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Step 10: Rework/Repair
December 31, 1969 |Estimated reading time: 8 minutes
By Paul Wood
BGAs and CSPs require particular care and attention if post-assembly rework is required, namely alternate handling and process control because of small size and high density, while their hiddenjoints demand a different approachto profiling and inspection.
The use of advanced (array) devices such as land grid arrays (LGA), chip scale packages (CSP) and miniature ball grid arrays (BGA) is rising fast as more users realize the size and performance benefits they offer. In line with this trend is a demand for better rework tools and techniques dedicated to advanced packages' technology and their substrates. Indeed, the ability to rework these components quickly, safely and effectively can be key toward using them profitably.
In concept, reworking a printed circuit board (PCB) having array-type packages calls for essentially the same procedures as those for standard boards. All rework processes, whether for standard or advanced surface mount devices (SMD), follow the same basic steps of desoldering, removal, site cleaning and reattachment. The main differences with the new packages are that they are heat-sensitive and have no visible leads, which complicates the process and demands greater control. Standard techniques such as touching-up the connections are not possible, while particular care must be taken over profiling, material deposition and final inspection.
Desoldering
To maintain the integrity of the component and substrate while achieving reliable solder connections, it is essential to desolder a device by applying uniform, controlled heat in a way that duplicates the original reflow profile — especially if a post-mortem examination of the component is to be performed.
One critical choice is whether to use convection (hot air) or conduction (soldering iron) tools:
- Conduction generally is not considered a practical choice for array-package rework. This is because the hidden interconnections call for heat to be applied gradually through the package to reflow the solder joints. If, however, the temperature, ramp and dwell times (2° to 3°C/sec) are not controlled strictly, simultaneous reflow will not occur and damage to the part and board likely will follow.
- Convection rework systems are the only real means for removing and replacing array-type packages. In operation, hot air is forced through a nozzle (which varies in shape and size with equipment and user preference) at the required temperature — determined by the thermal profile — to reflow the solder without damaging the substrate or neighboring components.
Because reflowing a faulty CSP or BGA on a densely packed board can affect adjacent components, a low-airflow system having a maximum rate of 24 liters/min is preferred. Depending on the rework system's sophistication (hand-held or semiautomatic), convection offers precise temperature control, component profiling, excellent vision systems and good component alignment. Lastly, the latest systems offer sharp Z-axis control for placing components at the height required automatically, while a single reflow/placement head improves temperature control and underside preheating, a critical step to prevent warpage (Figure 1).
Removal and Site Cleaning
Component removal generally is accomplished with a vacuum pick-up. A fairly straightforward process and identical to standard rework, extra care nevertheless must be taken to avoid excessive vacuum pressure: too much vacuum can cause the solder to collapse and adhere to the PCB, making cleanup slow and difficult.
Having removed the component, the next step is to check pad integrity and remove any residual solder. For many operators, the preferred method is with a soldering iron and desoldering braid or wick. Using braid is simple enough, although it does require some skill. The user joins the solder, iron and selected braid, making sure that the wick is kept between the tip of the soldering tool and the board itself. (Direct tip contact can damage the board.)
To prevent thermal damage and speed the process, many rework facilities are moving away from traditional soldering irons in favor of advanced tools that control tip temperature precisely and automatically without constant operator intervention or recalibration.
Reattachment
Because the joints of the advanced packages are hidden beneath the component, a good rework station will have the assistance of a "split-optics" vision system for correct replacements. With split-optics one may look up and down an alignment to view the ball contacts of the array package and the PCB site simultaneously. Once aligned, the package can be placed with the vacuum pick-up tube, followed by solder reflow.
Thermal profiles developed for removing the component can be used to facilitate reattachment. Without a carefully developed profile, the chance of board and component damage increases with each second of unnecessary time and each degree of excess heat. Slight profile adjustments may be needed, however, depending on the flux or paste type used.
Figure 1. A convection system for reworking array-type packages can handle boards up to 8 x 10" and features ±0.001" placement accuracy. The system provides critical board-warpage prevention via underside preheating.
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Profiling
Precise temperature control and ideal temperature profile establishment for each component are essential rework and repair steps if faults are to be avoided. Even minimal warping, e.g., a BGA lifted by just 0.005" across the component, is enough to cause an open circuit. Even without an obvious defect, the joint will be under constant strain as the board returns to its normal shape and poses a threat to reliability in the field. The best way to approach rework profiles is to emulate the method used in production, i.e., with preheat, soak, reflow (spike) and cooling zones. The required solder joint temperature is a function of the degree of heat applied to both the top and bottom sides. Incorrect combinations lead to low yields and, in some cases, catastrophic results, including board warpage from excess heat on the topside.
