auger style pump was used to dispense a .250mm (0.010”) diameter bump (of a Type VI solder paste). Figure 7 shows the dispenser integrated into the rework system. |
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| 6. Precise Thermal Management and Inert Atmosphere | back to top |
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As stated in the introduction, the size and substrate density dictates that careful thermal management is needed when reworking 0201 components. Surrounding passives can easily be disturbed by the hot gas used in a typical rework system. The system should be capable of precise and repeatable ramp rates. Studies have shown that narrow changes in ramp rates can prevent flux thinning and spreading that lead to oxidation and poor wetting.3
Precise and repeatable bottom heating is also important to any rework process. Keeping the board conditions consistent for each rework step is a fundamental need. Localized and full area bottom heating options are available on many systems.
Nitrogen was used in all of the testing referred to here. The system was capable of switching nitrogen on just before reaching reflow temperatures. Secondly, it would switch nitrogen off again while cooling, just below the liquid state. The use of nitrogen is a proven benefit in traditional rework, and remains so with small passive rework. It will improve wetting, solderability, and open the process window for the operator.
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With the adoption of lead-free alloys, the need for rework systems with precise thermal management and high placement accuracy becomes even greater. Higher reflow temperatures create a narrow process window with risk of component and board damage if not properly managed. Lead-free alloys do not produce the same “self-aligning” characteristics normally seen with tin-lead soldering. Therefore, placement accuracy of the component in lead-free solder takes on a higher importance. The same steps outlined here for successful 0201 rework will also apply to a lead-free process.
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Successful 0201 rework is truly a machine-based solution. The system must be capable of handling any of the 0201 defects outlined in this paper. Component and solder removal on a miniature scale is necessary. The ability to deposit new solder paste directly with the rework system provides a complete solution.
Our testing has shown that specialized tooling using heat conduction rather than traditional convection, will result in a controlled soldering environment that leaves neighboring components in place. The system must also be able to hold a narrow and repeatable process window to allow optimization of wetting ability and solder joint formation.
Since the component is hardly visible with the naked eye, a magnification system with high quality optics must be used to achieve a repeatable result that is largely operator independent.
The accuracy and magnification required in 0201 rework leads to systems similar to those used in micro-assembly or flip chip bonding. These systems will have placement accuracies in the order of 5 to 10 microns. This level of placement accuracy will carry the user into the future as lead- free alloys further reduce the process window for such rework. Moreover, the continued drive toward miniaturization will only lead to smaller component size and higher density. The 01005 passive component will soon be the next issue.
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I would like to thank my colleagues at Finetech GmbH in Berlin, Germany for their work on this 0201 integrated process solution.
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1 Feldman, K., “Basic Study on the Use of Lead-free Solder Materials in the Rework Process”, FAPS – TT GmbH and Finetech GmbH.
2 Brackell, P., “Pulse-Heated Reflow Soldering of Flexible Circuits,” Circuits Assembly (1999).
3 Borkes, Thomas, “A Statistical Approach to 0201 Component Package Utilization,” SMTA International Conference, Chicago, IL, Sept. 2002, p. 3
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Rework of 0201 Small Passives |
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