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Reworking Area-array Packages
December 31, 1969 |Estimated reading time: 18 minutes
By Patrick McCall
Higher process control levels and controlled heat application will be required when reworking or repairing the next generation of reduced sized device assemblies.
Over the last several years, standard area-array devices [ball grid arrays (BGA), chip scale packages (CSP), flip chip, etc.] have become preferred packages in design and manufacturing. Reasons for moving away from traditional leaded packages include increased input/output (I/O) per unit of printed circuit board (PCB) area, wider capabilities and functionalities, and higher production yields. The latest shift in packaging technology is the movement toward microelectronic packages (MEP), permiting manufacturers to fulfill the demands for smaller products, higher power and increased performance.
MEP Growth
MEP production is expected to increase by 200 to 300 percent this year and will increase by 150 to 200 percent in 2001. As volume increases, package costs will come down and the specialized technology to manufacture, attach and apply MEPs will become more accessible.
Underfill is used to strengthen the MEP's structural stability and PCB interconnection. When underfill is used, rework is affected significantly. Reworkable underfills have been introduced; however, as they find their way into products, repair and rework centers will have to deal with other MEPs' differences from standard packages. Some rework areas to be explored include: package removal and site preparation, placement, flux/paste application, bottomside heating, and heating-process considerations.
Package Removal
When removing packages from PCBs, the reason for the removal and whether the component will be reused should guide the approach. If the reason is package failure, the removal profile may use faster ramp rates and shorter times. However, when a device must be reused, a removal profile similar to that for installation with slow and even heating should be used. This also is the case when dealing with PCBs and packages made from materials that must adhere to ramp-rate guidelines. Regardless of the approach, proper preheating and thorough PCB warming are critical to success.
Figure 1. This example shows the maximum allowable package placement misregistration for solder self-alignment.
Removing packages by applying heat only from the top should be avoided because this can result in damaged or lifted pads. The amount of heat required to melt solder spheres via topical application often results in overheating the rework site as well as exceeding recommended heating ramps. This approach also causes significant temperature differences between the top and bottom of the device and the PCB, which can cause twisting, flexing, and damage to microvias and other delicate circuitry as well as package delamination ("popcorning").
All solder joints under the package must be in the liquidus state before lifting the package from the board. Rework equipment that uses devices to automatically lift a package after removal profile completion is common and generally work well. However, the removal profile must be validated with thermocouples to ensure proper temperatures are reached. If a solder joint does not reach liquidus and the automated head rises, pads can be pulled off.
Figure 2. VOS solder and land arrays images overlaid to verify whether patterns match.
If the rework system has a vacuum cup inside the nozzle, it can shield a portion of the component, resulting in uneven heating. Unlike larger packages with longer heating cycles that allow the entire package to reach a homogenous temperature, MEPs have shorter heating cycles, are thinner and have different material compositions than standard packages. They are more likely to develop cool spots under the vacuum cup, resulting in nonliquidus solder joints. To avoid pad damage, this anomaly can be resolved by increasing the reflow phase duration. As an aid, thermocouples should be used when developing and validating profiles.
Site Preparation
Once a package has been removed, the rework site must be prepared properly for the next installation. Excess solder on land patterns with pads larger than 0.8 mm can be removed using a conductive desoldering tool. Other techniques include using a solder wick or hot-gas heating. Removing excess solder from land sites with pads less than 0.5 mm requires a very delicate hand. If conductive desoldering tools are selected, a light touch and low temperatures must be used. Temperatures greater than 200°C, combined with pressure, can damage pads or the soldermask. Conductive systems that use Teflon tips at low temperatures and incorporate hot-gas-assisted heating are preferred. Using a solder wick is not recommended for MEP land preparation.
Figure 3. Full- and split-screen views of high-I/O MEP. Split vision permits full alignment by focusing on fewer data points.
Generally, PCBs featuring microvias are not good candidates for conductive desoldering techniques. When the desoldering tip is moved across the land pattern, the vias can fill with solder, which is not easy to clean attempts to do so usually result in via, soldermask or PCB damage. Bumping the lands is another technique for removing excess solder or adding additional solder to the rework site. This is quite common for placing leaded devices. The technique uses a soldering iron tip to reflow and level the solder on the pads. Typically, flux is applied to the land site and a tinned tip is drawn across the array. The solder's natural surface tension controls the solder volume left behind on the pad.
If pads are bumped, they will be rounded; a sticky or gel flux must be used to hold the package in position. During the profile's soak portion, when the flux is activated and driven off, the package usually will move and can become misregistered. Therefore, while bumping can work fairly well on standard packages, it is not recommended for MEP land-site preparation. If bumping is used on MEPs, numerous issues must be considered:
- The amount of solder involved with MEP interconnects is very small. Using old solder can result in poor quality joints, while adding too much may result in bridging.