Normally only the component being reworked is required to reflow, which involves heating a specific area of the PCB. If performed incorrectly, however, such selective heating can cause process failure even when all other parameters appear correct. Hence, good underside heating is an important factor to consider for an ideal rework system.
The optimum profile is not the same for all components. Most assemblies have different thermal characteristics across the board owing to the various components or component densities used. While board variations can lead to large thermal mass differences, it is possible via a rework system possessing closed-loop feedback and control software to establish a pattern and develop a profile range to take these factors into account (see table).
Reflow process control becomes even more critical when dealing with lead-free assemblies. The higher process temperatures (up to 437°F or 225°C) coupled with the thermal sensitivity of many array packages can be problematic without the ability to ramp temperatures at a rate that will avoid harming them. The latest rework systems attempt to remedy this by using up to four heating zones and one cooling zone vs. just the three-zone system typical of traditional models.
Figure 2. An improved process for solder or flux deposition uses specially designed plates with the identical size and shape as that applied by the stencil printer to deposit the materials.
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Material Deposition
The specific component type and composition of its solder spheres determines whether flux (using dip transfer) or solder paste should be used for reattachment. In either case, operators must pay close attention to accuracy of material (solder or flux) deposition since this directly affects the yield. If paste is used, it often is applied to the board using mini-stencils or hand-held dispensing tools. But problems with pad flatness are common and tight board tolerances make it difficult to work around adjacent components. An improved technique developed recently is to print the paste directly onto the array package before it is placed on the board. This is a relatively quick process that uses specially designed plates to apply paste with the same size and shape as originally performed by an in-line stencil printer. It also is suited for small components or restricted access rework (Figure 2).
Printing paste onto the component minimizes flux contamination on the board and eliminates the possibility of solder contamination of the vias owing to poor stencil cleanliness. The plates also are more durable than most mini-stencils. Templates are available to suit most devices with solder-sphere arrays.
Figure 3. Endoscopic optical inspection permits inspection of rows of solder joints by moving the tip of the optics around the perimeter of a package at board level.
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Inspection
After replacement and reflow, a final inspection is required to ensure individual solder joint integrity. Inspecting the integrity of the solder bumps on an array package means using either X-ray or optical inspection system technology to view the hidden joints. X-ray images will highlight faults as deformed joint shapes such as voids, shorts and under-soldered fillets. However, other major defects, such as component or joint cracking and partially soldered joints, are not located as easily using X-ray.
The recent development of visual optical inspection based on endoscopes has provided a cost-effective alternative for finding many array-package component defects. Such systems reportedly can identify 90 percent of all errors, eliminating the need for expensive X-ray inspection. (Manufacturers already using X-ray can use optical systems as the first stage of a thorough inspection process.)
Endoscopic optical inspection is the only way to actually see what is going on under a component, and presents an intuitive image format that is already familiar to most engineers. A typical system works by moving the tip of the optics around the outside of the package at board level. Adjusting the focus permits the operator to look down the rows of joints into the component, providing images that show what is occurring without the need for the skilled interpretation required for X-ray images (Figure 3).
Conclusion
The growing use of advanced array packages is forcing a rethinking of rework processes to achieve optimum yields and maximum return. Although the process steps are similar to standard surface mount rework, the best results can be obtained only by careful control of thermal profiles, materials deposition and inspection.
Hot-air convection is the preferred technology and rework systems are available offering state-of-the-art vision as well as closed-loop time/temperature/airflow control. Backed by accurate placement and effective software, these systems simplify the rework of complex packages.
Materials deposition has been improved by the development of templates for printing paste onto the component rather than the PCB. This approach avoids the possibility of contaminating the substrate while alleviating the constraints of restricted accessibility.
Finally, cost-effective imaging of the reflowed joints is now possible using endoscope-based inspection systems, which offer a defect-coverage alternative to X-ray as well as easy-to-interpret results.
For more information on the article's content, contact Sherilyn Hill at Metcal Inc., 1530 O'Brien Dr., Menlo Park, CA 94025; (650) 325-3291; Fax: (650) 325-5932; E-mail: shill@ metcal.com; Web site: www. metcal.com.