- MEP pads can be so small in diameter that the solder surface tension on the tip will pull the solder over the pad, leaving none behind. Additionally, drawing a hot iron tip over a MEP array can damage and lift pads.
- The solder bumps may not be perfectly level. When installing packages with bump diameters as small as 0.1 mm, an unlevel device is difficult to avoid.
Once the excess solder has been removed, it is important to properly clean the PCB. The pads must be clean and free from old solder and flux residue before proceeding with package replacement. When installing MEPs, proper cleaning is just as important to the process as the installation profile. Shorter reflow cycles and small flux amounts coupled with the limited solder volume present in each joint do not make for a forgiving reflow configuration.
Placement
Various package sizes, solder bump diameters and pitches exist in standard package configurations as well as MEPs. Placing standard packages is relatively easy and can be accomplished with commonly available techniques. For example, placement by hand by using a template or the silkscreen around the land pattern as a guide is an available option. A vision overlay system (VOS) can be used to ensure proper alignment. Typically, a minimum magnification of 35X is required to ensure proper alignment, and a placement precision of 0.1 to 0.2 mm in the Z travel usually is adequate.
To place MEPs successfully, a VOS with minimum magnification of 80X is required. (Often, 100X magnification is needed to accurately align flip chips.) Placement precision in the Z direction should be 50 µm or less; for flip chips, 25 µm are required.
Figure 4. Reflow profile of BGA 225 installation on a six-layer PCB without "spike" heating from below.
The precision level for placement in the Z-travel mechanism is related directly to the solder sphere diameter. Precision rating requirements should be calculated using the smallest diameter to be reworked. To ensure proper installation, the solder bump must cover at least 50 percent of the pad, or the accuracy must be at least 50 percent when the device is placed on the PCB to take advantage of the array packages' self-aligning properties (Figure 1). For example, placing a package that has solder sphere diameters of 0.5 mm, one can assume that the maximum misregistration is 0.25 mm (50 percent of sphere diameter). This means that the placement system's precision tolerance in the Z direction must be less than 0.25 mm over the distance of travel. A good rule of thumb is to use 30 percent to ensure proper placement. Looking at the example again, using the 30 percent rule, a Z travel precision of 0.15 mm (0.5 mm x 30 percent) over the distance of travel should be specified.
Typically, a VOS is comprised of a prism to collect two images, one from above and one from below. The images are projected onto a series of mirrors and reflected to the camera lens where they are displayed on a video monitor as two separate overlaid images. The component or the board is repositioned until the solder sphere and land array patterns match exactly (Figure 2).
Figure 5. Reflow profile of BGA 225 installation on a six-layer PCB with "spike" heating from below.
Most VOSs also have a "split vision" capability, which permits a view of only two (opposite) corners of the images at a higher magnification. This is valuable when placing devices that have hundreds of solder contacts. To align more than 300 data points, for example, the images can be "split" and magnified to view two opposite package corners, focusing on less than 100 points of data (Figure 3). Proper VOS care and maintenance is important to placement success. Calibration checks should be performed regularly or as indicated by the manufacturer. When adjustment is required, only those qualified should attempt the work.
An equally important piece in the placement puzzle is the table/board holder. It should be stable, durable, adjustable, spring-loaded, and able to hold various board shapes and sizes. (When reworking MEPs, the ability to securely hold small boards, down to 20 mm wide, is desirable.) Additionally, PCBs containing MEPs usually are thinner and require proper underside support to ensure planarity.
Flux/Paste Application
Flux must be used for successful package installation. It may be used alone or combined with solder paste. Applying the proper flux amount to the rework site is critical. Too little flux and the solder will not flow correctly; too much and out-gassing can occur, resulting in voids within the solder joint or residue formation that can cause resistivity and corrosion problems.
Figure 6. Profile of inner and outer solder sphere temperatures without "spike."
There are many methods for applying flux, including a brush or pen applicator, a piston-driven mechanism to dispense a gel or sticky material, and a flux-applicator tool. When applying flux to area-array packages, the ideal amount covers 1/3 of the solder sphere. This ensures that enough flux is present to clean and remove oxides while eliminating the potential for outgassing and excessive flux residue. With the introduction of gel flux, applying precise amounts of flux can be accomplished and repeated consistently.
Solder paste should always be used when:
- Bumps on the package bottom are made from 90/10 solder
- PCBs have bare copper pads
- The package contains an elastomer layer for coefficient of thermal expansion (CTE) differences
- Joint geometry and solder paste must match that of the production assembly
- No-clean flux cannot be used
- A specification exists for the joint standoff height for function or cleaning.
Solder paste can be applied using various techniques, including spot stencils, stencils to apply the paste to the component and dispensing equipment to apply solder paste dots to individual pads.
Solder paste with #4 sphere sizes should be used for small pad printing. When applying solder paste to standard packages, a paste thickness of 0.17 to 0.23 mm typically is specified. When using solder paste for MEPs, a paste thickness of 0.1 to 0.16 mm is desired. Similarly, stencils with apertures larger than 0.2 mm are round and lend themselves to good release. Apertures in stencils designed for paste deposits smaller than 0.2 mm often are diamond shaped, which generally permits an easier release.
Bottomside Heating
Bottomside heating is associated with the "preheat" phase of a solder reflow profile; however, bottomside heating is as important in the soak and reflow phases, exposing the PCB and package to minimal thermal stress.
In the preheat phase, bottomside heating ensures homogenous temperatures across the board. This keeps the PCB from warping, twisting or flexing during the process. Heat application from the bottom during the preheat phase also is used to warm the PCB so heat is not drawn away from the installation site during the remainder of the process.
Figure 7. Profile of inner and outer solder sphere temperatures with "spike."
In the soak phase, the bottomside heater should continue to operate while a relatively small amount of heat is added from the top heater. The combination of top- and bottomside heat application permits the installation site and package to reach a temperature between 140° and 160°C, and to stabilize. The stabilization should be maintained for 40 to 60 seconds, allowing the flux to activate and drive off any volatiles in the material. This eliminates the potential for outgassing and prepares the package and PCB for reflow.
During reflow, heat is applied from the top heater, which generally operates between 200° and 350°C. This can place great thermal stress on the package top. Therefore, it is important to apply heat slowly and evenly so the package warms to reflow temperature uniformly. Temperature differences across a package of as little as 7° to 10°C can cause damage.
The bottom heater's set temperature can be maintained or increased. Increasing the temperature by as little as 30°C during reflow dramatically impacts the package profile. One benefit of applying additional heat from the bottom or "spiking" during the reflow phase is that lower temperatures can be applied from the top.
Installations always should be achieved with the lowest temperatures possible to ensure package and PCB safety. Additionally, by subjecting the package to lower temperatures, there is less chance for temperature overshooting, which can result in significant temperature variations between the package top and bottom, solder joint, and PCB.
In Figures 4 and 5, reflow is achieved successfully. However, in Figure 4, the temperature variation between the package top and the solder sphere is significant enough to damage the package. Table 1 details spike impact analysis. In another comparison, the spike effect can be seen in lower reflow temperatures as well as in more even heating between outer and middle solder spheres in the array pattern. Here, holes are drilled through the center of two lands on a six-layer PCB, one on an outer row of bumps and one in the center of the component. Thermalcouples are positioned and secured even with the PCB top so the component solder spheres contact the thermocouples without affecting package position. Figures 6 and 7 show the spike effect in this scenario.
By controlling the heat application from the installation site's top and bottom, a more even heating is achieved. This can be seen in lower temperature variances between package top and bottom and between the solder bumps on the outer edge and in the middle of the array pattern (Table 2).
Heating-process Considerations
While standard BGAs and MEPs look similar in design, they actually are quite different and demand different profile configurations for installation and removal. Physically, they differ in several areas: materials, dimensions, mass and tolerances to temperature. In general, profiles used to install MEPs are shorter than those for standard packages. Also, lower temperatures can and should be used when installing MEPs. Because the packages are essentially the size of the silicon die or slightly larger, there is no significant encapsulation for protection and heat is applied/transferred directly to the silicon.
The MEP's smaller mass solder bumps coupled with a very thin package permit heat to transfer through the component very quickly. When standard BGAs are installed, heat must be driven through and around the package, requiring a longer heat application period. If a profile for a standard package is used on an MEP, the component will become super-heated, which must be avoided.
As mentioned, vacuum cups in the nozzle can affect profile results. To demonstrate their effect, a microBGA 46 is installed twice on a cellular phone PCB using the same profile parameters. A hole is drilled through a pad center and a thermocouple is installed flush with the board's top, contacting a solder bump. The only difference between the two trials is the addition of the vacuum cup.
The vacuum cup effects are dramatic (Figure 8): Soak and reflow ramp rates are slower; final soak temperature is lower; heating continues beyond the reflow phase; cooling is inhibited; and the solder spheres do not reflow. In this case, the vacuum cup used is round so the package edges and corners are exposed and subjected to more heat than experienced by the portion under the vacuum cup. The result is higher temperature variations across the entire package.
There are many methods for applying heat as well as monitoring and controlling it; arguments can be made for all. Heaters used in area-array rework equipment almost always are controlled through closed-loop structures. This means that the heater is cycled on and off based on the thermal sensor condition. In many cases, the variation between set and actual temperatures between rework equipment is because of sensor position.
Thermocouple Placement
Sensor placement can dramatically affect set temperature results. Profiles should always be created and validated using feedback from thermocouples.
Many systems permit settings to be adjusted while a profile cycle is running.
Usually a PCB will be populated with thermocouples in numerous locations, including the top of the device, at least one solder sphere (two is better, inner and outer bumps), and two on the PCB bottomside, directly under and away from the rework site.
Positioning thermocouples in these locations permits monitoring temperature variations between the package's top and bottom, between the middle and outer edge, and a means of monitoring the solder contacts' thermal environment.
Monitoring the temperature directly below the rework site is important. Too much heat can damage circuitry and microvias and cause PCB delamination. The thermocouple on the PCB bottom and away from the rework site ensures that the entire board is warmed properly and not exposed to high temperatures.
Profiling can be accomplished using a site containing a previously installed package or by performing an actual installation. However, there are caveats with each:
- If using a site where a package is already installed, the thermocouples must contact the existing solder joints. This is best accomplished by drilling through the board bottom into a solder joint and affixing the sensor. Affixing a thermocouple to the top of a package already soldered is easier in most cases.
- When developing profiles through an actual installation, it is important to make sure the solder spheres are touching the thermocouples throughout the entire process. Should a thermocouple lose contact, bad data will be collected. Additionally, it often is difficult to attach a thermocouple to the top of a loose package and still maintain contact with the PCB during the process.
Ramp Rates and Maximum Temperatures
Acceptable ramp rates and maximum temperatures can be obtained from the package manufacturer. It is wise to select a maximum temperature that permits a margin of safety.
Preheat: If a "step profile" is desired, the PCB top should reach a stable temperature of 95° to 105°C. When plotting a curve, the trace should level off at this temperature. If a linear slope is desired, preheat is merged with the soak phase. The package and PCB are warmed at a constant rate (usually 1° to 3°C per second) until the desired soak temperature is reached.
Figure 8. The effects of a microBGA in contact with a vacuum cup vs. no vacuum cup during reflow are dramatic.
Soak is a critical step in the process. It is important that a soak temperature between 145° and 165°C be reached. Ideally, a relatively stable temperature should be maintained for 40 to 60 seconds. This should be extended if liquid flux is brushed onto the PCB or when installing a large package. Soak also permits the entire package and PCB to come to a uniform temperature.
Reflow is the penultimate phase in the cycle, during which solder bumps reach the melting point and form the joint between the package and the pads. It is critical that all array areas reach solder melt together and that all spheres are in the liquidus state for at least 10 to 15 seconds. Generally, reflow for MEPs will have a shorter phase than standard packages. Additionally, lower temperatures can be used for MEPs because they are thinner and have less mass.
Cool-down, the last phase of the cycle, brings the temperature of the package, solder joints and PCB under the package below liquidus temperatures. Cooling should be controlled; a good rule of thumb is to use the same rate as ramp-up.
Once a profile initially has been defined, it is important to rerun the profile with the determined parameters in a static (unchanging) environment to ensure valid results.
Conclusion
MEP use will continue expanding and the ability to rework them will become a critical competency. During rework, it is extremely important to prepare the land array by removing excess solder and proper cleaning. Placing MEPs on the PCB is similar to placing standard BGAs. However, it can require more delicate and precise equipment. The proper application of flux and paste (if used) also is critical to the installation's success.
Bottomside heating is key to success. When used properly, reflow can be achieved at lower temperatures, exposing the package and PCB to less thermal stress. Additionally, increasing the bottom heater temperature during reflow permits a stable and uniform reflow. While it can be compensated for, it is important to be aware that the profile results can be affected if any additional mass is present inside the nozzle or if anything is contacting the package during reflow. Using thermocouples is paramount when developing and validating profiles. Using multiple thermocouples at multiple locations is preferred.
Guidelines for profile development exist and should be used. Manufacturers' specifications should be adhered to and it is good practice to add a safety margin to maximum temperature and ramp-rate specifications. Profiles used for standard BGA packages will not transfer directly to MEP. However, they can be used as starting points in developing the "perfect" profile.
WORKS CONSULTED
- Kevin Towle, "Line Configuration for Today's Highly Complex Manufacturing," SMT Magazine, November 1998.
- Report on the Technology Road Map for Advanced System Integration and Packaging, Japan Printed Circuit Association (JPCA), 1998.
- M. Bixenman and T. Fang, "Wafer Solder Bumping," Advanced Packaging, June 1999.
- M. Sauer and K. Bergman, "Rework & Repair," SMT Magazine, July 1999.
- P. Wood, "New Considerations in CSP Rework," Circuits Assembly, July 1999.
- M. Wang, K. Nakajima, et al., "Investigation of the Printing Process for CSP Assembly," SMTA 1999 Conference Proceedings.
PATRICK McCALL may be contacted at PACE Inc., 9893 Brewers Court, Laurel, MD 20732; (301) 490-9860; Fax: (301) 498-3252.